By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
SELECT REGEXP_REPLACE("[highlight]987[/highlight]", "(^|\\[\\/highlight\\]).*?(\\[highlight\\]|$)", "\\1")
| REGEXP_REPLACE("[highlight]987[/highlight]", "(^|\[\/highlight\]).*?(\[highlight\]|$)", "\1") |
|---|
| 987[/highlight] |