Removing everything between nested parentheses

Question

For removing everything between parentheses, currently i use:

SELECT 
REGEXP_REPLACE('(aaa) bbb (ccc (ddd) / eee)', "\\([^()]*\\)", "");

Which is incorrect, because it gives bbb (ccc / eee), as that removes inner parentheses only.

How to remove everynting between nested parentheses? so expected result from this example is bbb

Regular expressions are generally not able to match nested expressions. — Barmar, Oct 21 '22 at 20:56

holem · Answer 1 · 2022-10-22T17:38:33.130

1

In case of Google BigQuery, this is only possible if you know your maximum number of nestings. Because it uses re2 library that doesn't support regex recursions.

let r = /\((?:(?:\((?:[^()])*\))|(?:[^()]))*\)/g
let s = "(aaa) bbb (ccc (ddd) / eee)"
console.log(s.replace(r, ""))

edited Oct 22 '22 at 17:38

answered Oct 21 '22 at 21:21

holem

105
1
10

2

In that case you need to use regex recursion [like this](https://stackoverflow.com/a/19863847/8312809) but Google BigQuery uses re2 library that [doesn't support recursions](https://github.com/google/re2/wiki/Syntax) – holem Oct 21 '22 at 21:47

score 0 · Answer 2 · answered Oct 23 '22 at 16:04

If you can iterate on the regular expression operation until you reach a fixed point you can do it like this:

repeat {
  old_string = string
  string := remove_non_nested_parens_using_regex(string)
} until (string == old_string)

For instance if we have

((a(b)) (c))x)

on the first iteration we remove (b) and (c): sequences which begin with (, end with ) and do not contain parentheses, matched by \([^()]*\). We end up with:

((a) )x)

Then on the next iteration, (a) is gone:

( )x)

and after one more iteration, ( ) is gone:

x)

when we try removing more parentheses, there is no more change, and so the algorithm terminates with x).

Removing everything between nested parentheses

2 Answers2