DECODE-like behavior needed for substitution

Question

Hi:
Problem: In the context of a single query, I need to transform a user specified wildcarded string to something standard to SQL. The user's idea of what wildcards are does not jive with SQL standards. In the mind of the users, '*' means "any string" (and that is easy enough to "translate" from '*' to '%'). But also underscore '_' means underscore, not "any single character", so that needs to be escaped. The problem is how to do that inside the context of a single SQL statement.
I believe I need to translate all occurances of '_' to something like '\_' after which I can "escape '\'". So, if the user inputs "ab_cd_ef*", it needs to become "ab\_cd\_cd%". I tried decode(str,'_','\_'), but that just comes up with a null a-la...
select decode('ab_cd','_','\_') from dual;
It sems to need to match the entire string 'ab_cd' in order for a translation to occur. Maybe I'm not using decode correctly or don't fully understand it.
"translate" seems to be good at substituting single chars with other single chars (like'*' becomes '%'), but I can't see how it can substitute a single '_' to the 2 chars '\_'.
Another viable solution would be to override how sql interprets wildcards. If I could tell the SQL interpreter to swap '*' and '%', and ignore '_' as being a wildcard alltogether, that would do the trick too.
Any idesa?
Thanks

Answer 1

try
SELECT REPLACE('ab_cd','_','\_') FROM dual;