Re: [Boost-users] [regex] validating expressions

Subject: Re: [Boost-users] [regex] validating expressions
From: John Dlugosz (JDlugosz_at_[hidden])
Date: 2010-05-20 11:51:23

> 
> 	"[[[A-Z]]"
> 	"[A-Z]??"
> 
> Both Qt's QRegExp and http://www.regexplanet.com/simple/index.html

> report the first is invalid. Qt's QRegExp reports the second as
> invalid,
> while http://www.regexplanet.com/simple/index.html report it as valid.

The second one is using a feature that changes the greediness of the quantifiers.  You might look at the "perlre" man page for Perl 5.10 to get more complete documentation.

=== quote ===
By default, a quantified subpattern is "greedy", that is, it will match as many times as possible (given a particular starting location) while still allowing the rest of the pattern to match. If you want it to match the minimum number of times possible, follow the quantifier with a "?". Note that the meanings don't change, just the "greediness":

    *?     Match 0 or more times, not greedily
    +?     Match 1 or more times, not greedily
    ??     Match 0 or 1 time, not greedily
    {n}?   Match exactly n times, not greedily
    {n,}?  Match at least n times, not greedily
    {n,m}? Match at least n but not more than m times, not greedily
=== end quote ===

The first one is not an error because a ']' by itself is just a literal character, and '[' is fine inside the range.  [[[A-Z] is a range, just like [xxA-Z] is.  

--John


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