This is a parser to replace UBB style tags with their html equivalents.
It does not simply do some regex calls, but is complete stack based
parse engine. This ensures that all tags are properly nested, if not,
extra tags are added to maintain the nesting. This parser should only
produce xhtml 1.0 compliant code. All tags are validated and so are all
their attributes. It should be easy to extend this parser with your own
tags.

WWW: http://pear.php.net/package/HTML_BBCodeParser/