is (?: ... ) capturing

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

Dear Monks

consider the following example

my $str  = "abc def 123 456" ;
$str =~ s/(?:\s)\w/_/g ;
print $str ;
[download]

which gives me: abc_ef_23_56, but I expected abc _ef _23 _56
From this I would say that (?:\s) is capturing. So I must do something wrong. Any comments?

thnx a lot
LuCa

Comment on is (?: ... ) capturing Download Code

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Re: is (?: ... ) capturing by ysth (Canon) on Sep 07, 2009 at 07:35 UTC
Capturing refers to data that is put into $1, $2, etc. or returned from a list context m//. You aren't using the result of a capture there, so capturing or not isn't relevant. Even though (?: ) doesn't capture, it does consume part of the string and usually becomes part of what s/// will replace. You can avoid that with a lookbehind instead: `s/(?<=\s)\w/_/g`. Or, in 5.10+, the \K feature: `s/\s\K\w/_/g`. See perlre for details. -- A math joke: r = \| \|csc(θ)\|+\|sec(θ)\|-\|\|csc(θ)\|-\|sec(θ)\|\| \| Online Fortune Cookie Search Office Space merchandise	[reply] [d/l] [select]
Re^2: is (?: ... ) capturing by jeanluca (Deacon) on Sep 07, 2009 at 07:45 UTC
thnx a lot!! I'm using 5.6, so if I had to do something like `my $str = "abc def 123 456" ; $str =~ s/(?:\s+)\w/_/g ;` [download] I would have had to do it differently, correct ? cheers	[reply] [d/l]
Re^3: is (?: ... ) capturing by ysth (Canon) on Sep 07, 2009 at 07:47 UTC
Yes, because lookbehind has to be fixed-width, you'd need to do `s/(\s+)\w/${1}_/g`. -- A math joke: r = \| \|csc(θ)\|+\|sec(θ)\|-\|\|csc(θ)\|-\|sec(θ)\|\| \| Online Fortune Cookie Search Office Space merchandise	[reply] [d/l]
Re^4: is (?: ... ) capturing by ikegami (Patriarch) on Sep 07, 2009 at 09:30 UTC
Re^5: is (?: ... ) capturing by ysth (Canon) on Sep 07, 2009 at 17:18 UTC
Re^4: is (?: ... ) capturing by jeanluca (Deacon) on Sep 07, 2009 at 07:51 UTC
Re: is (?: ... ) capturing by Anonymous Monk on Sep 07, 2009 at 07:34 UTC
Capturing refers to storing values in $1,$2... is not capturing.	[reply]