Re: Why my Regex doesn't work

A \n regex backreference is not active in a character class; [^\1] is equivalent to [^1] (Update: Nope. Not quite. See Update below.)

Try:

c:\@Work\Perl\monks>perl -wMstrict -le
"my $s = 'CCHHHCHHCHC';
 print qq{'$s'};
 ;;
 $s =~ s/(\w) (?!\1). \1/$1$1$1/xgi;
 print qq{'$s'};
"
'CCHHHCHHCHC'
'CCHHHHHHHHC'
[download]

(note the use of the /x modifier — for clarity only).

Update: In fact, \1 in a character class is an octal escape sequence:

c:\@Work\Perl\monks\flappygoat>perl -wMstrict -le
"my $s = qq{\1\x01\o{001}\cA};
 print 'match' if $s =~ /[^\1]/;
 print 'count: ', $s =~ tr/\1//;
"
count: 4
[download]

(no match; nothing printed). Wonderful what you can find out if you actually test stuff.

Give a man a fish: <%-{-{-{-<

Comment on Re: Why my Regex doesn't work Select or Download Code

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Re^2: Why my Regex doesn't work by Anonymous Monk on Apr 11, 2017 at 15:28 UTC
The OP wanted CCHHHHHHHCC, not CCHHHHHHHHC. Maybe that's a typo, or maybe he wants all the surrounded characters substituted simultaneously, as in a cellular automaton. In that case, `s/(?<=(\w))(?!\1).(?=\1)/$1/g;` [download]	[reply] [d/l]
Re^3: Why my Regex doesn't work by Not_a_Number (Prior) on Apr 11, 2017 at 17:25 UTC
Yeah. The problem seems underspecified. For example, what output would the OP want from `CHCHC`? AnomalousMonk's snippet outputs `CCCHC`, while anonymonk's gives `CCHCC`...	[reply] [d/l] [select]
Re^3: Why my Regex doesn't work by flappygoat (Initiate) on Apr 11, 2017 at 17:36 UTC
thank you so much! That is exactly what I needed. I wonder tho if it's possible to do without "lookaround".	[reply]
Re^4: Why my Regex doesn't work by AnomalousMonk (Archbishop) on Apr 11, 2017 at 18:32 UTC
I wonder tho if it's possible to do without "lookaround". There is probably some tricky way to avoid lookarounds, but just as a matter of curiosity, why would you want to? Lookarounds have been a part of Perl 5 regex from the beginning. Was it only a matter of curiosity on your part? Give a man a fish: `<%-{-{-{-<`	[reply] [d/l]
Re^5: Why my Regex doesn't work by flappygoat (Initiate) on Apr 11, 2017 at 19:16 UTC
Re^6: Why my Regex doesn't work by Anonymous Monk on Apr 11, 2017 at 21:44 UTC
Some notes below your chosen depth have not been shown here
Re^2: Why my Regex doesn't work by flappygoat (Initiate) on Apr 11, 2017 at 17:34 UTC
Thank you. That is what I was trying to code even tho I see now that it has a problem, it won't match the last 3 chars CHC, but the other comment solved it.	[reply]