Tanktalus provided an excellent decription of how the regular expression attempted to match your string. If you would like to experiment with the regex some more I recommend using YAPE::Regex::Explain to get english descriptions of how a given regular expression works. It's much easier to read than the output of use re 'debug' 

use strict;
use warnings;
use YAPE::Regex::Explain;

my $regex = qr/foo       # Match starting at foo
 (?:       # Complex expression:
   (?!baz) #   make sure we're not at the beginning of baz
   .       #   accept any character
 )*        # any number of times
 bar       # and ending at bar
/x;


my $explanation = YAPE::Regex::Explain->new($regex)->explain;
print $explanation;

[download]
Outputs: 
The regular expression:

(?x-ims:foo        # Match starting at foo
 (?:        # Complex expression:
   (?!baz)  # make sure we're not at the beginning of baz
   .        # accept any character
 )*         # any number of times
 bar        # and ending at bar
)

matches as follows:
  
NODE                     EXPLANATION
----------------------------------------------------------------------
(?x-ims:                 group, but do not capture (disregarding
                         whitespace and comments) (case-sensitive)
                         (with ^ and $ matching normally) (with . not
                         matching \n):
----------------------------------------------------------------------
  foo                      'foo'
----------------------------------------------------------------------
  (?:                      group, but do not capture (0 or more times
                           (matching the most amount possible)):
----------------------------------------------------------------------
    (?!                      look ahead to see if there is not:
----------------------------------------------------------------------
      baz                      'baz'
----------------------------------------------------------------------
    )                        end of look-ahead
----------------------------------------------------------------------
    .                        any character except \n
----------------------------------------------------------------------
  )*                       end of grouping
----------------------------------------------------------------------
  bar                      'bar'
----------------------------------------------------------------------
)                        end of grouping
----------------------------------------------------------------------

[download]

In reply to Re: Regex look ahead by imp
in thread Regex look ahead by pglenski

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.