f2

F2 is a cross-platform tool for bulk renaming files and directories on Linux, macOS, and Windows. It simplifies the renaming process by offering powerful workflows while prioritizing safety to prevent data loss (such as overwriting files by accident).

lowercase, uppwercase, or titlecase files

f2 -r '{.lw}'

there is also {.up} and {.ti} for UPPERCASE and Title Case

zero-pad single-digit numbers

convert (1).txt, (2).txt into (01).txt, (02).txt

f2 -f '\((\d)\)\.txt' -r '({$1%02d}).txt'