REGEXP_REPLACE has a limitation in handling ASCII Chr(0), which is NULL. If you concatenate two strings using ASCII Chr(0), REGEXP_REPLACE cannot handle that input string. For example, consider the following code:
sel TD_SYSFNLIB.RegExp_Replace ('a'||chr(0)||'bc', '[b]', 'X', 1, 0) AS
regex_rep_input_string;
The function is supposed to replace the b in the input string "'a'||chr(0)||'bc'" with X. The result must be a Xc. However, because of the limitation, the result is a.