Why my Regex doesn't work

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

Hi everyone. I want to have a regex that matches all occurrences of a letter sandwiched between 2 same letters(the letter in the middle is different than the two outer letters) and changes the middle letter so that all 3 letters are the same. For instance it would change CCHHHCHHCHC into CCHHHHHHHCC. I only work with 3 chars C,H,E but I wrote for a general case, but it won't work the way I want. my regex is

s/(\w)[^\1]\1/$1$1$1/gi

I don't understand why it matches HHH if I wrote that the middle letter cannot be the same as first. Why is this wrong and how to make it work?

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Re: Why my Regex doesn't work by AnomalousMonk (Archbishop) on Apr 11, 2017 at 14:01 UTC
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: `<%-{-{-{-<`	[reply] [d/l] [select]
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
Re^5: Why my Regex doesn't work by flappygoat (Initiate) on Apr 11, 2017 at 19:16 UTC
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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]