This tag library wrap's the ORO regexp engine. It provides two tags that can be used as follows:
<regexp:match
var="foo"
text="ID1234"
expr="[A-Z][A-Z][0-9]{4}"/>
<regexp:contains
var="bar"
text="Hello World"
expr="World"/>
The code is not written by me, but by Christian Amor Kvalheim, so give the cred to him (he was too sloppy to submit this himself).