Re: [boost] [regex] empty expression issue

From: John Maddock (john_at_[hidden])
Date: 2007-05-31 04:16:30

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.