W3C

Speech Recognition Grammar Specification Version 1.0

W3C Recommendation 16 March 2004

This version:
http://www.w3.org/TR/2004/REC-speech-grammar-20040316/
Latest version:
http://www.w3.org/TR/speech-grammar/
Previous version:
http://www.w3.org/TR/2003/PR-speech-grammar-20031218/
Editors:
Andrew Hunt, ScanSoft
Scott McGlashan, Hewlett-Packard
Contributors:
See Acknowledgements

Please refer to the errata for this document, which may include some normative corrections.

See also translations.

Copyright © 2004 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.

Abstract

This document defines syntax for representing grammars for use in speech recognition so that developers can specify the words and patterns of words to be listened for by a speech recognizer. The syntax of the grammar format is presented in two forms, an Augmented BNF Form and an XML Form. The specification makes the two representations mappable to allow automatic transformations between the two forms.

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document has been reviewed by W3C Members and other interested parties, and it has been endorsed by the Director as a W3C Recommendation. W3C's role in making the Recommendation is to draw attention to the specification and to promote its widespread deployment. This enhances the functionaility and interoperability of the Web.

This specification is part of the W3C Speech Interface Framework and has been developed within the W3C Voice Browser Activity (activity statement) by participants in the Voice Browser Working Group (W3C Members only).

The design of SRGS 1.0 has been widely reviewed (see the disposition of comments) and satisfies the Working Group's technical requirements. A list of implementations is included in the SRGS 1.0 implementation report, along with the associated test suite.

Comments are welcome on [email protected] (archive). See W3C mailing list and archive usage guidelines.

The W3C maintains a list of any patent disclosures related to this work.

Table of Contents

1. Introduction

This document defines the syntax for grammar representation. The grammars are intended for use by speech recognizers and other grammar processors so that developers can specify the words and patterns of words to be listened for by a speech recognizer.

The syntax of the grammar format is presented in two forms, an Augmented BNF (ABNF) Form and an XML Form. The specification ensures that the two representations are semantically mappable to allow automatic transformations between the two forms.

Both the ABNF Form and XML Form have the expressive power of a Context-Free Grammar (CFG). A grammar processor that does not support recursive grammars has the expressive power of a Finite State Machine (FSM) or regular expression language. For definitions of CFG, FSM, regular expressions and other formal computational language theory see, for example, [HU79]. This form of language expression is sufficient for the vast majority of speech recognition applications.

This W3C standard is known as the Speech Recognition Grammar Specification and is modelled on the JSpeech Grammar Format specification [JSGF], which is owned by Sun Microsystems, Inc., California, U.S.A.

1.1 Grammar Processors and User Agents

A grammar processor is any entity that accepts as input grammars as described in this specification.

A user agent is a grammar processor that accepts user input and matches that input against a grammar to produce a recognition result that represents the detected input.

As the specification title implies, speech recognizers are an important class of grammar processor. Another class of grammar processor anticipated by this specification is a Dual-Tone Multi-Frequency (DTMF) detector. The type of input accepted by a user agent is determined by the mode or modes of grammars it can process: e.g. speech input for "voice" mode grammars and DTMF input for "dtmf" mode grammars.

For simplicity, throughout this document references to a speech recognizer apply to other types of grammar processor unless explicitly stated otherwise.

A speech recognizer is a user agent with the following inputs and outputs:

1.2 Scope

The primary use of a speech recognizer grammar is to permit a speech application to indicate to a recognizer what it should listen for, specifically:

Speech recognizers may also support the Stochastic Language Models (N-Gram) Specification [NGRAM]. Both specifications define ways to set up a speech recognizer to detect spoken input but define the word and patterns of words by different and complementary means. Some recognizers permit cross-references between grammars in the two formats. The rule reference element of this specification describes how to reference an N-gram document.

The grammar specification does not address a number of other issues that affect speech recognition performance. Most of the following capabilities are addressed by the context in which a grammar is referenced or invoked: for example, through VoiceXML 2.0 [VXML2] or through a speech recognizer API.

1.3 Grammar Conversions

The ABNF Form and XML Form are specified to ensure that the two representations are semantically mappable. It should be possible to automatically convert an ABNF Form grammar to an XML Form grammar (or the reverse) so that the semantic performance of the grammars are identical. Equivalence of semantic performance implies that:

  1. Both grammars accept the same language as input and reject the same language as input
  2. Both grammars parse any input string identically

The XSL Transformation document in Appendix F demonstrates automatic conversion from XML to ABNF. The reverse conversion requires an ABNF parser and a transformational program.

There are inherent limits to the automatic conversion to and From ABNF Form and XML Form.

1.4 Semantic Interpretation

A speech recognizer is capable of matching audio input against a grammar to produce a raw text transcription (also known as literal text) of the detected input. A recognizer may be capable of, but is not required to, perform subsequent processing of the raw text to produce a semantic interpretation of the input.

For example, the natural language utterance "I want to book a flight from Prague to Paris" could result in the following XML data structure. To perform this additional interpretation step requires semantic processing instructions that may be contained within a grammar that defines the legal spoken input or in an associated document.

  <book-flight>
    <depart>Prague</depart>
    <arrive>Paris</arrive>
  </book-flight>

The Speech Recognition Grammar Specification provides syntactic support for limited semantic interpretation. The tag construct and the tag-format and tag declarations provide a placeholder for instructions to a semantic processor.

The W3C Voice Browser Working Group is presently developing the Semantic Interpretation for Speech Recognition specification [SEM]. That specification defines a language that can be embedded in tags within SRGS grammars to perform the interpretation process. The semantic processing is defined with respect to the logical parse structure for grammar processing (see Appendix H). Other tag formats could be used but are outside the scope of the W3C activities.

For examples of semantic interpretation in the latest working draft see [SEM].

The output of the semantic interpretation processor may be represented using the Natural Language Semantics Markup Language [NLSML]. This XML representation of interpreted spoken input can be used to transmit the result, as input to VoiceXML 2.0 [VXML2] processing or in other ways.

The semantic interpretation carried out in the speech recognition process is typically characterized by:

It is this restricted form of semantic interpretation that this approach is intended to support. A VoiceXML application that receives a speech result with semantic interpretation will typically process the user input to carry out a dialog. The application may also perform deeper semantic analysis, for example resolving deictic or anaphoric references.

1.5 Embedded Grammars

The Speech Recognition Grammar Specification is designed to permit ABNF Form and XML Form grammars to be embedded into other documents. For example, VoiceXML 1.0 [VXML1] and VoiceXML 2.0 [VXML2] permit inline grammars [VXML2 §3.1.1.1] in which an ABNF Form grammar or XML Form grammar is contained within a VoiceXML document.

Embedding an XML Form grammar within an XML document can be achieved with XML namespaces [XMLNS] or by incorporating the grammar XML Schema definition or DTD into to enclosing document's schema or DTD.

An ABNF Form grammar may be embedded into any XML document as character data. ABNF grammars will often contain angle brackets which require special handling within XML. A CDATA section [XML §2.7] or the escape sequences of "&lt;" and "&gt;" may be required to create well-formed XML. Note: angle brackets ('<' and '>') are used in ABNF to delimit any URI, media type or repeat operator.

1.6 Terminology

Requirements terms
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. However, for readability, these words do not appear in all uppercase letters in this specification.

URI: Uniform Resource Identifier
A URI is a unifying syntax for the expression of names and addresses of objects on the network as used in the World Wide Web. A URI is defined as any legal 'anyURI ' primitive as defined in XML Schema Part 2: Datatypes [SCHEMA2 §3.2.17]. The XML Schema definition follows [RFC2396] and [RFC2732]. The syntax representation of a URI differs between the ABNF Form and the XML Form. Any relative URI reference must be resolved according to the rules given in Section 4.9.1.
  • ABNF URI: in the ABNF Form of this specification a URI is delimited by angle brackets ('<' '>'). For example, <http://www.example.com/file-path>
  • XML URI: in the XML Form of this specification any URI is provided as an attribute to an element; for example the ruleref and lexicon elements.

Media Type
A media type (defined in [RFC2045] and [RFC2046]) specifies the nature of a linked resource. Media types are case insensitive. A list of registered media types is available for download [TYPES]. In places where a URI can be specified a media type may be provided to indicate the content type of URI.

[See Appendix G for information on media types for the ABNF and XML Forms of the Speech Recognition Grammar Specification.]

  • ABNF URI with Media Type: in the ABNF Form a media type may be attached as a postfix to any URI. The media type is delimited by angle brackets ('<' '>') and the URI and media type are separated by a tilde character ('~') without intervening white space. For example,
    <http://example.com/file-path>~<media-type>
  • XML URI with Media Type: in the XML Form any element that carries a URI attribute may carry a type attribute.

Language identifier
A language identifier labels information content as being of a particular human language variant. Following the XML specification for language identification [XML §2.12] a legal language identifier in ABNF Form grammars and XML Form grammars is identified by an RFC 3066 [RFC3066] code. A language code is required by RFC 3066. A country code or other subtag identifier is optional by RFC 3066. A grammar's language declaration declares the language of a grammar. Additionally a legal rule expansion may be labeled by its language content.

White space
White space consists of one or more space (#x20) characters, carriage returns, line feeds, or tabs. The normative definition for both ABNF and XML follows the XML white space definition [XML §2.3]. ABNF processors must also follow the end-of-line handling of XML 1.0 [XML §2.11].

DTMF
DTMF (Dual Tone Multiple Frequency) is an ITU standard for telephony signaling. ITU Recommendation Q.23 defines DTMF generation. ITU Recommendation Q.24 [Q24] defines DTMF reception. A grammar processor that accepts DTMF input should implement Q.24.

2. Rule Expansions

A legal rule expansion is any legal token, rule reference, tag, or any logical combination of legal rule expansions as sequence, alternatives, repeated expansion or language-attributed expansion.

A rule expansion is formally a regular expression (see, for example, [HU79]).

A rule definition associates a legal rule expansion with a rulename.

2.1 Tokens

A token (a.k.a. a terminal symbol) is the part of a grammar that defines words or other entities that may be spoken. Any legal token is a legal expansion.

For speech recognition, a token is typically an orthographic entity of the language being recognized. However, a token may be any string that the speech recognizer can convert to a phonetic representation.

Token Content: In both the XML Form and ABNF Form any unmarked text within a rule definition, except example phrases (XML only) or tag content, is token content. The unmarked text is delimited by any syntactic construct of the grammar form (see below for details on the ABNF Form and XML Form). For each token content span in a grammar the grammar processor applies the following tokenization, white space normalization, token normalization and pronunciation lookup processes. All token content in both the XML Form and ABNF Form is treated as Characters in [XML]. (informative: XML specifies Characters by reference to ISO/IEC 10646 [ISO/IEC 10646] and Unicode [Unicode].)

Tokenization behavior: Text spans containing token sequences are delimited as follows:

Token type Form Example
Single unquoted token ABNF & XML hello
Single unquoted token: non-alphabetic ABNF & XML 2
Single quoted token: including white space ABNF & XML "San Francisco"
Single quoted token: no white space ABNF & XML "hello"
Two tokens delimited by white space ABNF & XML bon voyage
Four tokens delimited by white space ABNF & XML this is a test
Single XML token in <token> XML Only <token>San Francisco</token>

White Space Normalization: White space must be normalized when contained in any token delimited by a <token> elements or by double quotes. Leading and trailing white space characters are stripped. Any token-internal white space character or sequence is collapsed to a single space character (#x20). For example, the following are all normalized to the same string, "San Francisco".

  "San Francisco"
  " San Francisco "
  "San 
  Francisco"
  " San   Francisco "

Because the presence of white space within a token is significant the following are distinct tokens.

  "San Francisco"
  "SanFrancisco"
  "San_Francisco"

Token Normalization: Other normalization processes are applied to the white space normalized token according to the language and the capabilities of the speech recognizer.

Grammar processors may assume Early Uniform Normalization as defined in the Character Model for the World Wide Web 1.0 [CHARMOD §4].

Pronunciation Lookup: To match spoken (audio) input to a grammar a speech recognition must be capable of modelling the audio patterns of any token in a grammar. Speech recognizers employ a diverse set of techniques for performing this key recognition process. The following is an informative description of techniques that a speech recognizer may apply based on conventional large vocabulary speech recognition technology.

A large vocabulary speech recognizer converts each normalized token to a phoneme sequence or a set of possible phoneme sequences. Conversion of an orthographic form (token) to the spoken form (phonemes) is a highly language-specific process. In many cases the conversion is even specific to a national variant, regional dialect or other variant of the language. For example, for some tokens Parisian French, Quebec French and Swiss French will each convert to different pronunciations.

The text-to-phoneme conversion in a large vocabulary speech recognizer may involve some or all of the following sub-processes.

Any language is likely to have other specialized processes for determining a pronunciation for a token. For example, for Japanese special techniques are required for Kanji and each Kana form.

For any language and recognizer there may be variation in coverage and completeness of the language's tokens.

When a grammar processor handles a grammar containing a token that it cannot convert to phonemic form or otherwise use in the speech recognition processing of audio it should inform the hosting environment.

Limitations of token handling: the following is informative guidance to grammar developers.

The Pronunciation Lexicon activity [LEX] of the W3C Voice Browser Working Group will provide guidance on the token-handling processes outlined above.

Token handling will vary between recognizers and will vary between languages.

Grammar authors can improve document portability by avoiding characters and forms in tokens that do not have obvious pronunciations in the language. For English, the following are ways to handle some orthographic forms:

ABNF Form

Any plain text within a rule definition is token content. The ABNF Syntax (Appendix D) normatively defines the token parsing behavior.

A language attachment may be provided for any token. When attached to a token the language modifies the handling of that token only.

Informative

The rule expansion of a rule definition is delimited at the start and end by equals sign ('=') and semicolon (';') respectively. Any leading plain text of the rule expansion is delimited by ('=') and similarly any final plain text is closed by semicolon.

Within a rule expansion the following symbols have syntactic function and delimit plain text.

Within plain text regions delimited by these characters the tokenization, white space normalization, token normalization and pronunciation lookup processes described above apply.

XML Form

Any token element explicitly delimits a single token as described above. The token element may include an optional xml:lang attribute to indicate the language of the contained token.

Any other character data within a rule element (rule definition) or item element is token content. Note that character data within tag or example is not token text.

2.2 Rule Reference

Any legal rule reference is a legal rule expansion .

Rulenames: Every rule definition has a local name that must be unique within the scope of the grammar in which it is defined. A rulename must match the "Name" Production of XML 1.0 [XML §2.3] and be a legal XML ID. Section 3.1 documents the rule definition mechanism and the legal naming of rules.

