When you match a regex against a string, you're essentially asking the question "is there any position in the string at which the pattern matches?" The regex engine (RE) is looking for any match, and normally will be satisfied by the first match (it's possible to force the RE to go on looking). The rule is "the leftmost, longest match", and "leftmost" trumps "longest." 

c:\@Work\Perl\monks>perl -wMstrict -le
"for my $f ('xyz.pl', '.pl', 'abc.x', 'abc') {
   printf qq{%10s matches at position $-[0] \n}, qq{'$f'}
     if $f =~ m{ (?! [.]pl \z) }xms;
   }
"
  'xyz.pl' matches at position 0
     '.pl' matches at position 1
   'abc.x' matches at position 0
     'abc' matches at position 0

[download]

In most of these examples, a match can be found at the very start of the string, the first position "not followed by .pl and the end of the string". Even in the case of  '.pl' a match can be found, even though the RE has to go all the way to the second character position to find it.

In reply to Re: Regex negative lookahead by AnomalousMonk
in thread Regex negative lookahead by aeqr

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.