Say you have a phone number in various formats but you just want the numeric characters and all the non-numeric characters removed. Well use the SQL function REGEXP_REPLACE and look for characters with the regular expression [^\d] and replace it with empty character ”. [^\d] is a negated set which means match any characters not in between [^ and ]. \d is a short hand way of saying all digit characters 0-9.
SELECT PHONE_NUMBER, REGEXP_REPLACE(PHONE_NUMBER,'[^\d]’,”)
IBM documentation on REGEXP_REPLACE
Online regex tester tool