Re: [Boost-users] Boost-regex: Weird behaviour with non-greedy matching operator in regex_replace in boost 1.40?

Subject: Re: [Boost-users] Boost-regex: Weird behaviour with non-greedy matching operator in regex_replace in boost 1.40?
From: John Maddock (john_at_[hidden])
Date: 2009-09-23 12:54:33

> I have the following questions:
> - why does test 1 match the expected "hall" while test 2 matches "hallo"
> - why does test 1 match the whole string while test 4 matches only a
> part of it.

Because that's the way that Perl regexes work, if you have the expression 
(.*?)o? then for preference the .*? part will match *no characters at all*, 
so basically your expression either matches no characters, or one character 
if the next character is an "o".  So since you're doing a search and 
replace, the effect is:

* If the next character is not an "o", match a zero length string and output 
a null string (the contents of $1).
    * Since the last match was against a zero length string, then skip to 
the next character.
* Otherwise if the next character is an "o", match it and output $1 - again 
this is an empty string.
    * Move to the end of the string matched.
* Find the next match and output all unmatched text (everything from the end 
of the last match to the start of this one).
* Repeat.

So in effect we end up deleting all the letter "o"'s.

Or at least I think that's what's going on here after a very brief look ;-)

HTH, John.