Re: [boost] [xpressive] "quantifier not expected" exception when using non-capturing group

From: Eric Niebler (eric_at_[hidden])
Date: 2007-06-06 13:05:43

Jean-Francois Bastien wrote:
> Here is a simplification of my expression:
>   sregex expression = sregex::compile(
>     /* 1 */ "^("
>     /* 2 */   "c[^\r\n]*(?:\r?\n|\r)"
>     /* 3 */   "(?:"
>     /* 4 */     "(?!c)"
>    /* 5 */      "[^\r\n]*(?:\r?\n|\r)"
>    /* 6 */    ")*"
>    /* 7 */  ")"
>     );
> 
> Basically: match lines starting with a "c" and the line that follow,
> until another line starting with a "c" is found.
> 
> This gives a "quantifier not expected" exception. Changing line 3 to a
> normal group solves the problem. Changing lines 2 and 5's groups to
> normal groups also solves the problem.

Yes, it's a bug. This is a hard one to hit, too. It happens when:

- The quantifier applies to a group
- The group is non-capturing
- The first element within the non-capturing group is something that 
cannot be quantified, such as a zero-width assertion; eg., (?!c)

I've already fixed this on HEAD and added a test. The fix to the RC is 
more involved. I'll look into it. I do NOT believe this problem to be 
serious enough for 1.34.1, though. It happens quite rarely, and there is 
a work-around.

Thanks for the report.

-- 
Eric Niebler
Boost Consulting
www.boost-consulting.com