Re: [Boost-users] boost.regex doesn't implement character setcomplements?

From: Robert Mathews (rmathews_at_[hidden])
Date: 2005-06-29 09:06:17

Yes, that a negative lookahead assertions works very nicely. And you are
quite right that my perl wasn't really doing the right thing either.

Thank you very much!

"Eric Niebler" <eric_at_[hidden]> wrote in message
news:42C22F01.6060009_at_boost-consulting.com...
> Robert Mathews wrote:
> > I was trying to write "Match everything *except* Bar".
> >
> > I wrote this:
> >
> > bool aResult = boost::regex_match(L"Foo", boost::wregex(L"[^(Bar)]"));
> >
> > This doesn't match. But if I wrote the same thing in Perl, it does work,
ie:
> >
> > print "match\n" if( "Foo" =~ "[^(Bar)]");
> >
> > It looks a lot like sets can't contain subexpressions in boost::regex
... is
> > that really true?
> > Is there some other way to write the same thing?
>
>
> I guarantee that your perl code isn't doing what you think it's doing.
This:
>
>     "Foo" =~ /[^(Bar)]/
>
> will succeed because 'F' is not one of '(', 'B', 'a', 'r', or ')'. The
> same pattern means the same thing to Boost.Regex, but you're getting a
> different result because the regex_match algorithm only reports success
> if the pattern matches all of the input, which it doesn't in this case.
> Had you used regex_search, you'd have gotten the same results as perl.
> But it still wouldn't be doing what you want.
>
> What you're looking for is a negative lookahead assertion. If you want
> to match three characters that are not "Bar", you can do it like:
>
>      "(?!Bar)..."
>
> (?!Bar) asserts that the next three characters are not "Bar", but
> doesn't consume them. The ... matches the next three characters (which
> are not "Bar") and consumes them.
>
> HTH,
>
> -- 
> Eric Niebler
> Boost Consulting
> www.boost-consulting.com