Complex substr issue.

Question

'abcd def$ff>def$nsfdef$'
i have above string and need to replace 'def' with some string but only those occurance where space is not before it ..
output :-
'abcd def$ff>xyz$nsfdef$'

Answer 1

select regexp_replace('abcd def$ff>def$nsfdef$','([^ ])(def)','\1XXX') str
from dual;
abcd def$ff>XXX$nsfXXX$And if you need to replace only the first occurance (As your sample output says) use the below
select regexp_replace('abcd def$ff>def$nsfdef$','([^ ])(def)','\1XXX',1,1) str
from dual;
abcd def$ff>XXX$nsfdef$And without using regular expressions
--Assuming "~" will not occur in your string.
select replace(replace(replace('abcd def$ff>def$nsfdef$',' def',' ~~~'),'def','XXX'),'~~~','def') str
from dual;
abcd def$ff>XXX$nsfXXX$