Regular-Expressions.info - Regex Tutorial, Examples and Reference - Regexp Patterns
Regular Expressions Cheat Sheet - Cheat Sheets - ILoveJackDaniels.com

and finally,

preg_match()
preg_replace()

For "mytext", if that was the literal string, you could

echo preg_replace("~(mytext)~", "newtext", $szString);

...for a real basic example.
-m