how to match a string?

Experts,

I am trying to match a string using match_regex. For example if the input field is ABCDRTF, I want match_regex to match 'RT' from it.

I have written code as match_regex ('ABCDRTF','RT',null) and it is not working. However it works when RT is at the start. How to make it search the whole string?

Thanks

Pramod