Improve regex backref replace casts that need to add a postfix digit

[jlowe]

rapidsai/cudf#8841 allows us to specify a boundary around replacement groups so we can follow a group by a digit (e.g.: 0). This should allow us to simplify some of the regex replaces used for timestamp casting.

[jlowe]

@andygrove FYI, not high priority, but it may be useful if you run across this use case again in some new code.

[jlowe]

I don't think this applies to the current timestamp casting code. Even if it does, if we want to get serious about timestamp casting performance, we need to move away from regex searching.