This is cool idea. Why can't we extend this even more and use regular string functions, so instead "$u2" to write "ToUpper($2)" ?
Modifiers of replace strings
So, replacement part of RegExp should be taken as "replacement expression" that takes $N (N >= 1) as an input
Finally, the performance of StringReplace has been improved a lot, as well as the overall performance of the := operator when long strings are involved.