I have tried some basic regex but all I seem to be able to do is remove "thispart" and end up with the stuff I don't want.

If you show us the regex that removes "thispart", there are probably lots of Monks here who would be able to teach you how to edit it to make it keep just "thispart". And, as BrowserUK said, supplying a small amount of actual data will help the Monks customize their responses to your actual input data, rather than the alpha-only data you've shown, along with what you expect the output to be given that data.

As a hint while you're reading about perlre: if you create a matching group in your regex, the magic $1 variable will hold the contents of the first matching group; you can use that with a m// to just extract your ID (assuming the "thispart" is really a partial regex that matches what you're looking for), or you could use it with s/// to remove either what matches or remove what doesn't match.