SAML tranformation using regexreplace not working as expected

Question

SAML tranformation using regexreplace not working as expected
have usernames "user-123" that need to be "user_123" because of the application limitations.

SAML Attribute transformations on NameID
user attribute used: onpremisesamaccountname.
transformation1: Touppercase
transformation2: RegexReplace
----> match pattern '-'
----> replacement pattern '_'

i would expect the tranformation to ignore any string that doesnt contain '-' but instead i think it errors? as it just sends a guid for that SAML attribute value

can someone explain how this fuction is designed to work?
additionally there is not a way to run a transformation on the value if the "specify output if no match"
if my app cap sensitive i need this as well.