I often have requests to comment out BLOCKS of text, such as "comment out everything between and including each cat/^cat block. Example: 
mouse
cat
1
2
3
^cat 
deer
eel
cat
furbaby
elk
^cat
dog
fish

[download]
the result I want is 
mouse
#cat
#1
#2
#3
#^cat 
deer
eel
#cat
#furbaby
#elk
#^cat
dog
fish

[download]
This can be in an array (one element per line) or a scalar. Usually I just loop thru the list and set a flag when hit cat, and unset it after leaving ^cat, and add the # if flagged. Which is like how I would do this in C. Every time I solve a problem in my brain in C, I always think to myself surely Perl has a better approach!.. Fugly code: 
 my $comment=0;
 for (@a) {
  $comment++ if /^cat\b/;
  $_ = '#'.$_ if $comment;
  $comment-- if /^\#\^cat\b/;
 }

[download]

Can you suggest a brief solution with a single no-looping regex, OR a map for an array? Or maybe a clever mapped split?

Much thanks and happy Day of the Earth-Rodent-Forecaster!

In reply to can u suggest a regex for this ? by misterperl

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.