Implement toupper and tolower (or general char translation?) in regsub

[dirk-zimoch]

Mybe use PowerGrep syntax: \U1 = toupper(\1), \L1 (tolower), \F1 (first char upper rest lower) \I1 (first char of each word upper rest lower)
Also allow more than 9 sub-expressions. \{4}2 vs \42 vs to substitute 4th match followed by 2 instead of 42nd match.
Separate translate format? Example %|[a-z]|[A-Z]|

[dirk-zimoch]

\Un \Ln \un \ln with n=0...9 for toupper/lower whole subexpression/first letter implemented in commit ed30011.