I came up with the following:
use warnings;
use strict;

while (<DATA>) {

   s|^(?:<bl>)?(.*?)(?:</bl>)?$|<listitem>$1</listitem>|;
   print
}
__DATA__
<bl>List item 1
List item 2
<bl>List item a
List item b
List item c</bl>
List item 3</bl>

[download]
It does not produce exactly the intended output, but:
<listitem>List item 1</listitem>
<listitem>List item 2</listitem>
<listitem>List item a</listitem>
<listitem>List item b</listitem>
<listitem>List item c</listitem>
<listitem>List item 3</listitem>

[download]
Which, to me at least, looks more likely to be useful.

In reply to Re: Help in using Regular Expressions by cdarke
in thread Help in using Regular Expressions by rsriram

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.