How to insert text into existing tags?

I have a number of tracks where the ALBUM tag is in the format

CD 01 The Beach Boys CD 02 The Beatles CD 03 Ray Charles
and so on. How can I use Actions to make MP3Tag insert, say, a colon character after the CD number, so the tags instead read
CD 01: The Beach Boys CD 02: The Beatles CD 03: Ray Charles
Thanks.

Create an action of the type "Replace with regular expression" for ALBUM
Search string: CD (\d+) (.*)
Replace String: CD $1: $2)