Allow to update portions of values via processing rules

Have capability built into processing rules, similar to regular expressions, where we can specify a portion of a value and just update that.

For example... if a few pagenames passed in are:

a:x

a:y

a:z

If I want to update just the 1st character or all before the colon, etc to be a "b" and have just one rule render results:

b:x

b:y

b:z

that would be useful.