This table summarizes the various forms of rule reference that are possible within and across grammar documents.

Note: an XML Form grammar document must provide one and only one of the uri or special attributes on a ruleref element. There is no equivalent constraint in ABNF since the syntactic forms are distinct.

Reference type ABNF Form XML Form
2.2.1: Explicit local rule reference $rulename <ruleref uri="#rulename"/>
2.2.2: Explicit reference to a named rule of a grammar identified by a URI $<grammarURI#rulename> <ruleref uri="grammarURI#rulename"/>
2.2.2: Implicit reference to the root rule of a grammar identified by a URI $<grammarURI> <ruleref uri="grammarURI"/>
2.2.2: Explicit reference to a named rule of a grammar identified by a URI with a media type $<grammarURI#rulename>~<media-type> <ruleref uri="grammarURI#rulename" type="media-type"/>
2.2.2: Implicit reference to the root rule of a grammar identified by a URI with a media type $<grammarURI>~<media-type> <ruleref uri="grammarURI" type="media-type"/>
2.2.3: Special rule definitions $NULL
$VOID
$GARBAGE		 <ruleref special="NULL"/>
<ruleref special="VOID"/>
<ruleref special="GARBAGE"/>

2.2.1 Local References

When referencing rules defined locally (defined in the same grammar as contains the reference), always use a simple rulename reference which consists of the local rulename only. The ABNF Form and XML Form have a different syntax for representing a simple rulename reference.

ABNF Form

The simple rulename reference is prefixed by a "$" character.

$city
$digit

XML Form

The ruleref element is an empty element with a uri attribute that specifies the rule reference as a same-document reference URI [RFC2396]: that is, the attribute consists only of the number sign ('#') and the fragment identifier that indicates the locally referenced rulename.

<ruleref uri="#city"/>
<ruleref uri="#digit"/>

2.2.2 External Reference by URI

References to rules defined in other grammars are legal under the conditions defined in Section 3. The external reference must identify the external grammar by URI and may identify a specific rule within that grammar. If the fragment identifier that would indicate a rulename is omitted, then the reference implicitly targets the root rule of the external grammar.

Any externally-referenced rule may be activated for recognition. That is it may define the top-level syntax of spoken input. For instance, VoiceXML [VXML2] grammar activation may explicitly reference one or more public rules (see Section 3.2) and/or implicitly reference the root rule (see Section 4.7).

A URI reference is illegal if the referring document and referenced document have different modes. For instance, it is illegal to reference a "dtmf" grammar from a "voice" grammar. (See Section 4.6 for additional detail on modes).

A resource indicated by an URI reference may be available in one or more media types. The grammar author may specify the preferred media-type via the type attribute (XML form) or in angle braces following the URI (ABNF form).When the content represented by a URI is available in many data formats, a grammar processor may use the preferred media-type to influence which of the multiple formats is used. For instance, on a server implementing HTTP content negotiation, the processor may use the preferred media-type to order the preferences in the negotiation.

The resource representation delivered by dereferencing the URI reference may be considered in terms of two types. The declared media-type is the asserted value for the resource and the actual media-type is the true format of its content. The actual media-type should be the same as the declared media-type, but this is not always the case (e.g. a misconfigured HTTP server might return text/plain for an application/srgs+xml document). A specific URI scheme may require that the resource owner always, sometimes, or never return a media-type. The declared media-type is the value returned by the resource owner or, if none is returned, the preferred media type given in the grammar. There may be no declared media-type if the resource owner does not return a value and no preferred type is specified. Whenever specified, the declared media-type is authoritative.

Three special cases may arise. The declared media-type may not be supported by the processor; this is an error. The declared media-type may be supported but the actual media-type may not match; this is also an error. Finally, there may be no declared media-type; the behavior depends on the specific URI scheme and the capabilities of the grammar processor. For instance, HTTP 1.1 allows document introspection (see RFC 2616, section 7.2.1), the data scheme falls back to a default media type, and local file access defines no guidelines. The following table provides some informative examples:

HTTP 1.1 request
Local file access
Media-type returned by the resource owner text/plain application/srgs+xml <none> <none>
Preferred media-type appearing in the grammar Not applicable; the returned type takes precedence application/srgs+xml <none>
Declared media-type text/plain application/srgs+xml application/srgs+xml <none>
Behavior if the actual media-type is application/srgs+xml Error; the declared and actual types do not match The declared and actual types match; success if application/srgs+xml is supported by the grammar processor; otherwise an error Scheme specific; the grammar processor might introspect the document to determine the type.

See Appendix G for a summary of the status for media types for ABNF Form and XML Form grammars.

ABNF Form

In ABNF an external reference by URI is represented by a dollar sign ('$') followed immediately by either an ABNF URI or ABNF URI with media type. There must be no white space between the dollar sign and the URI.

// References to specific rules of an external grammar
$<http://grammar.example.com/world-cities.gram#canada>
$<http://grammar.example.com/numbers.gram#digit>

// Implicit reference to the root rule of an external grammar
$<../date.gram>

// References with associated media types
$<http://grammar.example.com/world-cities.gram#canada>~<application/srgs>
$<../date.gram>~<application/srgs>

Note: the media type of "application/srgs" has been requested for ABNF Form grammars. See Appendix G for details.

XML Form

An XML rule reference is represented by a ruleref element with a uri attribute that defines the URI of the referenced grammar and rule within it. If a fragment identifier is appended then the identifier indicates a specific rulename being referenced. If the fragment identifier is omitted then the reference is (implicitly) to the root rule of the referenced grammar.

The optional type attribute specifies the media type of the grammar containing the reference.

<!-- References to specific rules of an external grammar -->
<ruleref uri="http://grammar.example.com/world-cities.grxml#canada"/>
<ruleref uri="http://grammar.example.com/numbers.grxml#digit"/>

<!-- Implicit reference to the root rule of an external grammar -->
<ruleref uri="../date.grxml"/>

<!-- References with associated media types -->
<ruleref uri="http://grammar.example.com/world-cities.grxml#canada"
         type="application/srgs+xml"/>
<ruleref uri="../date.grxml" type="application/srgs+xml"/>

Note: the media type "application/srgs+xml" has been requested for XML Form grammars. See Appendix G for details on media types for grammars.

2.2.3 Special Rules

Several rulenames are defined to have specific interpretation and processing by a speech recognizer. A grammar must not redefine these rulenames.

In the ABNF Form a special rule reference is syntactically identical to a local rule reference. However, the names of the special rules are reserved to prevent a rule definition with the same name.

In the XML Form a special rulename is represented with the special attribute on a ruleref element. It is illegal to provide both the special and the uri attributes.

NULL
Defines a rule that is automatically matched: that is, matched without the user speaking any word.

ABNF Form: $NULL
XML Form: <ruleref special="NULL"/>

VOID
Defines a rule that can never be spoken. Inserting VOID into a sequence automatically makes that sequence unspeakable.

ABNF Form: $VOID
XML Form: <ruleref special="VOID"/>

GARBAGE
Defines a rule that may match any speech up until the next rule match, the next token or until the end of spoken input. A grammar processor must accept grammars that contain special references to GARBAGE. The behavior GARBAGE rule is implementation-specific. A user agent should be capable of matching arbitrary spoken input up to the next token but may treat GARBAGE as equivalent to NULL (match no spoken input).

ABNF Form: $GARBAGE
XML Form: <ruleref special="GARBAGE"/>

Informative example: given suitable definitions of US cities and states, a speech recognizer may implement the following ABNF and XML rule definitions to match "Philadelphia in the great state of Pennsylvania" as well as simply "Philadelphia Pennsylvania". 
$location = $city $GARBAGE $state;

<rule id="location">
  <ruleref uri="#city"/>
  <ruleref special="GARBAGE"/>
  <ruleref uri="#state"/>
</rule>

2.2.4 Referencing N-gram Documents (Informative)

The W3C Voice Browser Working Group has released a Working Draft for the Stochastic Language Models (N-Gram) Specification [NGRAM]. These two specifications represent different and complementary ways of informing a speech recognizer of which words and patterns of words to listen for.

A speech recognizer may choose to support the Speech Recognition N-Gram Grammar Specification in addition to the speech recognition grammar defined in this document.

If a speech recognizer supports both grammar representations it may optionally support references between the two formats. Grammars defined in the ABNF Form or XML Form may reference start symbols of N-Gram documents and vice versa.

The syntax for referencing an N-Gram is the same as referencing externally defined ABNF Form or XML Form grammar documents. A media type is recommended on a reference to an N-gram document. The Working Group has not yet applied for a type on N-gram documents so no example is given. The fragment identifier (a rulename when referencing ABNF Form and XML Form grammars) identifies a start symbol as defined by the N-Gram specification. If the start symbol is absent the N-Gram, as a whole, is referenced as defined in the N-Gram specification.

ABNF Form

URI references to N-Gram documents follow the same syntax as references to other ABNF or XML Form grammar documents. The following are examples of references to an N-Gram document via an explicit rule reference and an implicit reference to the root rule.

$<http://grammar.example.com/ngram.xml#StartSymbol>
$<http://grammar.example.com/ngram.xml>

XML Form

URI references to N-Gram documents follow the same syntax as reference to other ABNF Form and XML Form grammar documents. The following are examples of references to an N-Gram document via an explicit rule reference and an implicit reference to the root rule.

<ruleref uri="http://grammar.example.com/ngram.xml#StartSymbol"/>
<ruleref uri="http://grammar.example.com/ngram.xml"/>

2.3 Sequences and Encapsulation

A sequence of legal rule expansions is itself a legal rule expansion.

The sequence of rule expansions implies the temporal order in which the expansions must be detected by the user agent. This constraint applies to sequences of tokens, sequences of rule references, sequences of tags, parentheticals and all combinations of these rule expansions.

Both the ABNF Form and XML Form provide syntax for encapsulating any expansion. This is used, for example, to attach a repeat operator, a language identifier or to ensure correct precedence in parsing (ABNF only).

ABNF Form

A sequence of legal expansions separated by white space is a legal expansion.

A legal expansion surrounded by parentheses ('(' and ')') is a legal expansion.

this is a test           // sequence of tokens
$action $object          // sequence of rule references
the $object is $color    // sequence of tokens and rule references
(fly to $city)           // parentheses for encapsulation
Special cases

An empty parenthetical is legal as is a parenthetical containing only white space; e.g. '()' or '( )'. Both forms are equivalent to $NULL and a grammar processor will behave as if the parenthetical were not present.

// equivalent sequences
phone home
phone ( ) home

XML Form

A sequence of XML rule expansion elements ( <ruleref>, <item>, <one-of>, <token> <tag>) and CDATA sections containing space separated tokens must be recognized in temporal sequence. (The only exception is where one or more "item" elements appear within a one-of element.)

An item element can surround any expansion to permit a repeat attribute or language identifier to be attached. The weight attribute of item is ignored unless the element appears within a one-of element.

<!-- sequence of tokens -->
this is a test

<!--sequence of rule references-->
<ruleref uri="#action"/> <ruleref uri="#object"/>

<!--sequence of tokens and rule references-->
the <ruleref uri="#object"/> is <ruleref uri="#color"/>

<!-- sequence container -->
<item>fly to <ruleref uri="#city"/> </item>
Special cases

An empty item element is legal as is an item element containing only white space. Both forms are equivalent to a NULL reference and a grammar processor will behave as if the item were not present.

<!-- equivalent sequences -->
phone home
phone <item/> home
phone <item></item> home
phone <item>    </item> home

2.4 Alternatives

Any set of alternative legal rule expansions is itself a legal rule expansion. For input to match a set of alternative rule expansions it must match one of the set of alternative expansions. A set of alternatives must contain one or more alternatives.

Any set of alternatives may be labeled with a language attachment. In the XML Form an xml:lang attribute is present on the one-of element. In the ABNF Form to ensure correct precedence the set of alternatives must be delimited by parentheses with the ABNF language attachment immediately following.

2.4.1 Weights

A weight may be optionally provided for any number of alternatives in an alternative expansion. Weights are simple positive floating point values without exponentials. Legal formats are "n", "n.", ".n" and "n.n" where "n" is a sequence of one or many digits.

A weight is nominally a multiplying factor in the likelihood domain of a speech recognition search. A weight of 1.0 is equivalent to providing no weight at all. A weight greater than "1.0" positively biases the alternative and a weight less than "1.0" negatively biases the alternative.

[JEL98] and [RAB93] are informative references on the topic of speech recognition technology and the underlying statistical framework within which weights are applied.

Grammar authors and speech recognizer developers should be aware of the following limitations upon the definition and application of weights as outlined above.

ABNF Form

A set of alternative choices is identified as a list of legal expansions separated by the vertical bar symbol. If necessary, the set of alternative choices may be delimited by parentheses.

Michael | Yuriko | Mary | Duke | $otherNames
(1 | 2 | 3)

A weight is surrounded by forward slashes and placed before each item in the alternatives list.

/10/ small | /2/ medium |  large
/3.1415/ pie | /1.414/ root beer | /.25/ cola
Special Cases

It is legal for an alternative to be a reference to $NULL, an empty parenthetical or a single tag. In each case the input is equivalent to matching $NULL and as a result the other alternatives are optional.

// Legal
$rule1 = word | $NULL;
$rule2 = () | word;
$rule3 = word | {TAG-CONTENT};

An empty alternative (white space only) is not legal.

// ILLEGAL
$rule1 = a | | b;
$rule2 = | b;
$rule3 = a |;

The following construct is interpreted as a single weighted alternative.

// Legal
$rule1 = /2/ word;
$rule2 = /2/ {TAG-CONTENT};
$rule3 = /2/ $NULL;

XML Form

The one-of element identifies a set of alternative elements. Each alternative expansion is contained in a item element. There must be at least one item element contained within a one-of element. Weights are optionally indicated by the weight attribute on the item element.

<one-of>
  <item>Michael</item>
  <item>Yuriko</item>
  <item>Mary</item>
  <item>Duke</item>
  <item><ruleref uri="#otherNames"/></item>
</one-of>

<one-of><item>1</item> <item>2</item> <item>3</item></one-of>

<one-of>
  <item weight="10">small</item>
  <item weight="2">medium</item>
  <item>large</item>
</one-of>

<one-of>
  <item weight="3.1415">pie</item>
  <item weight="1.414">root beer</item>
  <item weight=".25">cola</item>
</one-of>
Special cases

A one-of element containing a single item is legal and requires that input match the single item. The single item may be optionally weighted.

<one-of>
  <item>word</item>
</one-of>

<one-of>
  <item weight="2.0">word</item>
</one-of>

Is it legal for an alternative to be a reference to NULL, an empty item or a single tag. In each case the input is equivalent to matching NULL and as a result the other alternatives are optional.

