It is a pretty crappy example but what it does is include some code in the match regex which gets executed. Basically we try for a match with (pseudocode)m/.{0,length $_}lori/ and use $i to remember how many chars we needed to eat up to find "lori". There are far easier ways to do this like using pos() and a positive lookahead assertion if you want a regex solution or just plain old index(): 

$_ = "lothlorien";

m/ (?{ $i = 0 })
   (.  (?{local $i = $i + 1; })  )*  
   lori
   (?{ $result = $i })
/xi;

print "\$result=$result \$i=$i\n";

# this provides the same answer but is IMHO much easier to understand
m/(.*)(?=lori)/ig;
print "pos=", pos;

# this is probably the best solution
print "index=", index($_, 'lori');

[download]

You can't localise and use $i++ in the same statement.

Update

Fixed technical inexactitude thanks jryan

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

In reply to Re: reg-ex frustration by tachyon
in thread reg-ex frustration by stratkid

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.