Personally I have no need to use backreferences in character classes. But I saw the following code: 
sub eraseCommet {
  my($all, $comment) = @_;
  return $all  if !$comment;
}
s/(<(\/)?((!--)|(script)|(style)|\w+)(?(4).*?-->|(\s+\w+(?:\s*=\s*(["'
+])?(?(8)[^\8]+?\8|\S+?(?=[>\s])))?)*?\s*\/?>(?(5).*?<\/script>|(?(6).
+*?<\/style>))))/eraseCommet($1,$4)/gixse;

[download]
That weird regex (I don't like it) was written by one person who want to eliminate comments from HTML. He knew about HTML::Parser, but he wanted to make it with regexps for his fun. I was trying to find a valid HTML where that code fails and I noted that he used \8 as a backreference within a character class. I knew that one can use variables [$var] but using of [^\8] appeared alarming for me. Such way I obtained that probably undocumented behavior.

In reply to Re^2: \1, \2, \3, ... inside of a character class by ccn
in thread \1, \2, \3, ... inside of a character class by ccn

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.