<one-of>
  <item>word</item>
  <item/>
</one-of>
<one-of>
  <item>word</item>
  
  <item> <ruleref special="NULL"/> </item>
</one-of>
<one-of>
  <item>word</item>
  <item> <tag>TAG-CONTENT</tag> </item>
</one-of>

2.5 Repeats

Any repeated legal rule expansion is itself a legal rule expansion.

Operators are provided that define a legal rule expansion as being another sub-expansion that is optional, that is repeated zero or more times, that is repeated one or more times, or that is repeated some range of times.

ABNF Form
Example		 XML Form
Example		 Behavior
<n>
<6>		 repeat="n"
repeat="6"		 The contained expansion is repeated exactly "n" times. "n" must be "0" or a positive integer.
<m-n>
<4-6>		 repeat="m-n"
repeat="4-6"		 The contained expansion is repeated between "m" and "n" times (inclusive). "m" and "n" must both be "0" or a positive integer and "m" must be less than or equal to "n".
<m->
<3->		 repeat="m-"
repeat="3-"		 The contained expansion is repeated "m" times or more (inclusive). "m" must be "0" or a positive integer. For example, "3-" declares that the contained expansion can occur three, four, five or more times.
<0-1>
[...]		 repeat="0-1" The contained expansion is optional.
Common Repeats

As indicated in the table above, an expansion that can occur 0-1 times is optional. Because optionality is such a common form the ABNF syntax provides square brackets as a special operator for representing optionality.

A repeat of "0-" indicates that an expansion can occur zero times, once or any number of multiple times. In regular expression languages this is often represented by the Kleene star ('*') which is reserved but not used in ABNF.

A repeat of "1-" indicates that an expansion can occur once or any number of multiple times. In regular expression languages this is often represented by the positive closure ('+') which is reserved but not used in ABNF.

Although both ABNF and XML support a grammar that permits an unbounded number of input tokens it is not the case that users will speak indefinitely. Speech recognition can perform more effectively if the author indicates a more limited range of repeat occurrences.

Special Cases

Where a number of possible repetitions (e.g. <m-> or <m-n> (n > 0) but not <0>) is expressed on a construct whose only content is one or more tag elements, the behavior of the grammar processor is not defined and will be specific to individual implementations.

Any number of non-optional repetitions (e.g., <m-n>; m>0) of VOID is equivalent to a single VOID.

The behavior of a grammar processor in handling any number of repetitions of NULL is not defined and will be specific to individual implementations.

If the number of repetitions for any expansion can be only zero (i.e. <0> or <0-0>) then the expansion is equivalent to NULL.

2.5.1 Repeat Probabilities

Any repeat operator may specify an optional repeat probability. The value indicates the probability of successive repetition of the repeated expansion.

A repeat probability value must be in the floating pointing range of "0.0" to "1.0" (inclusive). Values outside this range are illegal. The floating point format is one of "n", "n.", "n.nnnn", ".nnnn" (with any number of digits after the dot).

Note: repeat probabilities and weights are different logical entities and have a different impact upon a speech recognition search.

Informative example: A simple example is an optional expansion (zero or one occurrences) with a probability — say "0.6". The grammar indicates that the chance that the expansion will be matched is 60% and that the chance that the expansion will not be present is 40%.

When no maximum is specified in a range (m-) the probabilities decay exponentially.

Grammar authors and speech recognizer developers should be aware of the following limitations upon the definition and application of repeat probabilities as outlined above.

Useful references on statistical models of speech recognition include [JEL98] and [RAB93].

ABNF Form

The following are postfix operators: <m-n> <m-> <m>. A postfix operator is logically attached to the preceding expansion. Postfix operators have high precedence and so are tightly bound to the immediately preceding expansion (see Section 2.8).

Optional expansions may be delimited by square brackets: [expansion]. Alternatively, an optional expansion is indicated by the postfix operator "<0-1>".

The following symbols are reserved for future use in ABNF: '*', '+', '?'. These symbols must not be used at any place in a grammar where the syntax currently permits a repeat operator.

// the token "very" is optional
[very]
very <0-1>

// the rule reference $digit can occur zero, one or many times

$digit <0->

// the rule reference $digit can occur one or more times

$digit <1->

// the rule reference $digit can occur four, five or six times
$digit <4-6>

// the rule reference $digit can occur ten or more times
$digit <10->

// Examples of the following expansion
//   "pizza"
//   "big pizza with pepperoni"
//   "very big pizza with cheese and pepperoni"
[[very] big] pizza ([with | and] $topping) <0->

Repeat probabilities are only supported in the range form. The probability is delimited by slash characters and contained within the angle brackets: <m-n /prob/> and <m- /prob/>.

// the token "very" is optional and is 60% likely to occur
// and 40% likely to be absent in input
very <0-1 /0.6/>

// the rule reference $digit must occur two to four times 
// with 80% probability of recurrence
$digit <2-4 /.8/>

XML Form

The item element has a repeat attribute that indicates the number of times the contained expansion may be repeated. The following example illustrates the accepted values of the attribute.

<!-- the token "very" is optional -->

<item repeat="0-1">very</item>

<!-- the rule reference to digit can occur zero, one or many times -->

<item repeat="0-"> <ruleref uri="#digit"/> </item>

<!-- the rule reference to digit can occur one or more times -->

<item repeat="1-"> <ruleref uri="#digit"/> </item>

<!-- the rule reference to digit can occur four, five or six times -->
<item repeat="4-6"> <ruleref uri="#digit"/> </item>

<!-- the rule reference to digit can occur ten or more times -->
<item repeat="10-"> <ruleref uri="#digit"/> </item>

<!-- Examples of the following expansion -->
<!--   "pizza" -->
<!--   "big pizza with pepperoni" -->
<!--   "very big pizza with cheese and pepperoni" -->

<item repeat="0-1"> 
   <item repeat="0-1"> very </item>
   big 
</item> 
pizza
<item repeat="0-">
   <item repeat="0-1">
      <one-of>
         <item>with</item>
         <item>and</item>
      </one-of>
   </item>
   <ruleref uri="#topping"/>
</item>

The repeat-prob on the item element carries the repeat probability. Repeat probabilities are supported on any item element but are ignored if the repeat attribute is not also specified.

<-- The token "very" is optional and is 60% likely to occur. -->
<-- Means 40% chance that "very" is absent in input -->
<item repeat="0-1" repeat-prob="0.6">very</item>

<-- The rule reference to digit must occur two to four times -->
<-- with 80% probability of recurrence. -->
<item repeat="2-4" repeat-prob=".8">
   <ruleref uri="#digit"/> 
</item>

2.6 Tags

A tag is a legal rule expansion (a tag can also be declared in the grammar header - see S4.1).

A tag is an arbitrary string that may be included inline within any legal rule expansion. Any number of tags may be included inline within a rule expansion.

Tags do not affect the legal word patterns defined by the grammars or the process of recognizing speech or other input given a grammar.

Tags may contain content for semantic interpretation. The semantic interpretation processes may affect the recognition result.

Language attachments have no effect upon tags.

The tag format declaration indicates the content type of all tags in a grammar.

Special Cases

It is legal to use a tag as a stand-alone expansion. For example, a rule may expand to a single tag and no tokens.

  $rule = {TAG-CONTENT};

  <rule id="rule"><tag>TAG-CONTENT</tag></rule>

ABNF Form

A tag is delimited by either a pair of opening and closing curly brackets — '{' and '}' — or by the following 3-character sequences which are considered very unlikely to occur within a tag — '{!{' and '}!}'. A tag delimited by single curly brackets cannot contain the single closing curly bracket character ('}'). A tag delimited by the 3-character sequence cannot contain the closing 3-character sequence ('}!}').

The tag content is all text between the opening and closing character sequences including leading and trailing white space. The contents of the tag are not parsed by the grammar processor.

Tag precedence is the same as for rule references and tokens. In the first example below there is a sequence of six space-separated expansions (3 tokens, a tag, a token and a tag). In the second example, the alternative is a choice between a sequence containing a token and a tag or a sequence containing a rule reference and a tag.

$rule1 = this is a {TAG-CONTENT-1} test {TAG-CONTENT-2};

$rule2 = open {TAG-CONTENT-1} | $close {TAG-CONTENT-2};

$rule3 = {!{ a simple tag containing { and } needs no escaping }!};

XML Form

A tag element can be a direct child of the item and rule elements. The content of tag is CDATA.

<rule id="rule1">this is a <tag>TAG-CONTENT-1</tag> test <tag>TAG-CONTENT-2</tag> </rule>

<rule id="rule2">
   <one-of>
      <item> open <tag>TAG-CONTENT-1</tag> </item>
      <item> <ruleref uri="#close"/> <tag>TAG-CONTENT-2</tag> </item>
   </one-of>
</rule>

2.7 Language

Any legal rule expansion that has an attached language identifier is itself a legal rule expansion. Both the ABNF Form and the XML Form permit a legal language identifier to be attached to any token, sequence or set of alternatives (Note that rule reference does not permit a language identifier to be attached). The syntax for the ABNF Form and for the XML Form are provided below.

The language declaration for a rule expansion affects only the contained content. Moreover, the language declaration affects only the handling of tokens in the contained content and does not affect tags or rule references. The application of language to token handling and particularly to pronunciation lookup is described in Section 2.1.

By default a grammar is a single language document with a language identifier provided in the language declaration in the grammar header (see Section 4.5). All tokens within that grammar, unless otherwise declared, will be handled according to the grammar's language.

In situations where applications target a multilingual user community, grammars that contain words in more than one language may be needed. For example, in response to a prompt such as: "Do you want to talk to André Prévost?" (a combination of an English sentence with a French name), the response may be either "yes" or "oui".

The Speech Recognition Grammar Specification permits one grammar to collect input from more than one language. The specification also permits multiple grammars each with a separate single language to be used in parallel. The specification also permits a single input utterance to contain more than one language. Finally, the specification permits any combination of the above: for example, parallel grammars each with multi-lingual capability.

Not all user agents are required to support all languages, or indeed any or all of the multi-lingual capabilities. The conformance requirements regarding multi-lingual support for XML Form grammar processors and ABNF Form grammar processors are the same and are laid out in Section 5.4 and Section 5.6 respectively.

There is a related challenge for multilingual applications that deal with proper names (people, streets, companies, etc.) that may be spoken with different pronunciations or accents depending upon the language of origin and the speaking language. It is often impossible to predict the language that users will use to pronounce certain tokens. In fact, users may actually use different languages for different words in the same sentence, and in unpredictable ways. For instance, the name "Robert Jones" might be pronounced by a French-speaking user using the French pronunciation for "Robert" but an English pronunciation for "Jones", whereas a mono-lingual English speaker would use the English pronunciation for both words.

Language scoping: language declarations are scoped locally to a document and to a rule definition. In XML terminology, the language attribute is inherited down the document tree. Where a language change encompasses a reference to another grammar, the referenced rule and its containing grammar define the language of the reference expansion. The language in effect at the point of the rule reference does not have any effect upon the referenced rule.

Language and results: The language used in the recognition of a token is not considered a part of the speech result even in the case that a language declaration is associated with a token.

ABNF Form

In the ABNF Form a language identifier may be right-attached to any legal rule expansion except rule reference. The attachment is an exclamation point character ('!') followed by a legal language identifier without intervening white space.

The language attachment has higher precedence than sequences or alternatives. To attach a language to these rule expansion types the expansion should be delimited by parentheses (see Section 2.3).

#ABNF 1.0 ISO-8859-1;

// Default grammar language is US English
language en-US;

// Single language attachment to tokens
// Note that "fr-CA" (Canadian French) is applied to only
//  the word "oui" because of precedence rules
$yes = yes | oui!fr-CA;

// Single language attachment to an expansion
$people1 = (Michel Tremblay | André Roy)!fr-CA;

// Handling language-specific pronunciations of the same word
// A capable speech recognizer will listen for Mexican Spanish and
//   US English pronunciations.
$people2 = Jose!en-US; | Jose!es-MX;

/**
 * Multi-lingual input possible
 * @example may I speak to André Roy
 * @example may I speak to Jose
 */
public $request = may I speak to ($people1 | $people2);

XML Form

XML 1.0 [XML §2.12] defines the xml:lang attribute for language identification. The attribute provides a single language identifier for the content of the element on which it appears. The xml:lang attribute may be attached to one-of , token and item. It applies the token handling of scoped tokens.

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN"
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">
 
<!-- the default grammar language is US English -->
<grammar xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="en-US" version="1.0">
 
  <!-- 
     single language attachment to tokens
     "yes" inherits US English language
     "oui" is Canadian French language
  -->
  <rule id="yes">
    <one-of>
      <item>yes</item>
      <item xml:lang="fr-CA">oui</item>
    </one-of> 
  </rule> 
  
  <!-- Single language attachment to an expansion -->
  <rule id="people1">
    <one-of xml:lang="fr-CA">
      <item>Michel Tremblay</item>
      <item>André Roy</item>
    </one-of>
  </rule>
  
  <!--
     Handling language-specific pronunciations of the same word
     A capable speech recognizer will listen for Mexican Spanish 
     and US English pronunciations.
  -->
  <rule id="people2">
    <one-of>
      <item xml:lang="en-US">Jose</item>
      <item xml:lang="es-MX">Jose</item>
    </one-of>
  </rule>
  
  <!-- Multi-lingual input is possible -->
  <rule id="request" scope="public">
    <example> may I speak with André Roy </example>
    <example> may I speak with Jose </example>
  
    may I speak with
    <one-of>
      <item> <ruleref uri="#people1"/> </item>
      <item> <ruleref uri="#people2"/> </item>
    </one-of>
  </rule>
</grammar>

2.8 Precedence

This section defines the precedence of the ABNF rule expansion syntax. Because XML documents explicitly indicate structure there is no ambiguity and thus a precedence definition is not required. The precedence definitions for the ABNF Form are intended to minimize the need for parentheses.

ABNF Form

The following is the ordering of precedence of rule expansions. Parentheses may be used to explicitly control rule structure.

  1. A rule reference, a quoted token, an unquoted token or a tag.
  2. Parentheses ('(' and ')') for grouping and square brackets ('[' and ']') for optional grouping.
  3. Repeat operator (e.g. "<0-1>") and language attachment (e.g. "!en-AU") apply to the tightest immediate preceding rule expansion. (To apply them to a sequence or to alternatives, use `()' or `[]' for grouping.)
  4. Sequence of rule expansions.
  5. Set of alternative rule expansions separated by vertical bars ('|') with optional weights.

XML Form

None required. XML structure is explicit.

