Regex subpatterns - capturing and not capturing

Q: Could someone explain the difference between anumbered and unnumbered subpatterns? (?: subpattern) vs. (subpattern)

A: One is numbered (which means you can access it later) and one is not (which means you cannot access it later).

Reference

• http://forums.devshed.com/regex-programming-147/regular-expression-subpatterns-809343.html
• http://php.net/manual/en/regexp.reference.subpatterns.php
• http://stackoverflow.com/questions/3044026/php-regular-expression-subpattern-behaviour