Re: [Boost-users] Boost.Regex - problem with non-marking parenthesis

From: John Maddock (john_at_[hidden])
Date: 2005-12-13 11:09:27

> I have a regular expression that compiles & matches as expected, viz:
>
>   boost::regex rxBlah("([a-z](_?[a-z0-9])*)_dvs",
> boost::regex_constants::icase);
>
> I would like to make the inner parentheses non-marking, so I modified
> the regex as below:
>
>   boost::regex rxBlah("([a-z](?:_?[a-z0-9])*)_dvs",
> boost::regex_constants::icase);
>
> However, this throws a bad_expression exception on compilation, with
> the message "Invalid preceding regular expression". I've tested the
> second regex in another regex tool (The Regex Coach,
> http://weitz.de/regex-coach/) and it was accepted, and performed as
> expected - when presented with the text "a_dvs", it showed one
> capture, the text "a".
>
> So - help, what have I done wrong?

The flags you're passing to the expression are wrong: just passing icase is 
roughly the same as basic|icase and POSIX basic expressions don't support 
Perl style features like non-marking parenthesis, but:

boost::regex rxBlah("([a-z](?:_?[a-z0-9])*)_dvs",
   boost::regex_constants::perl | boost::regex_constants::icase);

will do what you want.

BTW from 1.33.0 onwards boost::regex_constants::perl now has a value of 0 
precisely to avoid this problem.

John.