3. Rule Definitions

A rule definition associates a legal rule expansion with a rulename. The rule definition is also responsible for defining the scope of the rule definition: whether it is local to the grammar in which it is defined or whether it may be referenced within other grammars. Finally, the rule definition may additionally include documentation comments and other pragmatics.

The rulename for each rule definition must be unique within a grammar. The same rulename may be used in multiple grammars.

A rule definition is referenced by a URI in a rule reference with the rulename being represented as the fragment identifier.

3.1 Basic Rule Definition

The core purpose of a rule definition is to associate a legal rule expansion with a rulename.

A legal rulename in either the XML Form or ABNF Form is a character sequence that:

Defined rulenames must be unique within a grammar. The schema enforces this by declaring the rulename as an XML ID.

Rulenames are case-sensitive in both XML and ABNF grammars. Exact string comparison is used to resolve rulename references.

A legal rulename cannot be one of the special rules: specifically "NULL", "VOID" or "GARBAGE".

ABNF Form

The rule definition consists of an optional scoping declaration (explained in the next section) followed by a legal rule name, an equals sign, a legal rule expansion and a closing semicolon. The rule definition has one of the following legal forms:

$ruleName = ruleExpansion;
public $ruleName = ruleExpansion;
private $ruleName = ruleExpansion;

For example:

$city = Boston | "New York" | Madrid;
$command = $action $object;
Special Cases

An empty rule definition is illegal.

It is legal to define a rule that expands to empty parentheses or $NULL (equivalent forms). It is legal to define a rule that expands to a single tag.

// Legal
$rule = ();
$rule = $NULL;
$rule = {TAG-CONTENT};

// ILLEGAL
$rule = ;

XML Form

A rule definition is represented by the rule element. The id attribute of the element indicates the name of the rule and must be unique within the grammar (this is enforced by XML). The contents of the rule element may be any legal rule expansion defined in Section 2. The scope attribute is explained in the next section.

<rule id="city">
   <one-of>
      <item>Boston</item>
      <item>"San Francisco"</item>
      <item>Madrid</item> 
   </one-of>
</rule>
<rule id="command">
   <ruleref uri="#action"/>
   <ruleref uri="#object"/>
</rule>
Special Cases

It is not legal to define an empty rule element or a rule element that contains only white space CDATA.

It is legal to define a rule that expands to an empty item or to a single rule reference to NULL.

It is legal to define a rule that expands to a single tag element.

<!-- Legal -->
<rule id="rule"><item/></rule>
<rule id="rule"><ruleref special="NULL"/></rule>
<rule id="rule"><tag>TAG-CONTENT</tag></rule>

<!-- ILLEGAL -->
<rule id="rule"/>
<rule id="rule"></rule>
<rule id="rule">   </rule>

3.2 Scoping of Rule Definitions

Each defined rule has a scope. The scope is either "private" or "public". If not explicitly declared in a rule definition then the scope defaults to "private".

A public-scoped rule may be explicitly referenced (using the fragment identifier syntax of a URI) in the rule definitions of other grammars and in other non-grammar documents. A private-scoped rule cannot be so referenced and is directly accessible only within its containing grammar. A private rule may be explicitly referenced only by other rules within the same grammar.

Informative: grammar authors may consider the following guidance when scoping the rules of a grammar.

ABNF Form

A rule definition may be annotated with the keywords "public" or "private". If no scope is provided, the default is "private".

$town = Townsville | Beantown;
private $city = Boston | "New York" | Madrid;
public $command = $action $object;

XML Form

The scope attribute of the rule element defines the scope of the rule definition. Defined values are public and private. If omitted, the default scope is private.

<rule id="town">
   <one-of>
      <item>Townsville</item>
      <item>Beantown</item> 
   </one-of>
</rule>
<rule id="city" scope="private">
   <one-of>
      <item>Boston</item>
      <item>"San Francisco"</item>
      <item>Madrid</item> 
   </one-of>
</rule>
<rule id="command" scope="public">
   <ruleref uri="#action"/>
   <ruleref uri="#object"/>
</rule>

3.3 Example Phrases

It is often desirable to include examples of phrases that match rule definitions along with the definition. Zero, one or many example phrases may be provided for any rule definition. Because the examples are explicitly marked, automated tools can be used for regression testing and for generation of grammar documentation.

ABNF Form

A documentation comment is a C/C++/Java comment that starts with the sequence of characters /** and which immediately precedes the relevant rule definition. Zero or more @example tags may be contained at the end of the documentation comment. The syntax follows the Tagged Paragraph of a documentation comment of the Java Programming Language [JAVA §18.4]. The tokenization of the example follows the tokenization and sequence rules defined in Section 2.1 and Section 2.3 respectively.

/**
 * A simple directive to execute an action.
 *
 * @example open the window
 * @example close the door
 */
public $command = $action $object;

XML Form

Any number of "example" elements may be provided as the initial content within a "rule" element. The tokenization of the example follows the tokenization and sequence rules defined in Section 2.1 and Section 2.3 respectively.

<rule id="command" scope="public">
    <!-- A simple directive to execute an action. -->
    <example> open the window </example>
    <example> close the door </example>
    <ruleref uri="#action"/> <ruleref uri="#object"/>
</rule>

4. Grammar Documents

A conforming stand-alone grammar document consists of a legal header followed by a body consisting of a set of legal rule definitions. All rules defined within that grammar are scoped within the grammar's rulename namespace and each rulename must be legal and unique.

It is legal for a grammar to define no rules. The grammar cannot be used for processing input since it defines no patterns for matching user input.

4.1 Grammar Header Declarations

A legal stand-alone grammar header consists of a number of required declarations and other optional declarations. In addition, the ABNF Form and XML Form each have additional requirements and capabilities of the header that are specific to each syntactic form. The ordering of header declarations is also specific to the two forms.

The table summarizes the information declared in a grammar header and the appropriate representation in the ABNF Form and XML Form.

Declaration Status ABNF Form XML Form
Grammar version Required §4.2: ABNF self-identifying header §4.3: version attribute on grammar element
XML Namespace Required (XML only) Not applicable §4.3: xmlns attribute on grammar element
Document type Recommended (XML only) Not applicable §4.3: XML DOCTYPE
Character encoding Recommended §4.4: ABNF self-identifying header §4.4: encoding attribute in XML declaration
Language Required in voice mode
Ignored in DTMF mode		 §4.5: language declaration §4.5: xml:lang attribute on grammar element
Mode Optional §4.6: mode declaration §4.6: mode attribute on grammar element
Root rule Optional §4.7: root declaration §4.7: root attribute on grammar element
Tag format Optional §4.8: tag-format declaration §4.8: tag-format attribute on grammar element
Base URI Optional §4.9: base declaration §4.9: xml:base attribute on grammar element
Pronunciation lexicon Optional. Multiple allowed. §4.10: lexicon declaration §4.10: lexicon element
Metadata Optional. Multiple allowed. §4.11.1: meta and http-equiv declarations §4.11.1: meta element
XML metadata Optional. (XML Only) Not applicable §4.11.2: metadata element
Tag Optional. Multiple allowed. §4.12: tag declaration §4.12: tag element

A grammar that complies to this specification must declare the version to be "1.0".

Note: the grammar version indicates the version of the specification implemented by the grammar and is not for versioning of the grammar content. A meta or metadata declaration may be used for content versioning.

ABNF Form: Header Summary

A legal header for a stand-alone ABNF document consists of a required ABNF self-identifying header including the grammar version and optional character encoding followed by these declarations in any order:

ABNF comments may appear between the declarations in the ABNF header after the ABNF self-identifying header.

The header declarations are followed by the rule definitions of the grammar.

The following are two examples of ABNF headers. Note that ordering of the declarations (except the ABNF self-identifying header) is unimportant.

#ABNF 1.0 ISO-8859-1;

language en;
mode voice;
root $myRule;
tag-format FORMAT-STRING;
base <http://www.example.com/base-file-path>;
lexicon <http://www.example.com/lexicon.file>;
lexicon <http://www.example.com/strange-city-names.file>~<media-type>;
meta "Author" is "Stephanie Williams";
http-equiv "Date" is "Fri, 10 Feb 2002 17:27:21 GMT";
{var x=1};

#ABNF 1.0;

// A French Canadian grammar
language fr-CA;

// It's a speech grammar
mode voice;

// Here's the root rule
root $QuebecCities;

XML Form: Header Summary

A legal stand-alone XML Form grammar document consists of:

  1. Legal XML Prolog
  2. Root grammar element with the following attributes
  3. grammar element containing any number of the following elements in any order:

Rule definitions follow the lexicon, meta, metadata and tag declarations.

The following are examples of XML Form grammars headers each including all declarations permitted on the grammar element and one with the DOCTYPE declaration.

<?xml version="1.0" encoding="ISO-8859-1"?>

<grammar version="1.0" 
         xml:lang="en"
         mode="voice"
         root="myRule"
         xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:base="http://www.example.com/base-file-path">

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN"
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar version="1.0"
         xml:lang="fr-CA"
         mode="voice"
         root="QuebecCities"
         xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:base="http://www.example.com/another-base-file-path">

4.2 ABNF Self-Identifying Header

The ABNF self-identifying header must be present in any legal stand-alone ABNF Form grammar document.

The first character of an ABNF document must be the "#" symbol (x23) unless preceded by an optional XML 1.0 byte order mark [XML §4.3.3]. The ABNF byte order mark follows the XML definition and requirements. For example, documents encoded in UTF-16 must begin with the byte order mark.

The optional byte order mark and required "#" symbol must be followed immediately by the exact string "ABNF" (x41 x42 x4d x46) or the appropriate equivalent for the document's encoding (e.g. for UTF-16 little-endian: x23 x00 x41 x00 x42 x00 x4d x00 x46 x00). If the byte order mark is absent on a grammar encoded in UTF-16 then the grammar processor should perform auto-detection of character encoding in a manner analogous to auto-detection of character encoding in XML [XML §F].

Next follows a single space character (x20) and the required version number which is "1.0" for this specification (x31 x2e x30).

Next follows an optional character encoding. Section 4.4 defines character encodings in more detail. If present, there must be a single space character (x20) between the version number and the character encoding.

The self-identifying header is finalized with a semicolon (x3b) followed immediately by a newline. The semicolon must be the first character following the version number or the character encoding if is present.

For the remaining declarations of the ABNF header white space is not significant.

4.3 XML Form Prolog and Root Element

A legal stand-alone XML Form grammar document must have a legal XML Prolog [XML §2.8].

The XML prolog in an XML Form grammar comprises the XML declaration and an optional DOCTYPE declaration referencing the grammar DTD. It is followed by the root grammar element. The XML prolog may also contain XML comments, processor instructions and other content permitted by XML in a prolog.

The version number of the XML declaration indicates which version of XML is being used. The version number of the grammar element indicates which version of the grammar specification is being used — "1.0" for this specification. The grammar version is a required attribute.

The grammar element must designate the grammar namespace. This can be achieved by declaring an xmlns attribute or an attribute with an "xmlns" prefix. See [XMLNS] for details. Note that when the xmlns attribute is used alone, it sets the default namespace for the element on which it appears and for any child elements. The namespace for XML Form grammars is defined as http://www.w3.org/2001/06/grammar.

It is recommended that the grammar element also indicate the location of the grammar schema (see Appendix C) via the xsi:schemaLocation attribute from [SCHEMA1]. Although such indication is not required, to encourage it this document provides such indication on all of the examples:

<grammar version="1.0"
         xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd">
  ...
</grammar>

If present, the optional DOCTYPE must reference the standard DOCTYPE and identifier.

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN"
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

The character encoding is defined on the XML declaration as defined by the XML specification. See Section 4.4 for detail.

The language is defined by the xml:lang attribute on the grammar element. See Section 4.5 for details.

The grammar mode is defined on the grammar element. See Section 4.6 for details.

The root rule is defined on the grammar element. See Section 4.7 for details.

The tag-format is defined on the grammar element. See Section 4.8 for details.

The base URI for the document is defined by the xml:base attribute on the grammar element. See Section 4.9 for details.

4.4 Character Encoding

The character encoding declaration indicates the scheme used for encoding character data in the document. For example, for US applications it would be common to use US-ASCII, UTF-8 (8-bit Unicode) or ISO-8859-1. For Japanese grammars, character encodings such as EUC-JP and UTF-16 (16-bit Unicode) could be used.

Except for the different syntactic representation, the ABNF Form follows the character encoding handling defined for XML. XML grammar processors must accept both the UTF-8 and UTF-16 encodings of ISO/IEC 10646 and may support other character encodings. This follows from an XML grammar processor being a compliant XML processor and thus required to support those character encodings. For consistency, ABNF grammar processor must also accept both the UTF-8 and UTF-16 encodings of ISO/IEC 10646 and may support other character encodings.

For both XML Form and ABNF Form grammars the declaration of the character encoding is optional but strongly recommended. XML defines behavior for XML processors that receive an XML document without a character encoding declaration. For consistency an ABNF grammar processor must follow the same behavior (with adjustments for the different syntax). (Note the character encoding declaration is optional only in cases where it is optional for a legal XML document.)

ABNF Form

The character encoding declaration is part of the self-identifying grammar header defined in Section 4.1 and is processed in combination with the byte order mark, if present, using the same procedure as XML 1.0 [XML §4.3.3].

The following are examples of ABNF self-identifying grammar headers with and without the character encoding declaration.

Note: the ABNF Form syntax does not provide a character reference syntax for entry of a specific character, for example, one not directly accessible from available input devices. This contrasts with XML 1.0 syntax for character references [XML §4.1]. For development requiring character references the XML Form of the specification is recommended.

#ABNF 1.0 ISO-8859-1;

#ABNF 1.0 EUC-JP;

#ABNF 1.0;

XML Form

XML declares character encodings as part of the document's XML declaration on the first line of the document.

The following are examples of XML headers with and without the character encoding declaration.

<?xml version="1.0" encoding="ISO-8859-1"?>

<?xml version="1.0" encoding="EUC-JP"?>

<?xml version="1.0"?>

4.5 Language

The language declaration of a grammar provides the language identifier that indicates the primary language contained by the document and optionally indicates a country or other variation. Additionally, any legal rule expansion may be labeled with a language identifier.

The language declaration is required for all speech recognition grammars: i.e. all grammars for which the mode is "voice". (Note that mode defaults to voice if there is no explicit mode declaration in ABNF or mode attribute in XML.)

If an XML Form grammar is incorporated within another XML document — for example, as supported by VoiceXML 2.0 — then the xml:lang attribute is optional on the grammar element and the xml:lang attribute must be inherited from the enclosing document.

In DTMF grammars a language declaration must be ignored if present.

