That filter will include messages that match your regular expression, but it will not modify the message prior to displaying it.
If you’re familiar with vbscript, there is an example script in the scripts folder that you can modify and use (it replaces the word “cat” with “dog).
What you would want to do is use MID and InStr to substring everything after the : .
I’m don’t have my kiwi server handy at the moment or I’d substring it for you, but essentially replace the entire message text with your substring and there you go. If you’re not comfortable messing with vbscript, let me know and I will write up a quick script for you.