Eric Niebler wrote:
Empty alternatives are not allowed (these are almost always a mistake), but if you really want an empty alternative use (?:) as a placeholder, for example:
"|abc" is not a valid expression, but "(?:)|abc" is and is equivalent, also the expression: "(?:abc)??" has exactly the same effect.
This would seem to be non-compliant behavior according to TR1, which references ECMA-262, which describes the regex syntax in section 15.10.1 as:
Quite possibly, although I note that Perl 6 (last time I checked anyway) was planning to disallow these, on the grounds that they are a persistent source of buggy regular expressions. And as noted there are alternatives: |abc == (?:abc)?? == (?:)|abc abc| == (?:abc)? == abc|(?:) John.