SQL & PL/SQL

I need to search in all the triggers which ones contain in their own body the string MYSTRING.
Unlike the USER_SOURCE table for procedures and packages where it's possible to execute a

select distinct name from user_source where upper(text) like '%MYSTRING%';

how can I do the same for trigger where the trigger body is in a LONG type? I can't execute a upper(trigger_body) like '%MYSTRING%'; on USER_TRIGGERS, so how can I do?

Thanks!