Oracle Constraint Letters Only SQL REGEXP - sql

Question

I already have my tables full of data, I am trying to add a constraint to only allow letters to be entered into the attribute I have named as studentfname. I have tried various different REGEXP but cannot seem to get any to work without getting an error when inserting them.
Im not sure if its a syntax error or what!
Edit: Here is what I have tried and the error I am getting:
ALTER TABLE STUDENT
ADD CONSTRAINT
check_name
CHECK(regexp_like(studentfname,'^[A-Za-z''-]+$'));
Error:
QL Error: ORA-02293: cannot validate (JEIGH7.CHECK_NAME) - check constraint violated
02293. 00000 - "cannot validate (%s.%s) - check constraint violated"
*Cause: an alter table operation tried to validate a check constraint to
populated table that had nocomplying values.
*Action: Obvious

Answer 1

Apparently you can try to run this below query to check if any non integral data resides in the above mentioned column which is preventing you from applying a check constraints. Once you get the output from this query try UPDATING/DELETING the row and then apply the constraint. Hope this helps.
SELECT studentfname nm FROM STUDENT a
WHERE NOT regexp_like(a.nm,'^[A-Za-z''-]+$');