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with t as (
select 'AA/BB' myval from dual
union all select 'AA/BB1' from dual
union all select 'AA/BB-1' from dual
union all select 'BB' from dual
union all select 'BB' from dual
union all select 'BB1' from dual
union all select 'BB-1' from dual
union all select 'BB/BB' from dual
)
select myval, regexp_replace(myval, 'BB($|[^/])', 'XX\1') newval from t
| MYVAL | NEWVAL |
|---|---|
| AA/BB | AA/XX |
| AA/BB1 | AA/XX1 |
| AA/BB-1 | AA/XX-1 |
| BB | XX |
| BB | XX |
| BB1 | XX1 |
| BB-1 | XX-1 |
| BB/BB | BB/XX |