Re: [boost] Re: Simple regex problem

From: John Maddock (john_at_[hidden])
Date: 2005-02-03 06:30:36

> Trust me, the regex works.  I changed my flags to
> boost::regex_constants::normal, and it all worked just fine, however it
> does not explain why when I was using just the flags
> boost::regex_constants::char_classes | boost::regex_constants::intervals,
> it refused to match more than two words.

The problem is that you need to specify what kind of regular expression it 
is (basic, extended, Perl etc), if you don't specify anything it defaults to 
something like POSIX-Basic semantics, which leads to leftmost longest 
matching being selected.

Now onto your expression, the problem here is the [[:space:]]* part: because 
this can match zero times, your expression could be reduced down to 
something equivalent to:  "([[:alnum:]]+)+"  *in the worst case*, and this 
is the classic "may take forever to match example".

It works when you use Perl matching semantics, because the matcher stops as 
soon as a match is found, if no match is found, then it may well thrash 
indefinitely (leading to an exception eventually).  When POSIX matching 
semantics are selected, the leftmost longest rule causes the matcher to 
thrash looking for the "best" possible match, again leading to the 
exception.

So to conclude: specify that you want Perl-style regexes (unless you really 
want POSIX leftmost longest rules).
And, change your expression to something like: 
"^([[:alpha:]][-[:alnum:]]*(?:[[:space:]]+|$))+$"

Hope this helps,

John.