This function replaces the characters out of from_string with the corresponding character out of to_string in the string, expr.
- The characters in expr which do not exist in from_string are not replaced.
- If from_string is longer than to_string, then the relevant characters are deleted and not replaced.
- If to_string is longer than from_string, the relevant characters are ignored during the replacement process.
- If one of the parameters is the empty string, then the result is NULL.