Passing regex match to a script function

[MHolmes]

In a Replace Files step, I need to pass the result of a regex match. Basically, I need to match a filename in a file using the following regex:

^.*"OutputFilename"\s*=\s*"(.*)".*$

I capture the complete path specified, but I want to get JUST the filename part (so if it's C:\My Files\somefile.txt, I want somefile.txt) so that I can mutate the final output path at build time. I have a project level function called GetFileNamePart which uses the FSO to return the filename, but I can't call it as:

[GetFileNamePart($1)]

In the replace text. I know that regexp normally uses backreferences for that, but how can I get a match and pass it to a script function?