Some handy regular expressions (and attendant PHP code) that I've come across in my travels.

Useful.Regexes.php

<?php
  //
  // some useful regular expressions and sample usage
  //

  // convert Twitter usernames into profile links
  // cred. Simon Granade (http://bit.ly/FihuS)
  $text = preg_replace('/(^|\s)@(\w+)/',
    '\1@<a href="http://twitter.com/\2">\2</a>', $text;

  // convert Twitter hashtags into search links
  // cred. as above
  $text = preg_replace('/(^|\s)#(\w+)/',
    '\1#<a href="http://search.twitter.com/search?q=%23\2">\2</a>', $text;

  // a slightly improved hashtag regex - won't highlight pure-numeric hashtags, e.g. #123
  // this seems to be the "correct" behaviour according to how Twitter does it
  // cred. http://bit.ly/fve6Rv (Stack Overflow question referring to this very Gist, ooh!)
  $text = preg_replace('/(^|\s)#(\w*[a-zA-Z_]+\w*)/',
    '\1#<a href="http://search.twitter.com/search?q=%23\2">\2</a>', $strTweet);


  // convert URLs into links
  // includes URLs without http:// prefix
  // cred. Allen Shaw + webmancers.com
  $text = preg_replace('/\b([a-zA-Z]+:\/\/[\w_.\-]+\.[a-zA-Z]{2,6}[\/\w\-~.?=&%#+$*!]*)\b/i',
    "<a href=\"$1\">$1</a>", $text);
  $text = preg_replace('/\b(?<!:\/\/)(www\.[\w_.\-]+\.[a-zA-Z]{2,6}[\/\w\-~.?=&%#+$*!]*)\b/i',
    "<a href=\"http://$1\">$1</a>", $text);

  // convert URLs into links, truncating long links (helps prevent wrapping fail)
  // includes URLs without http:// prefix
  // uses /e modifier to include functions in replacement - very handy
  // regex itself cred. Allen Shaw + webmancers.com
  $text = preg_replace('/\b([a-zA-Z]+:\/\/[\w_.\-]+\.[a-zA-Z]{2,6}[\/\w\-~.?=&%#+$*!]*)\b/ie',
    "\"<a href='\\1'>\".(strlen('\\1') > 32 ? substr('\\1', 0, 32).'&hellip;' : '\\1').\"</a>\"",
    $text);
  $text = preg_replace('/\b(?<!:\/\/)(www\.[\w_.\-]+\.[a-zA-Z]{2,6}[\/\w\-~.?=&%#+$*!]*)\b/ie',
    "\"<a href='http://\\1'>\".(strlen('\\1') > 32 ? substr('\\1', 0, 32).'&hellip;' : '\\1').\"</a>\"",
    $text);

  // match a reasonable selection of ASCII penises
  $text = preg_replace('/(8|B|\(\)\))(=+)(D)(~*)/', '', $text);

  // and for the ladies...
  $text = preg_replace('/(\()( *)(\.)( *)(\)\(|\) *\(|Y)( *)(\.)( *)(\))/', '', $text);