unfortunately, i've seen this bug occur on non-empty strings, too. i think the question is whether or not the regex matches zero or more of something. for example this yields different results, too:

var re = /[a-z]*/g
re.lastIndex = 1
re.exec('s')
re.lastIndex === 2 // in IE6, expected empty string match at pos 1 instead
re.lastIndex = 1
re.exec('s1')
re.lastIndex === 2 // in IE6, expected empty string match at pos 1 instead

for a patch i'm thinking (not tested):

var hasLastIndexBug = (function () {
  var re = /.*/g
  re.exec('')
  return re.lastIndex !== 0
}())

if (hasLastIndexBug) {
  var nativeExec = RegExp.prototype.exec
  RegExp.prototype.exec = function exec (string) {
    var match = nativeExec.call(this, string)
    if (match && !match[0].length) this.lastIndex--
    /* or alternatively
    if (match && !match[0].length) this.lastIndex = match.index
     * or most explicit
    if (match && !match[0].length) this.lastIndex = match.index + match[0].length
     */
    return match
  }
}

it seems the state-machine forgets to reset its look-ahead pointer before it bails out.

regex lastIndex #403

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