Las expresiones regulares son (ver: wikipedia, ).
http://www.regular-expressions.info/
http://regexlib.com/
http://www.amk.ca/python/howto/regex/
http://weblogtoolscollection.com/regex/regex.php
http://www.php.net/manual/en/book.pcre.php
http://www.php.net/manual/en/book.regex.php
http://www.php.net/manual/en/reference.pcre.pattern.differences.php
