Hi there @Jeff,
to escape backslash character in regex function you need 4 backslashes! So something like this should work:
regexReplace($column1$,"c:\\\\MyDir\\\\(Dir1|Dir2|Dir3)\\\\Save\\\\","")
More about that here.
Also you can use following syntax in String Manipulation node to obtain file name together with extension:
substr($column1$,indexOf($column1$,"\\","b") + 1)
Basically it says get me everything after last backslash.
Hope this helps!
Br,
Ivan