Xml namespaces replacing

The regexp is not working properly, it edits the content of posts too,
removing all the colons. As far as I've tested (not much, being honest)
it's not an easy task to do with regular expressions (multiple tags AND
attributes inside tags), probably needing to use the expat extension
(http://php.net/book.xml.php).

The easy way: just doing some string replacements.