Re: [Boost-users] Lookbehind assertions problem with 1.33

From: Gregory Nisnevich (gregnis_at_[hidden])
Date: 2005-07-17 16:12:03

I found another problem, this time, I'm afraid, without a workaround:

regex: (?<=^).{2}|(?<=^...).{2}
string: 1234567890

I get, using boost::regex, only one match: 

12

whereas, for example, .Net Regex engine expectedly finds

12

and

45

Is this a bug? If so, any hopes for fixing it in 1.33?

Thank you,

-Gregory

--- John Maddock <john_at_[hidden]> wrote:

> > I'm trying to use lookbehind assertions, and here's a problem: the
> > following syntax
> >
> > (?<=^.{2}).{4}
> >
> > gives me the error:
> >
> > '(?<=^.{2}).{4}' is a bad regular expression: Unmatched [ or [^
> >
> > while the following works:
> >
> > (?<=^..).{4}
> >
> > matching, as I expected, 4 characters after the first 2 characters
> > in a line.
> >
> > Shouldn't the above two regexes work the same way?
> 
> Probably, but the lookbehind assertion code is pretty dumb at present: it 
> needs to be able to figure out exactly how many characters to look behind in 
> advance, and currently bounded repeats aren't supported.  It's probably not 
> too hard to support that case, but it's too late for 1.33 I'm afraid.
> 
> John. 
> 
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> Boost-users mailing list
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> 

                
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