Make that "or else the Regex engine may consider it an array". This is actually one of the more magical parts of Perl parsing. Consider: 

#!/usr/bin/perl -w
use strict;

my $m= 'aeiou';
my @m= qw( a e i o u );
my $at= 'tye@perlmonks';
my $dollar= 'tye$perlmons';
sub Try {
    local( $" )= ",";
    print "(@_)\n";
}
Try $at =~ /[@m]+/g;
Try $at =~ /[m@]+/g;
Try $dollar =~ /[$m]+/g;
Try $dollar =~ /[m$]+/g;
__END__
Outputs:
(e,e,o)
(@,m)
(e,e,o)
/[m5.006+/: unmatched [] in regexp at reinterp.pl line 12.

[download]

The "rules" that control this defy simple explanation so I'll just refer you to the source code. Simply search for "weight" in the Perl source code (it is only ever mentioned in the code for this parsing which is located in the file toke.c).

        - tye (but my friends call me "Tye")

In reply to (tye)Re: Expression matching by tye
in thread Expression matching by amearse

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.