Decomposing a regex into component parts makes conceptualization and maintenance easier. 

perl -wMstrict -le
"my $beginning  = qr{ \A FOO }xms;
 my $not_ending = qr{ (?! (?: BAR | BAZ) \z ) }xms;
 my $ending     = qr{ .*                 \z   }xms;
 for my $string (@ARGV) {
  if ($string =~ m{ $beginning $not_ending ($ending) }xms) {
   print qq($string accepted ('$1' to right))
   }
  else {
   print qq($string rejected)
   }
  }
" FOOABC FOO_BAR FOOBA FOOBAZZZZ "FOOBAR BAZ" FOOBAR FOOBAZ BARBAZ
FOOABC accepted ('ABC' to right)
FOO_BAR accepted ('_BAR' to right)
FOOBA accepted ('BA' to right)
FOOBAZZZZ accepted ('BAZZZZ' to right)
FOOBAR BAZ accepted ('BAR BAZ' to right)
FOOBAR rejected
FOOBAZ rejected
BARBAZ rejected

[download]

To make your life simpler, avoid as the plague the use of capturing groups in the decomposed regexes (the  qr{ ... } regexes); only use them in the matching regexes (e.g., the  m{ ... }), where it's a lot easier to keep track of them.

In reply to Re: Zero-width look-ahead regexp question by Anonymous Monk
in thread Zero-width look-ahead regexp question by rovf

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.