This is an iterative approach as well but more efficient: 
#!/usr/bin/perl -wl
use strict;
my $str = "17341234173412341734123417341234";
{
    local *_ = \$str;
    *_ = \substr $_, pos()-4 while s/(.)(...)\1/A$2B/;
}
print $str;

[download]

I am exploiting the fact that assigning a ref to something to a glob creates an alias to that something; this also works for a ref to an lvalue such as substr returns. Only when a match happens, the substitution is restarted, and then in order to avoid rescanning, I hide the already processed front of the string by aliasing $_ to the unprocessed portion. In the ideal case of no match, this means the RE engine will only be invoked once, regardless of the string's length.

Unfortunately, it doesn't work in this simple form because alas, s/// does not leave pos in a useful state after a substitution. The real code thus needs an ugly temporary variable and a messy addition to the substitution:

    my $i = 0;
    *_ = \substr $str, $i while s/(.)(...)\1(?{ $i += pos() - 4 })/A$2
+B/;

[download]

The result is A7AAB2BBA7AAB2BBA7AAB2BBA7AAB2BB.

Update: I just noticed this is essentially the same as diotalevi is doing. Doh.

Makeshifts last the longest.

In reply to Re^2: Replace zero-width grouping? (s/// against substr) by Aristotle
in thread Replace zero-width grouping? by tinypig

Title:
Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post, it's "PerlMonks-approved HTML":


  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.