Look Behind issues

dominic01 has asked for the wisdom of the Perl Monks concerning the following question:

I have a line like
$Line = "Please;, read these°, before you post 180;, if you're not absolutely –, sure you're posting";

I am trying to replace the ";," with ";". However it should not replace if the comma follows an entity.

My output should be
$Line = "Please; read these°, before you post 180; if you're not absolutely –, sure you're posting";

Appreciate any help on framing the regex. I am using perl 5, version 20, subversion 1 (v5.20.1) built for MSWin32-x64-multi-thread

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Re: Look Behind issues by AnomalousMonk (Archbishop) on Mar 05, 2015 at 06:25 UTC
This approach is a variation on `AnonyMonk`'s above: `c:\@Work\Perl>perl -wMstrict -le "use 5.010; ;; my $s = qq{Please;, read these&\x23xB0;, before you post 180;, if not + absolutely &\x23x2013;, sure.}; print qq{'$s'}; ;; my $entity = qr{ & [\x23] x [[:xdigit:]]+ ; }xms; ;; $s =~ s{ (?: $entity , (SKIP) (FAIL))? ; \K , }{}xmsg; print qq{'$s'}; " 'Please;, read these°, before you post 180;, if not absolutely &# +x2013;, sure.' 'Please; read these°, before you post 180; if not absolutely &#x2 +013;, sure.'` [download] Some notes (update: now with more numbers for your indexing enjoyment): My REPL does not like the # character at all, so it has to be represented as `\x23` in strings and regexes, but it prints and otherwise operates as expected; The substitution regex uses features added with Perl version 5.10: `(SKIP) (FAIL)` (see Special Backtracking Control Verbs), and `\K` (see Extended Patterns); The `$entity` regex is very naive (as is the one used by `AnonyMonk`) and you will have to come up with one that covers all your cases; The `(?: $entity , (SKIP) (FAIL))?` sub-pattern acts as a variable width negative look-behind by 'consuming' (i.e., skipping past) something that is needed for a subsequent match, in this case a comma following an entity. Give a man a fish: `<%-(-(-(-<`	[reply] [d/l] [select]
Re^2: Look Behind issues by dominic01 (Sexton) on Mar 05, 2015 at 15:40 UTC
This worked well for me. I learned something new (SKIP) (FAIL). Thank you	[reply]
Re: Look Behind issues by Anonymous Monk on Mar 05, 2015 at 05:11 UTC
`s/&#x\w+;,\K\|;\K,//g` [download]	[reply] [d/l]
Re^2: Look Behind issues by dominic01 (Sexton) on Mar 05, 2015 at 15:37 UTC
This worked well for me. Thank you.	[reply]
Re: Look Behind issues (just matching) by Anonymous Monk on Mar 05, 2015 at 04:26 UTC
How about no look behind? `match entity # $1 commatarget # $2 and replace entity with itself and commatarget with comma, and you're +done :)` [download]	[reply] [d/l]
Re^2: Look Behind issues (just matching) by Anonymous Monk on Mar 05, 2015 at 04:32 UTC
As soon as you try making it any valid entity format, you're back to no look behind because its fixed width only http://perldoc.perl.org/perlre.html#%28?%3C!pattern%29 `my $f = 'blah;, ente°, blah;, blah.'; $f =~ s{(?<!&[#]x[A-Za-z0-9]{2});,}{,}g; print "$f\n"; __END__ blah, ente°, blah, blah.` [download]	[reply] [d/l]