The conformance definition in Section 5 defines the behavior of a grammar processor when it encounters a language variant that it does not support.

ABNF Form

The ABNF header must contain zero or one language declaration. It consists of the keyword "language", white space, a legal language identifier, optional white space and a terminating semicolon character (';').

language en-US;

language fr;

XML Form

Following the XML 1.0 specification [XML §2.12] the language identifier is indicated by an xml:lang attribute on the root grammar element.

<grammar xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="en-US" 
         version="1.0">

<grammar xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="fr"
         version="1.0">

4.6 Grammar Mode

The mode of a grammar indicates the type of input that the user agent should be detecting. The default mode is "voice" for speech recognition grammars. An alternative input mode defined in Appendix E is "dtmf" input.

The mode attribute indicates how to interpret the tokens contained by the grammar. Speech tokens are expected to be detected as speech audio that sounds like the token. Behavior with DTMF input, if supported, is defined in Appendix E.

It is often the case that a different user agent is used for detecting DTMF tones than for speech recognition. The same may be true for other modes defined in future revisions of the specification.

The specification does not define a mechanism by which a single grammar can mix modes: that is, a representation for a mixed "voice" and "dtmf" grammar is not defined. Moreover, it is illegal for a rule reference in one grammar to reference any grammar with a different mode.

A user agent may, however, support the simultaneous activation of more than one grammar including both "voice" and "dtmf" grammars. This is necessary, for example, for DTMF-enabled VoiceXML browsers [VXML2]. (Note: parallel activation implies disjunction at the root level of the grammars rather than mixing of modes within the structure of the grammars.)

ABNF Form

The ABNF header must contain zero or one mode declaration. It consists of the keyword "mode", white space, either "voice" or "dtmf" optional white space and a terminating semicolon character (';'). If the ABNF header does not declare the mode then it defaults to voice.

  mode voice;

  mode dtmf;

XML Form

The mode declaration is provided as an optional mode attribute on the root grammar element. Legal values are "voice" and "dtmf". If the mode attribute is omitted then the value defaults to voice.

<grammar mode="voice"
         version="1.0"
         xml:lang="en-US"
         xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"> 

<grammar mode="dtmf" 
         version="1.0"
                  xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd">

4.7 Root Rule

Both the XML Form and ABNF Form permit the grammar header to optionally declare a single rule to be the root rule of the grammar. The rule declared as the root rule must be defined within the scope of the grammar. The rule declared as the root rule may be scoped as either public or private.

An implicit rule reference to the root rule of a grammar is legal. The syntax for implicitly referencing root rules is defined in Section 2.2. It is an error to reference a grammar implicitly by its root if that grammar does not declare a legal root rule.

Although a grammar is not required to declare a root rule it is good practice to declare the root rule of any grammar.

ABNF Form

The ABNF header must contain zero or one root rule declaration. It consists of the keyword "root", white space, the legal rulename of a rule defined within the grammar prefixed by the dollar sign ('$'), optional white space and a terminating semicolon character (';'). If the ABNF header does not declare the root rule then it is not legal to implicitly reference the grammar by its root.

root $rulename;

XML Form

The root rulename declaration is provided as an optional root attribute on the grammar element. The root declaration must identify one rule defined elsewhere within the same grammar. The value of the root attribute is an XML IDREF (not a URI) and must not include the number sign ('#').

<grammar root="rulename" ...>

4.8 Tag Format

The tag-format declaration is an optional declaration of a tag-format identifier that indicates the content type of all rule tags and header tags contained within a grammar.

The tag-format identifier is a URI. It is recommended that the tag format identifier indicate both the content type and a version. Tags typically contain content for a semantic interpretation processor and in such cases the identifier, if present, should indicate the semantic processor to use.

Tag-format identifier values beginning with the string "semantics/x.y" (where x and y are digits) are reserved for use by the W3C Semantic Interpretation for Speech Recognition specification [SEM] or future versions of the specification.

Grammar processor handling of tags is undefined if the tag format declaration is omitted.

ABNF Form

The ABNF header must contain zero or one tag format declaration. It consists of the keyword "tag-format", white space, a tag format identifier (an ABNF URI), optional white space and a terminating semicolon character (';').

Informative example ("semantics/1.0" is a reserved identifier) :

tag-format <semantics/1.0>;

XML Form

The tag-format is an optional attribute of the grammar element and contains a tag format identifier.

<grammar tag-format="semantics/1.0" ...>

4.9 Base URI

Relative URIs are resolved according to a base URI, which may come from a variety of sources. The base URI declaration allows authors to specify a document's base URI explicitly. See Section 4.9.1 for details on the resolution of relative URIs.

The path information specified by the base URI declaration only affects URIs in the document where the element appears.

The base URI declaration is permitted but optional in both the XML Form and the ABNF Form.

Note: the base URI may be declared in a meta declaration but the explicit base declaration is recommended for both the ABNF Form and XML Form.

ABNF Form

The ABNF header must contain zero or one base URI declaration. It consists of the keyword "base", white space, a legal ABNF URI, optional white space and a terminating semicolon character (';').

base <http://www.example.com/base-file-path>;

base <http://www.example.com/another-base-file-path>;

XML Form

The base URI declaration follows [XML-BASE] and is indicated by a xml:base attribute on the root grammar element.

<grammar xmlns="http://www.w3.org/2001/06/grammar"
         xml:base="http://www.example.com/base-file-path" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         version="1.0">

<grammar xmlns="http://www.w3.org/2001/06/grammar"
         xml:base="http://www.example.com/another-base-file-path"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         version="1.0">

4.9.1 Resolving Relative URIs

User agents must calculate the base URI for resolving relative URIs according to [RFC2396]. The following describes how RFC 2396 applies to grammar documents.

User agents must calculate the base URI according to the following precedences (highest priority to lowest):

  1. The base URI is set by the xml:base attribute on the grammar element or the base declaration in the ABNF header (see Section 4.9).
  2. The base URI is provided in a meta declaration.
  3. The base URI is given by metadata discovered during a protocol interaction, such as an HTTP header (see [RFC2616]).
  4. By default, the base URI is that of the current document. Not all grammar documents have a base URI (e.g., a valid grammar document may appear in an email and may not be designated by a URI). Such grammar documents are not valid if they contain relative URIs and rely on a default base URI.

4.10 Pronunciation Lexicon

A grammar may optionally reference one or more external pronunciation lexicon documents. A lexicon document is identified by a URI with an optional media type.

The pronunciation information contained within a lexicon document is used only for tokens defined within the enclosing grammar.

The W3C Voice Browser Working Group is developing the Pronunciation Lexicon Markup Language [LEX]. The specification will address the matching process between tokens and lexicon entries and the mechanism by which a speech recognizer handles multiple pronunciations from internal and grammar-specified lexicons. Pronunciation handling with proprietary lexicon formats will necessarily be specific to the speech recognizer.

Pronunciation lexicons are necessarily language-specific. Pronunciation lookup in a lexicon and pronunciation inference for any token may use an algorithm that is language-specific. (See Section 2.1 for additional information on token handling and pronunciations.)

ABNF Form

The ABNF header may contain any number of pronunciation lexicon declarations (zero, one or many). The lexicon declaration consists of the "lexicon" keyword followed by white space, an ABNF URI or ABNF URI with media type, optional white space and a closing semicolon (';'). (Note that a lexicon URI is not preceded by a dollar sign as is the case for ABNF rule references.) Example:

   #ABNF V1.0 ISO-8859-1;
   language en-US;
   
   lexicon <http://www.example.com/lexicon.file>;
   lexicon <http://www.example.com/strange-city-names.file>~<media-type>;
   ...

XML Form

Any number of lexicon elements may occur as immediate children of the grammar element. The lexicon element must have a uri attribute specifying a URI that identifies the location of the pronunciation lexicon document.

The lexicon element may have a type attribute that specifies the media type of the pronunciation lexicon document.

<grammar xmlns="http://www.w3.org/2001/06/grammar" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="en" version="1.0">
   
   <lexicon uri="http://www.example.com/lexicon.file"/>
   <lexicon uri="http://www.example.com/strange-city-names.file" type="media-type"/>
   ...

4.11 Meta Data

Grammar documents let authors specify metadata — information about a document rather than document content — in a number of ways.

A meta declaration in either the ABNF Form or XML Form may be used to express metadata information in both XML Form and ABNF Form grammars or to reference metadata available in an external resource. The XML Form also supports a metadata element that provides a more general and powerful treatment of metadata information than meta. Since metadata requires an XML metadata schema which cannot be expressed in ABNF, there is no equivalent of metadata in the ABNF Form of grammars.

4.11.1 Meta and HTTP-Equiv

A meta declaration in either ABNF Form or the XML Form associates a string to declared meta property or declares "http-equiv" content.

The seeAlso property is the only defined meta property name. It is used to specify a resource that might provide additional metadata information about the containing grammar. This property is modelled on the rdfs:seeAlso property of Resource Description Framework (RDF) Schema Specification 1.0 [RDF-SCHEMA §2.3.4].

It is recommended that for general metadata properties that grammar authors follow the metadata properties defined in the Dublin Core Metadata Initiative [DC]. For example, "Creator" to identify the entity primarily responsible for making the content of the grammar, "Date" to indicate creation date, or "Source" to indicate the resource From which a grammar is derived (e.g. when converting an XML Form grammar to the ABNF Form, use "Source" to provide the URI for the original document.)

ABNF Form

The ABNF header may contain any number of meta declarations and http-equiv declarations (zero, one or many). Each declaration consists of the "meta" or "http-equiv" keyword followed by white space, the name string delimited by quotes, the keyword "is", white space, the content string delimited by quotes, optional white space and a closing semicolon (';').

The name string and the content string must be delimited by either a matching pair of double quotes ('"') or a matching pair of single quotes ("'").

Informative example:

#ABNF 1.0;

meta "Creator" is "Stephanie Williams";
meta "seeAlso" is "http://example.com/my-grammar-metadata.xml";

http-equiv "Expires" is '0';
http-equiv "Date" is "Thu, 12 Dec 2000 23:27:21 GMT";

XML Form

A metadata property is declared with a meta element. Either a name or http-equiv attribute is required. It is illegal to provide both name and http-equiv attributes. A content attribute is required. The meta, metadata and lexicon elements must occur before all rule elements contained with the root grammar element. There are no constraints on the ordering of the meta, metadata and lexicon elements.

Informative example:

<?xml version="1.0"?> 
<grammar version="1.0" xml:lang="en-US"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xmlns="http://www.w3.org/2001/06/grammar"> 

  <meta name="Creator" content="Stephanie Williams"/> 
  <meta name="seeAlso" content="http://example.com/my-grammar-metadata.xml"/> 

  <meta http-equiv="Expires" content="0"/> 
  <meta http-equiv="Date" content="Thu, 12 Dec 2000 23:27:21 GMT"/> 
  ... 
</grammar>

4.11.2 XML Metadata (XML Only)

The metadata element is container in which information about the document can be placed using a metadata schema. Although any metadata schema can be used with metadata, it is recommended that the Resource Description Format (RDF) schema [RDF-SCHEMA] is used in conjunction with the general metadata properties defined in the Dublin Core Metadata Initiative [DC].

RDF is a declarative language and provides a standard way for using XML to represent metadata in the form of statements about properties and relationships of items on the Web. Content creators should refer to W3C metadata Recommendations [RDF-SYNTAX] and [RDF-SCHEMA] when deciding which metadata RDF schema to use in their documents. Content creators should also refer to the Dublin Core Metadata Initiative [DC], which is a set of generally applicable core metadata properties (e.g., Title, Creator, Subject, Description, Copyrights, etc.).

This specification only defines an XML representation for this form of metadata declaration. There is no ABNF equivalent for metadata. A conversion of an XML Form grammar to the ABNF Form may extract the XML metadata into a separate document that is referenced with a "seeAlso" meta declaration in the ABNF document. Note: an agent that searches XML documents for metadata represented with RDF would be unable to locate RDF even if it were represented in ABNF. Thus, support for RDF in ABNF was considered low utility.

XML Form

Document properties declared with metadata element can use any metadata schema. The metadata, meta, and lexicon elements must occur before all rule elements contained with the root grammar element. There are no constraints on the ordering of the metadata, meta and lexicon elements.

Informative: This is an example of how metadata can be included in an XML grammar document using the Dublin Core version 1.0 RDF schema [DC] describing general document information such as title, description, date, and so on:

<?xml version="1.0"?> 
<grammar
    xmlns="http://www.w3.org/2001/06/grammar" version="1.0" 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
    xml:lang="en-US">
    
  <metadata>
   <rdf:RDF
       xmlns:rdf = "http://www.w3.org/1999/02/22-rdf-syntax-ns#"
       xmlns:rdfs = "http://www.w3.org/TR/1999/PR-rdf-schema-19990303#"
       xmlns:dc = "http://purl.org/metadata/dublin_core#">

   <!-- Metadata about the grammar document -->
   <rdf:Description about="http://www.example.com/meta.grxml"
       dc:Title="Digit Grammar"
       dc:Description="Digit Grammar in W3C XML Form"
       dc:Publisher="W3C"
       dc:Language="en"
       dc:Date="2002-02-14"
       dc:Rights="Copyright 2002 Jan Smith"
       dc:Format="application/srgs+xml" >                
       <dc:Creator>
          <rdf:Seq ID="CreatorsAlphabeticalBySurname">
             <rdf:li>Jackie Crystal</rdf:li>
             <rdf:li>Jan Smith</rdf:li>
          </rdf:Seq>
       </dc:Creator>
   </rdf:Description>
  </rdf:RDF>
 </metadata>

</grammar>

4.12 Tag

A grammar may optionally specify one or more tag declarations in the header. The content of a tag in the header, just like a tag in rule expansions, is an arbitrary string which may be used for semantic interpretation.

ABNF Form

The ABNF header may contain any number of tag declarations (zero, one or many).

The tag declaration consists a string delimited as described in S2.6 ABNF Form, followed by a closing semicolon (';').

The tag content is all text between the opening and closing delimiters including leading and trailing whitespace. The contents of the tag are not parsed by the grammar processor.

   #ABNF V1.0 ISO-8859-1;
   language en-US;
   {TAG-CONTENT-1};
   {!{TAG-CONTENT-2}!};
   
   $rule = . . .;
   ...

XML Form

Any number of tag elements may occur as immediate children of the grammar element. The content of tag is CDATA.

<grammar xmlns="http://www.w3.org/2001/06/grammar" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="en" version="1.0">
   
   <tag>TAG-CONTENT-1<tag>
   <tag>TAG-CONTENT-2<tag>

   ...

