YAPE::Regex::Explain creates a human-readable explanation of regexen. 

use strict;
use warnings;

use YAPE::Regex::Explain;

my $re = qr/(?=(\d\d\d))/;

print YAPE::Regex::Explain->new($re)->explain;

[download]
Output: 
The regular expression:

(?-imsx:(?=(\d\d\d)))

matches as follows:

NODE                     EXPLANATION
----------------------------------------------------------------------
(?-imsx:                 group, but do not capture (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n) (matching whitespace and #
                         normally):
----------------------------------------------------------------------
  (?=                      look ahead to see if there is:
----------------------------------------------------------------------
    (                        group and capture to \1:
----------------------------------------------------------------------
      \d                       digits (0-9)
----------------------------------------------------------------------
      \d                       digits (0-9)
----------------------------------------------------------------------
      \d                       digits (0-9)
----------------------------------------------------------------------
    )                        end of \1
----------------------------------------------------------------------
  )                        end of look-ahead
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]
See perlre for more information on pattern extensions (like '?=') and modifiers (such as 'g').

In reply to Re: Explanation for Reg Expr by bobf
in thread Explanation for Reg Expr by decoder

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.