sanitize string wih gsub / pattern matching

pie

Hi, I think it's a stupid question but I can't wrap my head around this: I guess I am missing something trivial.

I need to sanitize two user inserted strings:

sanitize A should keep every alphanumeric character, spaces and punctuation

sanitize B should keep only lowercase a-z: no spaces, no punctuation, no numbers

I wonder why my gsub never works:
this should be my implementation of sanitize B, where the output in my dreams should be "elephant"

local testring = "134àelephantù#.!$ 4" --string.gsub(testring,"[^a-z]", "") testring:gsub("[^a-z]", "") print("TEST STRING", testring)

thank you

rrraptor

gsub returns new string

testring = testring:gsub("[^a-z]", "")