I don't think anyone has pointed out something which I think Sulferic was confused about, that the \1, \2 etc can overlap when there are brackets inside brackets.

Here's an example:

$string = 'my name is mister smith';

if($string =~ /(mr\.|mister (\w+))/){
  #  if the string contains either 'mr.' or
  #  'mister', then a space, then some word chars
  #  matching for instance 'mister jones' but not
  #  'hey mister!'
  print "surname is $2";
  #  $1 contains the whole thing, $2 just the surname
}
[download]

I came up with this one for an auto-html syntax thing.

Paste the URL and put optional link text after it in brackets.

$text='Learning Perl? http://www.perlmonks.org/ <this> is the place';

  $text =~ s!(http://\S+)
                      (\s+<([^>]+)>)?
                     !
                      defined($3)?
                        qq(<a href="$1">$3</a>):
                        qq(<a href="$1">$1</a>)
                     !gex;
print $text;
[download]

---

($_='kkvvttuubbooppuuiiffssqqffssmmiibbddllffss')
 =~y~b-v~a-z~s;                             print
[download]