text mask filter

[automaton]

I've been looking at examples posted but I can't figure out how the XThreads text mask filter works for text entered.  It appears to be similar to the rewrite rules Apache use but I'm not sure.  Can anyone point me in the right direction to help me figure out how text mask filters work?

I'd like to take the text entered in a textbox and attach some text at the beginning and end of the entered text, i.e. say I ask someone to enter which site they use to get to the page they can enter 'google', then the text is filtered to show http://www.google.co.uk or something.

thanks.