This bit can be tricky -- when you are using a KM prompt to set a variable to make a regex to paste into an app you get some funky text processing flows. See this post from @peternlewis for a better explanation than I could ever manage!
Given that, and the fact you'll be filling in a dialog whatever you do, I'd echo @drdrang and start by automating everything but search/replace string insertion. You could then deep-dive into @peternlewis's token processing explanation if you found you wanted to take things further.