Re: Supress similar chars in the string

You can accomplish your desired result using Backreferences. With your sample input, I would use the code:

#!/usr/bin/perl
use strict;
use warnings;

$_ = 'testing 1234567 ........ aaaaaaaasssssss __________ ++++++++++ -
+-------- testing testing ';

s/((.)\2\2)\2*/$1/g;
print;
[download]

to output

testing 1234567 ... aaasss ___ +++ --- testing testing

YAPE::Regex::Explain describes the regex as:

  (                        group and capture to \1:
----------------------------------------------------------------------
    (                        group and capture to \2:
----------------------------------------------------------------------
      .                        any character except \n
----------------------------------------------------------------------
    )                        end of \2
----------------------------------------------------------------------
    \2                       what was matched by capture \2
----------------------------------------------------------------------
    \2                       what was matched by capture \2
----------------------------------------------------------------------
  )                        end of \1
----------------------------------------------------------------------
  \2*                      what was matched by capture \2 (0 or more
                           times (matching the most amount possible))
[download]

Note I have kept the third 'testing' in the string, which seems to follow spec but not your example.

Update: As AnomalousMonk points out below, swapping \2* to \2+ is more rigorously correct and avoids some unnecessary no-ops. However, I didn't think this particular optimization was likely to have sufficient impact to warrant changing the post content. For a cleaner version, you can also use s/(.)\1{3,}/$1 x 3/eg; (or s/(.)\1{$n,}/$1 x $n/eg;).

Comment on Re: Supress similar chars in the string Select or Download Code

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Re^2: Supress similar chars in the string by toolic (Bishop) on Jan 18, 2011 at 18:10 UTC
And if the OP ever had the need to scale it up to more than 3 characters: `use strict; use warnings; my $n = 3; # number of adjacent characters $n--; $_ = 'testing 1234567 ........ aaaaaaaasssssss __________ ++++++++++ - +-------- testing testing '; s/((.)\2{$n})\2*/$1/g; print;` [download]	[reply] [d/l]
Re^3: Supress similar chars in the string by Jim (Curate) on Jan 18, 2011 at 18:50 UTC
Is this a case where adding the `/o` modifier to the `s///` operator might be helpful? It's an efficiency measure intended to ensure (or help ensure, I think) the regex is only compiled once. `s/((.)\2{$n})\2*/$1/go;` [download]	[reply] [d/l] [select]
Re^4: Supress similar chars in the string by Anonymous Monk on Jan 24, 2011 at 02:38 UTC
Is this a case where adding the /o modifier to the s/// operator might be helpful? No, /o is mostly misused and obsolete , see //o of any help?	[reply]
Re^3: Supress similar chars in the string by AnomalousMonk (Archbishop) on Jan 18, 2011 at 22:26 UTC
`s/((.)\2{$n})\2/$1/g;` [download] Shouldn't the `\2` (zero or more of...) term in the regex above be `\2+` (one or more of...) instead? I.e.: `s/((.)\2{$n})\2+/$1/g;` Won't the `\2` version lead to useless replacement of sub-strings of n identical characters with the same n-character sub-string (Update:* where n is the maximum number of contiguous identical characters originally defined)? Update: This really should have been posted as a reply to Re: Supress similar chars in the string.	[reply] [d/l] [select]
Re^2: Supress similar chars in the string by Anonyrnous Monk (Hermit) on Jan 18, 2011 at 17:44 UTC
Newer Perls (>=5.10) also allow you to say `s/(.)\1\1\K\1*//g;` [download] i.e., through the use of `\K`, you don't need to copy the part of the match you want to keep (`$1` in your regex) into the substitution.	[reply] [d/l] [select]
Re^3: Supress similar chars in the string by AnomalousMonk (Archbishop) on Jan 18, 2011 at 22:36 UTC
`s/(.)\1\1\K\1//g;` [download] Shouldn't `\1` in regex above be `\1+` for reason similar to that discussed in Re^3: Supress similar chars in the string: useless replacement of empty string with empty string? (Or is regex engine smart enough to avoid this null operation?)	[reply] [d/l] [select]
Re^2: Supress similar chars in the string by AnomalousMonk (Archbishop) on Jan 19, 2011 at 18:44 UTC
... more rigorously correct ... optimization ... For me, the issue is not rigor or optimization, but correctness. Insofar as I understand it, the OP requires an overall match on more-than-n contiguous identical characters (although an n-character match may be a component of the overall match). The regexes to which I object match equal-to-or-more-than-n such characters overall, and so are incorrect. The incorrectness of the matches is obscured, but not ameliorated, by the subsequent substitutions, which may replace a string S (which may be empty) with an identical string S. The point can be exemplified by looking at what is matched by the `qr{ (.)\1\1 \K \1* }xms` regex (the most elegant, IMHO, in this thread) in a context that does not involve substitution. (Similar examples could be constructed for the other objectionable regexes.) All the empty strings are, in my view, unneeded (and improper) matches. `>perl -wMstrict -le "my $s = 'aaa...bbbb...ccccc...dddddd.x..yy'; print qq{'$2'} while $s =~ m{ (.)\1\1 \K (\1*) }xmsg; " '' '' 'b' '' 'cc' '' 'ddd'` [download] Anyway, that's my story and I'm sticking to it.	[reply] [d/l] [select]
Re^3: Supress similar chars in the string by kennethk (Abbot) on Jan 19, 2011 at 20:51 UTC
I would politely disagree with your interpretation, as the two approaches are functionally equivalent. There was no mention of side effects/counting in the OP, and so judging the proposed solution on those grounds is inappropriate. It is an inferior algorithm and likely a bad habit, but the code does the job and does it in a straight-forward fashion. If I'd spent more brain power considering the question, I would have substituted `+` for `*`. The modification actually occurred to me after I posted but before I'd read your comments. However, I try to avoid correcting posts for stylistic reasons alone to avoid versioning confusion and race conditions. I agree that the `\K` construct is the most elegant and appropriate solution, but not everyone runs 5.10 - my external server runs 5.8. The alternate choice option I list seems the most transparent for a neophyte to my eye, but I expect to be the the most expensive given the `e` modifier.	[reply] [d/l] [select]