My guess: Perl is looking for exact strings from both ends, your regexes include a trailing "b" but your string $futile doesn't

This simplified demo seems to support my theory

D:\tmp\pm>perl -Mre=debug -E"'aaaa' =~/a*a*b/"
Compiling REx "a*a*b"
synthetic stclass "ANYOF[ab]".
Final program:
   1: STAR (4)
   2:   EXACT <a> (0)
   4: STAR (7)
   5:   EXACT <a> (0)
   7: EXACT <b> (9)
   9: END (0)
floating "b" at 0..9223372036854775807 (checking floating) stclass ANY
+OF[ab] min
len 1
Matching REx "a*a*b" against "aaaa"
Intuit: trying to determine minimum start position...
  doing 'check' fbm scan, [0..4] gave -1
  Did not find floating substr "b"...
Match rejected by optimizer
Freeing REx: "a*a*b"

[download]

please note Did not find floating substr "b"... Match rejected by optimizer

Cheers Rolf
(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery

In reply to Re^2: Is it safe to use external strings for regexes? by LanX
in thread Is it safe to use external strings for regexes? by stevieb

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.