Why does this simple grouping regex not match?

december has asked for the wisdom of the Perl Monks concerning the following question:

Hello fellow monks,

A quick question:

Why doesn't the first regex match while the second does?

$ perl -e '$_ = "(b)."; /\((?=a|b|c)\)/ && do { print "match!\n"; };'
$ perl -e '$_ = "(b)."; /\((?=a|b|c)/ && do { print "match!\n"; };'
match!
[download]

It's probably very simple and it has something to do with the use of grouping brackets instead of regular capturing brackets, but I don't seem to get why one works and the other doesn't – it's Monday and so far my brain has refused to accept that fact.

Thanks for any pointers!

EDIT: It's a typo. I overlooked the fact I typed an equal sign instead of a colon, accidently making it a look-ahead regex. Monday just isn't my day. Sorry folks!

Comment on Why does this simple grouping regex not match? Download Code

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Re: Why does this simple grouping regex not match? by moritz (Cardinal) on Feb 28, 2011 at 13:34 UTC
`(?= ...)` is not a "simple grouping", but a look-ahead (ie it looks for the presence of the regex within the group, but doesn't consume any characters). Use `(?: ... )` for just grouping. (Update: fixed markup) Perl 6 - second systems done right	[reply] [d/l] [select]
Re^2: Why does this simple grouping regex not match? by december (Pilgrim) on Feb 28, 2011 at 13:59 UTC
I'm an idiot. It's a typo, I meant to use a colon in my code. I just kept on copying the same typo. Sorry folks!	[reply]
Re^3: Why does this simple grouping regex not match? by Anonymous Monk on Feb 28, 2011 at 14:10 UTC
a healthy colon is vital to good health and vitality	[reply]
Re^4: Why does this simple grouping regex not match? by december (Pilgrim) on Mar 01, 2011 at 11:41 UTC
Re: Why does this simple grouping regex not match? by fisher (Priest) on Feb 28, 2011 at 13:38 UTC
`(?=a\|b\|c)` [download] I'm not sure how it iterprets exactly, but I think you need actually `perl -e '$_ = "(b)."; /$[abc]$/ && do { print "match!\n"; };'` [download] or `perl -e '$_ = "(b)."; /$(a\|b\|c)$/ && do { print "match!\n"; };'` [download]	[reply] [d/l] [select]
Re: Why does this simple grouping regex not match? by Anonymous Monk on Feb 28, 2011 at 13:58 UTC
`use re 'debug';` `perl -Mre=debug -e '$_ = q!(b).!; /$(?=a\|b\|c)$/ && do { print qq{mat +ch! $&\n}; };' perl -Mre=debug -e '$_ = q!(b).!; /\((?=a\|b\|c)/ && do { print qq{match +! $&\n}; };'` [download] Read more... (2 kB)	[reply] [d/l] [select]
Re^2: Why does this simple grouping regex not match? by december (Pilgrim) on Mar 01, 2011 at 11:46 UTC
Interesting... didn't know about that. Noted for future use. Thanks!	[reply]
Re: Why does this simple grouping regex not match? by umasuresh (Hermit) on Feb 28, 2011 at 13:35 UTC
Is this what you are looking for? `$ perl -e '$_ = "(b)."; /($?=a\|b\|c$)/ && do { print "match!\n"; };' match!` [download] The outer parenthesis captures the match and the inner should be escaped! UPDATE Never mind, didn't pay attention to the atomic grouping!	[reply] [d/l]


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