4.13 Comments

Comments may be placed in most places in a grammar document. For XML, use XML comments. For ABNF there are documentation comments and C/C++/Java-style comments.

ABNF Form

C/C++/Java comments are permitted. Documentation comments are permitted before grammar and language declarations and before any rule definition.

Section 3.3 defines the format for representing examples in documentation comments before a rule definition.

// C++/Java-style single-line comment
/* C/C++/Java-style comment */
/** Java-style documentation comment */

XML Form

An XML comment has the following syntax.

<!-- comment -->

4.14 Grammar Fetching

The fetching and caching behavior of both ABNF Form and XML Form grammar documents is defined primarily by the environment in which the grammar processor operates. For instance, VoiceXML 1.0 and VoiceXML 2.0 define certain fetching and caching behaviors that apply to grammars activated by a VoiceXML browser. Similarly, any API for a recognizer that supports ABNF Form or XML Form grammars may apply fetching and caching behaviors.

Grammar processors are recommended to support the following interpretation of "rendering" a grammar for the purpose of determining document freshness.

Activation of a grammar is the point at which the recognizer begins detection of user input matching the grammar and is therefore analogous to the action of visual or audio rendering of system output. As with output rendering, grammar freshness should be checked close to the moment of grammar activation.

4.15 ABNF Keywords

ABNF keywords are case sensitive. The keywords of the ABNF language are not reserved. The keywords with specified meaning in ABNF are:

Context Keywords
Language declaration "language"
Mode declaration "mode"
Root declaration "root"
Tag format declaration "tag-format"
Base URI declaration "base"
Pronunciation lexicon "lexicon"
Meta or HTTP-equiv declaration "meta", "http-equiv", "is"
Rule definition "public", "private"

Since keywords are not reserved they may be used as rulenames and as tokens. The following is a legal grammar that accepts as input a sequence of one or more "public" tokens.

#ABNF 1.0 ISO-8859-1;

language en-AU;
root $public;
mode voice;

public $public = public $public | public;

5. Conformance

This section is normative.

Different sets of grammar conformance criteria exist for:

5.1: Conforming XML Form Grammar Fragments

An XML Form grammar document fragment is a Conforming XML Form Grammar Fragment if:

5.2: Conforming Stand-Alone XML Form Grammar Document

A document is a Conforming Stand-Alone XML Form Grammar Document if it meets both the following conditions.

The XML Form grammar specification and these conformance criteria provide no designated size limits on any aspect of grammar documents. There are no maximum values on the number of elements, the amount of character data, or the number of characters in attribute values.

5.3 Using XML Form Grammars with other Namespaces

The grammar namespace may be used with other XML namespaces as per the Namespaces in XML Recommendation [XMLNS]. Future work by W3C will address ways to specify conformance for documents involving multiple namespaces.

5.4 Conforming XML Form Grammar Processors

An XML Form grammar processor is a program that can parse and process XML Form grammar documents. Examples include speech recognizers and DTMF detectors that accept the XML Form.

In a Conforming XML Form Grammar Processor, the XML parser must be able to parse and process all XML constructs defined by XML 1.0 [XML] and Namespaces in XML [XMLNS]. This XML parser is not required to perform validation of a grammar document as per its schema or DTD; this implies that during processing of an XML Form grammar document it is optional to apply or expand external entity references defined in an external DTD.

A Conforming XML Form Grammar Processor must correctly understand and apply the semantics of each possible grammar feature defined by this document.

A Conforming XML Form Grammar Processor must meet the following requirements for handling of languages:

When a Conforming XML Form Grammar Processor encounters elements or attributes in a non-grammar namespace it may:

A Conforming XML Form Grammar Processor is not required to support recursive grammars, that is, grammars in which rule references include direct or indirect self-reference.

There is, however, no conformance requirement with respect to performance characteristics of the XML Form Grammar Processor. For instance, no statement is required regarding the accuracy, speed or other characteristics of a speech recognizer or DTMF detector. No statement is made regarding the size of grammar or size of grammar vocabulary that an XML Form Grammar Processor must support.

5.5: Conforming Stand-Alone ABNF Form Grammar Documents

An ABNF grammar document is a Conforming ABNF Document if it adheres to the specification described in this document (Speech Recognition Grammar Specification) including the Formal ABNF Specification.

5.6: Conforming ABNF Form Grammar Processor

An ABNF Grammar Processor is a program that can parse and process ABNF grammar documents. Examples include speech recognizers and DTMF detectors that accept the ABNF Form.

A Conforming ABNF Grammar Processor must correctly understand and apply the semantics of each possible grammar feature defined by this document.

A Conforming ABNF Grammar Processor must follow the same language handling requirements as outlined in Section 5.4 for Conforming XML Form Grammar Processors.

A Conforming ABNF Grammar Processor should inform its hosting environment if it encounters an illegal grammar document or other grammar content that it is unable to process.

A Conforming ABNF Grammar Processor is not required to support recursive grammars, that is, grammars in which rule references include direct or indirect self-reference.

There is, however, no conformance requirement with respect to performance characteristics of the ABNF Grammar Processor. For instance, no statement is required regarding the accuracy, speed or other characteristics of a speech recognizer or DTMF detector. No statement is made regarding the size of grammar or size of grammar vocabulary that an ABNF Grammar Processor must support.

5.7: Conforming ABNF/XML Form Grammar Processor

A Conforming ABNF/XML Form Grammar Processor must meet all the conformance criteria defined in Section 5.4 and in Section 5.6.

Additionally an ABNF/XML Form Grammar Processor must be able to resolve and apply references from XML Form Grammars to ABNF Form Grammars, and references from ABNF Form Grammars to XML Form Grammars.

5.8: Conforming User Agent

A conforming user agent is a Conforming XML Form Grammar Processor, Conforming ABNF Form Grammar Processor or Conforming ABNF/XML Form Grammar Processor that is capable of accepting user input of the mode of a grammar (i.e. speech input in "voice" mode, DTMF input "dtmf" mode) and:

  1. Is capable of determining when a sequence of user input exactly matches a grammar,
  2. Is capable of producing an output representation that indicates how the input matches the grammar.

Current speech recognition technology is statistically based. Since the output is not deterministic and cannot be guaranteed to be a correct representation of the input there is no conformance requirement regarding accuracy. A conformance test may, however, require some examples of correct recognition of speech input to determine conformance.

6. Acknowledgements

This document was written with the participation of the members of the W3C Voice Browser Working Group (listed in alphabetical order):

Mike Brown, Lucent Bell Labs
Dan Burnett, Nuance Communications
Emily Candell, Comverse
Jerry Carter, Invited Expert
Debbie Dahl, Invited Expert
Debajit Ghosh, Nuance Communications
Andrew Hunt, ScanSoft
Stefan Krause, ScanSoft
Sol Lerner, ScanSoft
Bruce Lucas, IBM
Jens Marschner, ScanSoft
Scott McGlashan, Hewlett-Packard
Yves Normandin, Locus Dialogue
Brad Porter, Tellme
Dave Raggett, W3C/Canon
David Ramsthaler, Cisco
Luc Van Tichelen, ScanSoft
Kuansan Wang, Microsoft
Laura Werner, BeVocal

Appendix A: References

A.1: Normative References

[RFC2119]
Key words for use in RFCs to Indicate Requirement Levels, S. Bradner. IETF RFC 2119. See http://www.ietf.org/rfc/rfc2119.txt
[RFC2045]
Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies, N. Freed and N. Borenstein. IETF RFC 2045. November, 1996. See http://www.ietf.org/rfc/rfc2045.txt
[RFC2046]
Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types, N. Freed and N. Borenstein. IETF RFC 2046. November 1996. See http://www.ietf.org/rfc/rfc2046.txt
[RFC2396]
Uniform Resource Identifiers (URI): Generic Syntax, T. Berners-Lee, R. Fielding, U.C. Irvine, L. Masinter. IETF RFC 2396. 1998. See http://www.ietf.org/rfc/rfc2396.txt
[SCHEMA1]
XML Schema Part 1: Structures, H. S. Thompson, et al. W3C Recommendation, May 2001. See http://www.w3.org/TR/xmlschema-1/
[SCHEMA2]
XML Schema Part 2: Datatypes, P.V. Biron, A. Malhotra. W3C Recommendation, May 2001 See http://www.w3.org/TR/xmlschema-2/
[TYPES]
List of media types (MIME types) registered with IANA. See http://www.iana.org/assignments/media-types/index.html
[XML]
Extensible Markup Language (XML) 1.0 (Second Edition), World Wide Web Consortium. W3C Recommendation, 6 October 2000. See http://www.w3.org/TR/2000/REC-xml-20001006
[XML-BASE]
XML Base, J. Marsh, editor. W3C Recommendation, June 2001. See http://www.w3.org/TR/2001/REC-xmlbase-20010627/
[XMLNS]
Namespaces in XML, World Wide Web Consortium. W3C Recommendation. See http://www.w3.org/TR/REC-xml-names/

A.2: Informative References

[CHARMOD]
Character Model for the World Wide Web 1.0, World Wide Web Consortium. W3C Working Draft, 20 December 2001. See http://www.w3.org/TR/charmod/
[DC]
Dublin Core Metadata Initiative, See http://dublincore.org/.
[JAVA]
The Java Language Specification (First Edition), James Gosling, Bill Joy, and Guy Steele. Addison-Wesley. See http://java.sun.com/docs/books/jls/
[JSGF]
JSpeech Grammar Format, Sun Microsystems. Sun Microsystems submission to W3C. See http://www.w3.org/TR/jsgf/
[HTML]
HTML 4.01 Specification., World Wide Web Consortium. W3C Recommendation, 24 December 1999. See http://www.w3.org/TR/1999/REC-html401-19991224/
[HU79]
Introduction to Automata Theory, Languages, and Computation, J.E. Hopcroft and J.D. Ullman, Addison-Wesley, 1979.
[ISO/IEC 10646]
ISO/IEC 10646-1993 (E). Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane, ISO (International Organization for Standardization). [Geneva]: International Organization for Standardization, 1993 (plus amendments AM 1 through AM 7).
[JEL98]
Statistical methods for speech recognition, F. Jelinek. MIT Press, 1998. ISBN: 0-262-10066-5.
[NGRAM]
Stochastic Language Models (N-Gram) Specification, World Wide Web Consortium. W3C Working Draft. See http://www.w3.org/TR/ngram-spec/
[NLSML]
Natural Language Semantics Markup Language., World Wide Web Consortium, W3C Working Draft, 20 November 2000. See http://www.w3.org/TR/nl-spec/
[LEX]
Pronunciation Lexicon Markup Requirements, World Wide Web Consortium, W3C Working Draft, 12th March 2001. See http://www.w3.org/TR/lexicon-reqs/
[Q24]
Multifrequency push-button signal reception, International Telecommunication Union. ITU Recommendation approved 1988-11. See http://www.itu.int/home/index.html
[RAB93]
Fundamentals of Speech Recognition, L. Rabiner and B.H. Juang, Prentice Hall, 1993. ISBN: 0-13-015157-2.
[RDF-SYNTAX]
Resource Description Framework (RDF) Model and Syntax Specification, Ora Lassila and Ralph R. Swick. W3C Recommendation, 22 February 1999. See http://www.w3.org/TR/1999/REC-rdf-syntax-19990222/
[RDF-SCHEMA]
Resource Description Framework (RDF) Schema Specification 1.0, Dan Brickley and R.V. Guha. W3C Candidate Recommendation, March 2000. See http://www.w3.org/TR/2000/CR-rdf-schema-20000327/
[RFC1766]
Tags for the Identification of Languages, H. Alvestrand. IETF RFC 1766. See http://www.ietf.org/rfc/rfc1766.txt
[RFC2616]
Hypertext Transfer Protocol — HTTP/1.1, R. Fielding, et al. IETF RFC 2616. 1999. See http://www.ietf.org/rfc/rfc2616.txt
[RFC2732]
Format for Literal IPv6 Addresses in URL's, R. Hinden, B. Carpenter, L. Masinter. IETF RFC 2732. 1999. See http://www.ietf.org/rfc/rfc2732.txt
[RFC3066]
Tags for the Identification of Languages, H. Alvestrand. IETF RFC 3066. See http://www.ietf.org/rfc/rfc3066.txt
[SEM]
Semantic Interpretation for Speech Recognition, World Wide Web Consortium, W3C Working Draft, 16 November 2001. See http://www.w3.org/TR/semantic-interpretation/
[TALKML]
Introduction to TalkML, Dave Raggett. W3C Note. See http://www.w3.org/Voice/TalkML/
[Unicode]
The Unicode Standard, Version 2.0, The Unicode Consortium. Reading, Mass.: Addison-Wesley Developers Press, 1996.
[Unicode3]
The Unicode Standard, Version 3.0, The Unicode Consortium. Reading, Mass.: Addison-Wesley Developers Press, 2000. ISBN 0-201-61633-5.
[VXML1]
Voice eXtensible Markup Language (VoiceXML) version 1.0, VoiceXML Forum. W3C Note, 05 May 2000. See http://www.w3.org/TR/2000/NOTE-voicexml-20000505/
[VXML2]
Voice Extensible Markup Language (VoiceXML) Version 2.0, World Wide Web Consortium. W3C Working Draft, 23 October 2001. See http://www.w3.org/TR/2001/WD-voicexml20-20011023/

Appendix B: DTD for XML Form Grammars

This appendix is informative.

The grammar DTD is located at http://www.w3.org/TR/speech-grammar/grammar.dtd

Appendix C: XML Schema Definition For XML Form Grammars

This appendix is normative.

The grammar schema is located at http://www.w3.org/TR/speech-grammar/grammar.xsd

Note: the grammar schema includes the no-namespace core schema (below).

The no-namespace core schema for grammars is located at http://www.w3.org/TR/speech-grammar/grammar-core.xsd. It may be used as a basis for specifying XML Form Grammar Fragments embedded in non-grammar namespace schemas.

Appendix D: Formal Syntax for Augmented BNF Form Grammars

This appendix is normative.

The notation used here follows the EBNF notation (Extended Backus-Naur Form) defined in the XML 1.0 Recommendation [XML §6].

The white space handling of the ABNF Form follows white space and end-of-line handling of XML (see Section 1.6).

Lexical Grammar for ABNF

The lexical grammar defines the lexical tokens of the ABNF format and has single characters as its terminal symbols. As a consequence neither white space characters nor ABNF comments are allowed in lexical tokens unless explicitly specified.

