xsd reg expression for wildcard

Please can anyone tell me the regular expression to use within my XSD
to specify that an element value must always begin with STRING_ plus
any number of other alphanumeric characters. E.g. of valid entries:
STRING_dfafaa3423
STRING_12
STRING_fsdklljlasd12454

I have trawled the newsgroups and web but can't find the answer to
what should be a simple question.

Any advice would be appreciated.
Redge.

Red Ogden

Many thanks Colin!