Re: [boost] regexes matching empty strings

From: John Maddock (john_at_[hidden])
Date: 2005-02-18 12:08:27

> Why can I not construct a regex of the form: "|abc" (throws exception 
> based on REG_EMPTY) for use with regex_match, if I want to match either 
> "abc" or the empty string "". I understand thatn in a regex_search the 
> empty string would always match (per Perl like semantics), and abc would 
> never be considered. But, I want to use this with regex_match, where the 
> empty string would have to match the entire string being searched. Work 
> arounds such as "(?:)|abc" and "^$|abc" seem to get around this 
> deficiency, but I am wondering why it is there in the first place.

It's deliberate, based on the assumption that empty alternatives are almost 
always a mistake (as an aside, although these are allowed in Perl 5, Perl 6 
will make them an error as well).  POSIX regular expressions also requires 
them to be diagnosed as errors.  As you say the workaround is to use (?:) as 
a placeholder for the empty string (I do need to update the docs to make 
this clear though).

John.