Is there a way to capitalize a string using batch rename regex in Bridge?

Has anyone had success using more advanced pieces of regex to properly rename strings? I want to Capitalize the 3rd back reference in a file name, and although I believe I'm following the proper way of doing so via Replacement Text Case Conversion, Bridge is just giving me a capital U instead. Please see what I am doing below. Additionally, does anyone know what flavor regex Bridge uses? Thanks!