SelfIdentHeader  ::=
    '#ABNF' #x20 VersionNumber (#x20 CharEncoding)? ';'
       [Additional constraints:
          - The semicolon (';') must immediately be followed
            by an end-of-line.
       ]

VersionNumber    ::=
    '1.0'
     
CharEncoding     ::=
    Nmtoken

BaseURI          ::=
    ABNF_URI

LanguageCode     ::=
    Nmtoken
        [Additional constraints:
          - The language code must be a valid language identifier.
        ]

RuleName         ::=
    '$' ConstrainedName

ConstrainedName  ::=
    Name - (Char* ('.' | ':' | '-') Char*)

TagFormat        ::=
    ABNF_URI

LexiconURI       ::=
    ABNF_URI | ABNF_URI_with_Media_Type

SingleQuotedCharacters ::=
    ''' [^']* '''
     
DoubleQuotedCharacters ::=
    '"' [^"]* '"'
     
QuotedCharacters ::=
    SingleQuotedCharacters | DoubleQuotedCharacters

Weight           ::=
    '/' Number '/'


Repeat           ::=
    [0-9]+ ('-' [0-9]*)?
        [Additional constraints:
          - A number to the right of the hyphen must not be
            greater than the number to the left of the hyphen.
        ]


Probability      ::=
    '/' Number '/'
        [Additional constraints:
          - The float value must be in the range of "0.0"
            to "1.0" (inclusive).
        ]

Number            ::=
    [0-9]+  |  [0-9]+ '.' [0-9]*  |  [0-9]* '.' [0-9]+

ExternalRuleRef  ::=
    '$' ABNF_URI | '$' ABNF_URI_with_Media_Type
        [Additional constraints:
          - The referenced grammar must have the same mode
            ("voice" or "dtmf") as the referencing grammar.
          - If the URI reference contains a fragment
            identifier, the referenced rule must be a
            public rule of another grammar.
          - If the URI reference does not contain a fragment
            identifier, i.e. if it is an implicit root rule reference,
            then the referenced grammar must declare a root
            rule.
        ]

Token            ::=
    Nmtoken | DoubleQuotedCharacters

LanguageAttachment ::=
    '!' LanguageCode

Tag              ::=
    '{' [^}]* '}'
    | '{!{' (Char* - (Char* '}!}' Char*)) '}!}'

------------------------------------------------------------

ABNF_URI
 and ABNF_URI_with_Media_Type
 are defined
in Section 1.6 Terminology.

Name is defined by the XML Name production [XML §2.3].

Nmtoken is defined by the XML Nmtoken production [XML §2.3].

NameChar is defined by the XML NameChar production [XML §2.3].

Char is defined by the XML Char production [XML §2.2].

Note: As mentioned in Section 2.5 the symbols "*", "+" and "?", which are often used in regular expression languages, are reserved for future use in ABNF and must not be used at any place in a grammar where the syntax currently permits a repeat operator.

Syntactic Grammar for ABNF

The syntactic grammar has lexical tokens defined by the lexical grammar as its terminal symbols. Between two lexical tokens any number of white spaces or ABNF comments may appear.

grammar         ::=
    SelfIdentHeader declaration* ruleDefinition*

declaration     ::=
    baseDecl | languageDecl | modeDecl | rootRuleDecl
    | tagFormatDecl | lexiconDecl | metaDecl | tagDecl

baseDecl     ::=
    'base' BaseURI ';'
        [Additional constraints:
          - A base declaration must not appear more than
            once in grammar.
        ]

languageDecl    ::=
    'language' LanguageCode ';'
        [Additional constraints:
          - A language declaration must not appear more than
            once in grammar.
          - A language declaration is required if the
            grammar mode is "voice".
        ]

modeDecl        ::=
    'mode' 'voice' ';' | 'mode' 'dtmf' ';'
        [Additional constraints:
          - A mode declaration must not appear more than
            once in grammar.
        ]

rootRuleDecl    ::=
    'root' RuleName ';'
        [Additional constraints:
          - A root rule declaration must not appear more
            than once in grammar.
          - The root rule must be a rule that is defined
            within the grammar.
        ]

tagFormatDecl   ::=
    'tag-format' TagFormat ';'
        [Additional constraints:
          - A tag-format declaration must not appear more
            than once in grammar.
        ]

lexiconDecl     ::=
    'lexicon' LexiconURI ';'

metaDecl        ::=
   'http-equiv' QuotedCharacters 'is' QuotedCharacters ';'
   | 'meta' QuotedCharacters 'is' QuotedCharacters ';'


tagDecl        ::=
   Tag ';'


ruleDefinition  ::=
    scope? RuleName '=' ruleExpansion ';'
        [Additional constraints:
          - The rule name must be unique within a grammar,
            i.e. no rule must be defined more than once
            within a grammar.
        ]

scope           ::=
    'private' | 'public'

ruleExpansion   ::=
    ruleAlternative ( '|' ruleAlternative )*

ruleAlternative ::=
    Weight? sequenceElement+

sequenceElement ::=
    subexpansion | subexpansion repeatOperator

subexpansion    ::=
    Token LanguageAttachment?
    | ruleRef 
    | Tag
    | '(' ')'
    | '(' ruleExpansion ')' LanguageAttachment?
    | '[' ruleExpansion ']' LanguageAttachment?

ruleRef         ::=
    localRuleRef | ExternalRuleRef | specialRuleRef

localRuleRef    ::=
    RuleName
        [Additional constraints:
          - The referenced rule must be defined within the
            same grammar.
        ]

specialRuleRef  ::=
    '$NULL' | '$VOID' | '$GARBAGE'


repeatOperator  ::=
    '<' Repeat Probability? '>'

Appendix E: DTMF Grammars

This appendix is normative.

This section defines a normative representation of a grammar consisting of DTMF tokens. A DTMF grammar can be used by a DTMF detector to determine sequences of legal and illegal DTMF events. All grammar processors that support grammars of mode "dtmf" must implement this Appendix. However, not all grammar processors are required to support DTMF input.

If the grammar mode is declared as "dtmf" then tokens contained by the grammar are treated as DTMF tones (rather than the default of speech tokens).

There are sixteen (16) DTMF tones. Of these twelve (12) are commonly found on telephone sets as the digits "0" through "9" plus "*" (star) and "#" (pound). The four DTMF tones not typically present on telephones are "A", "B", "C", "D".

Each of the DTMF symbols is a legal DTMF token in a DTMF grammar. As in speech grammars, tokens must be separated by white space in a DTMF grammar. A space-separated sequence of DTMF symbols represents a temporal sequence of DTMF entries.

In the ABNF Form the "*" symbol is reserved so double quotes must always be used to delimit "*" when defining an ABNF DTMF grammar. It is recommended that the "#" symbol also be quoted. As an alternative the tokens "star" and "pound" are acceptable synonyms.

In any DTMF grammar any language declaration in a grammar header is ignored and any language attachments to rule expansions are ignored.

In all other respects a DTMF grammar is syntactically the same as a speech grammar. For example, DTMF grammars may use rule references, special rules, tags and other specification features.

The following is a simple DTMF grammar that accepts a 4-digit PIN followed by a pound terminator. It also permits the sequence of "*" followed by "9" (e.g. to receive a help message).

#ABNF 1.0 ISO-8859-1;

mode dtmf;

$digit = 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9;
public $pin = $digit <4> "#" | "*" 9;

<?xml version="1.0"?>

<grammar mode="dtmf" version="1.0" 
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xmlns="http://www.w3.org/2001/06/grammar">

<rule id="digit">
 <one-of>
   <item> 0 </item>
   <item> 1 </item>
   <item> 2 </item>
   <item> 3 </item>
   <item> 4 </item>
   <item> 5 </item>
   <item> 6 </item>
   <item> 7 </item>
   <item> 8 </item>
   <item> 9 </item>
 </one-of>
</rule>

<rule id="pin" scope="public">
 <one-of>
   <item>
     <item repeat="4"><ruleref uri="#digit"/></item>
     #
   </item>
   <item>
     * 9
   </item>
 </one-of>
</rule>

</grammar>

Appendix F: XSLT Style Sheet to Convert XML Form Grammars to ABNF Form

This appendix is informative.

The transformation provided below is illustrative of the conversion of an XML Form grammar to the Augmented BNF Form. Known limitations:

The source for this transformation is located at http://www.w3.org/TR/speech-grammar/grammar-transformer.xsl.

Appendix G. Media Types and File Suffix

This appendix is informative.

The W3C Voice Browser Working Group has applied to IANA to register a media type each for the ABNF Form and XML Form of this Speech Recognition Grammar Specification.

The W3C Voice Browser Working Group has adopted the convention of using the ".gram" filename suffix for ABNF grammar documents and the ".grxml" filename suffix for XML Form grammar documents.

Appendix H. Logical Parse Structure

This appendix is informative.

This section defines an informative representation of a parsed result of speech recognition or other user agent processing. This representation may be used as the basis for subsequent processing of user input, in particular, semantic interpretation. For instance, the W3C Semantic Interpretation for Speech Recognition specification [SEM] is defined around the logical parse structure.

H.1 Terminology and Notation

This Appendix adopts the terminology and nomenclature of Introduction to Automata Theory, Languages, and Computation [HU79].

Denote the tokens of the alphabet of all tokens accepted by a grammar as t1, t2....

An input or output token sequence is a space separated string of tokens. The logical parse structure contains white-space-normalized tokens. The tokens in the logical parse structure are optionally delimited by double quotes so that white space and others characters can be parsed unambiguously. e.g. t1,t2,"t3 with space". (For consistency, all examples in this Appendix include double quotes.)

Let ε (epsilon) or "" denote the unique string of length 0, also known as the empty string.

Denote the tags of the alphabet of all tags accepted by a grammar as {tag1}, {tag2}, ....

Denote a legal expansion as E. (A legal expansion is defined in Section 2.)

The expressive power of a rule expansion is a Regular Expression (see HU79) and has an equivalent Finite Automaton (see HU79). [The handling of rule references requires special treatment: see Section H.2.] The expressive power of the grammar specification consists of:

We formalize the logical parse structure by creating a Finite Automaton with Output (see HU79). This construct is also referred to as a Finite State Transducer.

We define the transitions for tokens and tags as producing an output symbol.

We represent parse output as an ordered array of output entities: [e1,e2,e3,...].

An entity e may be a token, a tag or a rule expansion (see H.2).

The empty output array is represented as [ε] or simply [].

Special Cases

A $NULL reference is equivalent to a transition that accepts as input ε and produces as output ε. In the notation of HU79: ε/ε.

A $VOID reference is logically equivalent to a missing transition. It accepts no input and produces no output.

A $GARBAGE reference is equivalent to a transition that accepts platform specific input and produces as output ε.

Ambiguity

An ambiguity occurs when for a specified sequence of input tokens matched to a specified rule of a grammar there is more than one distinct logical parse structure that can be produced.

An ambiguity can occur at points of disjunction (choice) in a grammar. Disjunction exists with the use of alternatives and repeats.

A grammar processor may preserve any number of ambiguous logical parse structures to create a set of alternative logical parse structures for the input. It is legal for a grammar processor to maintain all possible logical parse structures or to dispose of all but one of the alternatives. There is no specified behavior for selection of ambiguities amongst possibilities by a grammar processors. As a result grammars that contain ambiguity do not guarantee portability of performance. Developers and grammar tools should be ambiguity-aware.

This Appendix does not illustrate all forms of ambiguous expansions but provides examples of some of the form common forms.

Examples

Matching a token to a token produces an array of 1 token.

Expansion t1
Input t1
Output ["t1"]

A $NULL reference is matched by an empty input sequence and output is an empty array.

Expansion $NULL
Input ""
Output []

A tag is matched by an empty input sequence and output is an array of 1 tag.

Expansion {tag} or {!{tag}!}
Input ""
Output [{!{tag}!}]

Concatenation: An expansion consisting of a token and a tag is matched by input containing the token and produces as output a token, tag array.

Expansion t1 {tag1}
Input t1
Output ["t1",{!{tag1}!}]

Concatenation: an expansion consisting of a sequence of tokens, tags and $NULLs is matched by input that consists of the contained tokens. Output consists of the sequence of tokens and tags with order preserved. e.g.

Expansion t1 $NULL {tag1} t2 {tag2} t3
Input t1 t2 t3
Output ["t1",{!{tag1}!},"t2",{!{tag2}!},"t3"]

Parenthetical structure is not preserved in the result. The following is the same sequence as the previous example but with parentheticals added to the expansion definition.

Expansion ((t1) $NULL) {tag1} (t2 {tag2} t3)
Input t1 t2 t3
Output ["t1",{!{tag1}!},"t2",{!{tag2}!},"t3"]

Alternatives: a set of many alternative tokens is matched by input of a single token and produces as output a single token.

Expansion t1 | t2 |t3
Input t2
Output ["t2"]

Alternatives: if any single expansion in a set of alternatives can be matched by null input then the set of alternatives may be matched by null input and the output is the output of null-accepting expansion. ($NULL, {tag} and repeat counts of zero all permit null input.)

Expansion t1 | t2 | $NULL
Input ""
Output []

With a different null-accepting expansion:

Expansion t1 | t2 | {tag}
Input ""
Output [{!{tag}!}]

Alternatives and ambiguity: several examples of ambiguous expansions with the ambiguity arising from alternatives that accept the same input but produce different output.

Expansion t1 {tag1} | t1 {tag2} | t2
Input t1
Output 1 ["t1",{!{tag1}!}]
Output 2 ["t1",{!{tag2}!}]

In this example null input is ambiguous.

Expansion {tag1} | {tag2} | $NULL
Input ""
Output 1 [{!{tag1}!}]
Output 2 [{!{tag2}!}]
Output 3 []

The following is not ambiguous because the different paths through the expansion produce the same output.

Expansion t1 | t1 | t2
Input t1
Output 1 ["t1"]
Output 2 ["t1"]

Repeats: an optional expansion can be either matched by an empty token sequence or by any token sequence that matches the expansion contained within the optional.

Expansion t1 <0-1>
Input 1 ""
Output 1 []
Input 2 t1
Output 2 ["t1"]

Repeats: order is preserved upon multiple expansions.

Expansion (t1 {tag1}) <0-3>
Input 1 ""
Output 1 []
Input 2 t1
Output 2 ["t1",{!{tag1}!}]
Input 3 t1,t1,t1
Output 3 ["t1",{!{tag1}!},"t1",{!{tag1}!},"t1",{!{tag1}!}]

Repeats and null input: If the contents of an optional expansion can be matched by an empty input sequence AND the output of matching the contained expansion is always an empty array then the output of matching the optional expansion by an empty sequence is also an empty array.

Expansion $NULL <0-1>
Input ""
Output []

Ambiguous repeats: If a repeated or optional expansion can be matched by an empty input sequence BUT the output of matching the contained expansion may contain tags then the parse is ambiguous. It is recommended that the parse be minimal: Output 1 is preferred.

