Say, a string has embedded palindromes (PD); a PD in this case has odd (at least 3) number of letters. I want a piece of code to be executed exactly once for every PD including nested ones. Let "piece of code" be 'say'. Regex to extract a PD is well-known, and of course TIMTOWTDI, the question is not how to better solve the task.

Almost accidentally I found a solution, which I'm puzzled about. I don't know if it could be a simpler SSCCE without recursion.

say 'match' if 'abABCDCBAdeXYZYXfg' =~ /
    (?>^.*?)
    ( 
        (
            (.)(?-3)\g-1 | (.).\g-1 
        )
        (?{ 
            say $2
        })
    )
/x;

[download]

Output:

YZY
XYZYX
CDC
BCDCB
ABCDCBA

[download]

Bingo, just what I need. Match fails -- it's irrelevant. But HOW? If either of the following is removed: (1) "^" anchor; (2) independence (s/>/:/), or (3) entire 1st pair of parens, then there are multiple visits to each PD i.e. not what I want. In fact, for (3) the PD groups are also visited in reversed order, this I don't understand neither. And why both PD groups (not only 1st one) are visited.

Being greedy (s/\*\?/*/) produces no output, this I understand (I hope).

If, in case of code above i.e. without removing anything, the engine gives characters back one by one (I thought this is how backtracking works) before it ultimately fails when it (suddenly, somehow) remembers about independence, then I'd expect YZY is found and said; and then XYZYX is found and embedded code executed for both YZY and XYZYX, i.e. both are said (i.e. YZY said twice in total).

P.S. I've seen "Embedded Code Execution Frequency" section in perlre.

In reply to Possessive sub-pattern with non-greedy content + recursion: WHY does this work?? by Anonymous Monk

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.