Alphanumeric character string check function?

I’m trying to confirm a string field contains only A-Z a-z and 0-9 characters. I’ve found the replace function but it seems to only replace a single string to another string within the function (e.g. REPLACE(FIELD1,‘a’,’’) ). My plan was to remove all alphanumeric characters and then trim and confirm the field = ‘’. There must be a better way? Also I don’t want to next the replace function 26+26+10 (26+10 if I upcase first) times in order to remove all the characters I’d like to remove.

Thank you!