Expansion {tag} <0->
Input ""
Output 1 []
Output 2 [{!{tag}!}]
Output 3 [{!{tag}!},{!{tag}!}]
Output N [{!{tag}!},{!{tag}!},{!{tag}!},...]

A similar ambiguity arises if the repeated expansion contains a alternative expansion that has a null-accepting expansion.

Expansion (t1 | {tag}) <0-3>
Input t1
Output 1 ["t1"]
Output 2 ["t1",{!{tag}!}]
Output 3 [{!{tag}!},"t1"]
Output 4 ["t1",{!{tag}!},{!{tag}!}]
Output 5 [{!{tag}!},"t1",{!{tag}!}]
Output 6 [{!{tag}!},{!{tag}!},"t1"]

A sequence with two repeat expansion can be ambiguous if the two repeated expansions can accept the same input but produce different output.

Expansion (t1 {tag1}) <0-2> (t1 {tag2}) <0-2>
Input t1,t1,t1
Output 1 ["t1",{!{tag1}!},"t1",{!{tag1}!},"t1",{!{tag1}!}
Output 2 ["t1",{!{tag1}!},"t1",{!{tag1}!},"t1",{!{tag2}!}
Output 3 ["t1",{!{tag1}!},"t1",{!{tag2}!},"t1",{!{tag2}!}
Output 4 ["t1",{!{tag2}!},"t1",{!{tag2}!},"t1",{!{tag2}!}

H.2 Parsing Rule References

A rule reference is a legal rule expansion (see Section 2.2).

We denote output obtained by matching the token sequence "t1,t2,..." against the expansion $rulename as $rulename[e1,e2,...] where "e1,e2,..." is the entity sequence obtained by matching that token sequence against the rule expansion defined for $rulename. Where a rule reference to an external rule is used the ABNF syntax for the rule reference is used (without any media type). For example, $<http://www.example.com/grammar.grxml#rulename">[e1,e2,...] or an implicit root rule reference $<http://www.example.com/grammar.grxml">[e1,e2,...]. For brevity, all the examples below use only local rule references.

The rulename of the top-level rule should enclose the logical parse structure.

A distinct structure for matching rule references maintains the parse tree for the result. This structure may be utilized in the semantic interpretation process or other computational processes that derive from the parse output structure.

There is no distinction between local rule references (within the same grammar) and external rule references.

There is no distinction between a root reference and a reference to a named grammar.

Examples

The following is a simple rule reference example.

Rule $x = t1 t2 t3;
Expansion $x
Input t1,t2,t3
Output $x["t1","t2","t3"]

The following is a rule reference in sequence.

Rule $x = t2 t3 t4;
Expansion t1 $x t5
Input t1,t2,t3,t4,t5
Output ["t1",$x["t2","t3","t4"],"t5"]

The following includes a reference to a rule that outputs a tag.

Rule $x = t2 {tag};
Expansion t1 $x t3
Input t1,t2,t3
Output ["t1",$x["t2",{!{tag}!}],"t3"]

Multiple references to the same rule are permitted.

Rule $x = t1 {tag1};
Expansion $x $x $x
Input t1,t1,t1
Output [$x["t1",{!{tag1}!}],$x["t1",{!{tag1}!}],$x["t1",{!{tag1}!}]]

Rule references may be repeated.

Rule $x = t1 {tag};
Expansion $x <0->
Input t1,t1,t1
Output [$x["t1",{!{tag1}!}],$x["t1",{!{tag1}!}],$x["t1",{!{tag1}!}]]

H.3 Recursion

The Speech Recognition Grammar Specification has the expressive power of a Context Free Grammar. This arises because the language permits a rule to directly or indirectly reference itself. [Note: a Conforming XML Form Grammar Processor or Conforming ABNF Form Grammar Processor is not required to support recursive grammars.]

There is no distinct representation for a recursive rule reference.

Examples

Simple right recursion. Note: this grammar can be written in a non-recursive (regular expression) form.

Rule $x = t1 {last} | t1 $x;
Expansion $x
Input t1,t1,t1
Output [$x["t1",$x["t1",$x["t1",{!{last}!}]]]]

Embedded recursion. Note that this matches any sequence of n t1's followed by n t2's.

Rule $x = {bottom} | (t1 $x t2);
Expansion $x
Input t1,t1,t2,t2
Output [$x["t1",$x["t1",$x[{!{bottom}!}],"t2"],"t2"]]

Appendix I. Features under Consideration for Future Versions

This appendix is informative.

The following features are under consideration for versions of the Speech Recognition Grammar Specification after version 1.0:

Appendix J: Example Grammars in ABNF Form and XML Form

This appendix is informative.

J.1: Simple Examples (English)

The following shows a simple grammar that supports commands such as "open a file" and "please move the window". It references a separately-defined grammar for politeness which is not shown here.

ABNF Form

#ABNF 1.0 UTF-8;

language en;
mode voice;
root $basicCmd;

meta "author" is "Stephanie Williams";

/**
 * Basic command.
 * @example please move the window
 * @example open a file
 */

public $basicCmd = 
          $<http://grammar.example.com/politeness.gram#startPolite>
          $command
          $<http://grammar.example.com/politeness.gram#endPolite>;

$command = $action $object;
$action = /10/ open {TAG-CONTENT-1} | /2/ close {TAG-CONTENT-2} 
        | /1/ delete {TAG-CONTENT-3} | /1/ move {TAG-CONTENT-4};
$object = [the | a] (window | file | menu);

XML Form

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN"
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar xmlns="http://www.w3.org/2001/06/grammar" xml:lang="en"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         version="1.0" mode="voice" root="basicCmd">

<meta name="author" content="Stephanie Williams"/>

<rule id="basicCmd" scope="public">
  <example> please move the window </example>
  <example> open a file </example>

  <ruleref uri="http://grammar.example.com/politeness.grxml#startPolite"/>

  <ruleref uri="#command"/>
  <ruleref uri="http://grammar.example.com/politeness.grxml#endPolite"/>

</rule>

<rule id="command">
  <ruleref uri="#action"/> <ruleref uri="#object"/>
</rule>

<rule id="action">
   <one-of>
      <item weight="10"> open   <tag>TAG-CONTENT-1</tag> </item>
      <item weight="2">  close  <tag>TAG-CONTENT-2</tag> </item>
      <item weight="1">  delete <tag>TAG-CONTENT-3</tag> </item>
      <item weight="1">  move   <tag>TAG-CONTENT-4</tag> </item>
    </one-of>
</rule>

<rule id="object">
  <item repeat="0-1">
    <one-of>
      <item> the </item>
      <item> a </item>
    </one-of>
  </item>

  <one-of>
      <item> window </item>
      <item> file </item>
      <item> menu </item>
  </one-of>
</rule>

</grammar>

J.2: Cross-Reference Examples (English)

These two grammars illustrate referencing between grammars. The same grammar is shown in both XML Form and ABNF Form.

ABNF: http://www.example.com/places.gram

#ABNF 1.0 ISO-8859-1;

language en;
mode voice;
root $city_state;

public $city = Boston | Philadelphia | Fargo;

public $state = Florida | North Dakota | New York;

// References to local rules
// Artificial example allows "Boston, Florida!"

public $city_state = $city $state;

ABNF: http://www.example.com/booking.gram

#ABNF 1.0 ISO-8859-1;

language en;
mode voice;

// Reference by URI syntax
public $flight = I want to fly to
   $<http://www.example.com/places.gram#city>;

// Reference by URI syntax
public $exercise = I want to walk to $<http://www.example.com/places.gram#state>;

// Implicit reference to root rule by URI
public $wet = I want to swim to $<http://www.example.com/places.gram>;

XML Form Grammar: http://www.example.com/places.grxml

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN"
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="en" version="1.0" root="city_state" mode="voice">

   <rule id="city" scope="public">
     <one-of>
       <item>Boston</item>
       <item>Philadelphia</item>
       <item>Fargo</item>
     </one-of>
   </rule>

   <rule id="state" scope="public">
     <one-of>
       <item>Florida</item>
       <item>North Dakota</item>
       <item>New York</item>
     </one-of>
   </rule>

   <!-- Reference by URI to a local rule -->
   <!-- Artificial example allows "Boston, Florida"! -->
   <rule id="city_state" scope="public">
     <ruleref uri="#city"/> <ruleref uri="#state"/>
   </rule>
</grammar>

XML Form Grammar: http://www.example.com/booking.grxml

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN"
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="en" version="1.0" mode="voice">

   <!-- Using URI syntax -->
   <rule id="flight" scope="public">
     I want to fly to 
     <ruleref uri="http://www.example.com/places.grxml#city"/>
   </rule>

   <!-- Using URI  syntax -->
   <rule id="exercise" scope="public">
     I want to walk to <ruleref uri="http://www.example.com/places.grxml#state"/>

   </rule>

   <!-- Implicit reference to root rule of a grammar by URI  -->
   <rule id="wet" scope="public">
     I want to swim to <ruleref uri="http://www.example.com/places.grxml"/>

   </rule>
</grammar>

J.3: Korean Examples

The following two grammars are XML Form grammars with Korean yes/no content. The first represents the Korean symbols as Unicode characters and has UTF-8 encoding. The second represents the same Unicode characters using character escaping.

ABNF Form Grammar with Unicode Characters in UTF-8 Encoding

#ABNF 1.0 UTF-8;

language ko;
mode voice;
root $yes_no_ko;

/* 
 * Simple Korean yes/no grammar 
 * @example 예
 */

public $yes_no_ko = 예 | 아니오 ;

XML Form Grammar with Unicode Characters in UTF-8 Encoding

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN" 
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar xml:lang="ko" version="1.0" mode="voice"
         xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         root="yes_no_ko">

  <!-- yes/no grammar -->
  <rule id="yes_no_ko" scope="public">
    <example>예</example>
    <one-of>
      <item>예</item>
      <item>아니오</item>
    </one-of>
  </rule>
</grammar>

XML Form Grammar with Character Escaping of Unicode Characters

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN" 
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar xml:lang="ko" version="1.0" mode="voice"
         xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         root="main">

  <!-- yes/no grammar -->
  <rule id="yes_no_ko" scope="public">
    <example>&#50696;</example>
    <one-of>
      <item>&#50696;</item>
      <item>&#50500;&#45768;&#50724;</item>
    </one-of>
  </rule>
</grammar>

J.4: Chinese Examples

The following two grammars are XML Form grammars with Chinese number content. The first represents the Chinese symbols as Unicode characters with the UTF-8 encoding. The second represents the same Unicode characters using character escaping.

ABNF Form Grammar with Unicode Characters in UTF-8 Encoding

#ABNF 1.0 UTF-8;

language zh;
mode voice;
root $main;

public $main = $digits1_9;

/*
 * @example 四
 */

private $digits1_9 = 一 | 二 | 三 | 四 | 五 | 六 | 七 | 八 | 九;

XML Form Grammar with Unicode Characters in UTF-8 Encoding

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN" 
          "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar version="1.0" xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="zh" mode="voice" root="main">

  <rule id="main" scope="public">
    <ruleref uri="#digits1_9"/>
  </rule>

  <rule id="digits1_9" scope="private">
    <example>四</example>
    <one-of>
      <item>一</item>
      <item>二</item>
      <item>三</item>
      <item>四</item>
      <item>五</item>
      <item>六</item>
      <item>七</item>
      <item>八</item>
      <item>九</item>
    </one-of>
  </rule>
</grammar>

XML Form Grammar with Character Escaping of Unicode Characters

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN" 
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar version="1.0" xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="zh" mode="voice" root="main">

  <rule id="main" scope="public">
    <ruleref uri="#digits1_9"/>
  </rule>

  <rule id="digits1_9" scope="private">
    <example>&#22235;</example>
    <one-of>
      <item>&#19968;</item>
      <item>&#20108;</item>
      <item>&#19977;</item>
      <item>&#22235;</item>
      <item>&#20116;</item>
      <item>&#20845;</item>
      <item>&#19971;</item>
      <item>&#20843;</item>
      <item>&#20061;</item>
    </one-of>
  </rule>
</grammar>

J.5: Swedish Examples

This Swedish XML Form grammar provides a comprehensive set of forms of "yes" and "no". All characters are contained within the ISO-8859-1 (Latin-1) character set.

XML Form Grammar with ISO-8859-1

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE grammar PUBLIC "-//W3C//DTD GRAMMAR 1.0//EN" 
                  "http://www.w3.org/TR/speech-grammar/grammar.dtd">

<grammar version="1.0" xmlns="http://www.w3.org/2001/06/grammar"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
         xsi:schemaLocation="http://www.w3.org/2001/06/grammar 
                             http://www.w3.org/TR/speech-grammar/grammar.xsd"
         xml:lang="sv" mode="voice" root="main">

  <rule id="main" scope="public">
    <example>ja det är rätt</example>
    <example>nej det är fel</example>
    <one-of>
      <item>
        <ruleref uri="#yes_rule"/>
      </item>
      <item>
        <ruleref uri="#no_rule"/>
      </item>
    </one-of>    
  </rule>

  <rule id="yes_rule" scope="private">
    <example>ja det är rätt</example>
    <one-of>
      <item>exakt</item>
      <item>javisst</item>
      <item>
        ja
        <item repeat="0-1">
          <ruleref uri="#yes_emphasis"/>
        </item>
      </item>
      <item>jepp</item>
      <item>korrekt</item>
      <item>okej</item>
      <item>rätt</item>
      <item>si</item>
      <item>säkert</item>
      <item>visst</item>
    </one-of>
  </rule>

  <rule id="yes_emphasis" scope="private">
    <example>det stämmer</example>
    <one-of>
      <item>det gjorde jag</item>
      <item>
        <item repeat="0-1">det</item>
        stämmer
      </item>
      <item>det är rätt</item>
      <item>det är korrekt</item>
      <item>det är riktigt</item>
    </one-of>
  </rule>

  <rule id="no_rule" scope="private">
    <example>nej det är fel</example>
    <one-of>
      <item>icke</item>
      <item>fel</item>
      <item>
        nej
        <item repeat="0-1">
          <ruleref uri="#no_emphasis"/>
        </item>
      </item>
      <item>nix</item>
      <item>no</item>
    </one-of>
  </rule>

  <rule id="no_emphasis" scope="private">
    <example>det är fel</example>
    <one-of>
      <item>det gjorde jag inte</item>
      <item>
        <item repeat="0-1">det</item>
        stämmer inte
      </item>
      <item>det är fel</item>
      <item>absolut inte</item>
      <item>inte alls</item>
    </one-of>
  </rule>
</grammar>