Remove text outside of [] {} () bracket

Question

I have a string, from which I want to keep text inside a pair of brackets and remove everything outside of the brackets:

Hello [123] {45} world (67)
Hello There (8) [9] {0}

Desired output:
[123] {45} (67) (8) [9] {0}

Code tried but fails:

$re = '/[^()]*+(\((?:[^()]++|(?1))*\))[^()]*+/';
$text = preg_replace($re, '$1', $text);

Answer 1

If the values in the string are always an opening bracket paired up with a closing bracket and no nested parts, you can match all the bracket pairs which you want to keep, and match all other character except the brackets that you want to remove.

(?:\[[^][]*]|\([^()]*\)|{[^{}]*})(*SKIP)(*F)|[^][(){}]+

Explanation

• (?: Non capture gorup
  • \[[^][]*] Match from [...]
  • | Or
  • \([^()]*\) Match from (...)
  • | Or
  • {[^{}]*} Match from {...}
• ) Close non capture group
• (*SKIP)(*F)| consume characters that you want to avoid, and that must not be a part of the match result
• [^][(){}]+ Match 1+ times any char other than 1 of the listed

Regex demo | Php demo

Example code

$re = '/(?:\[[^][]*]|\([^()]*\)|{[^{}]*})(*SKIP)(*F)|[^][(){}]+/m';
$str = 'Hello [123] {45} world (67)
Hello There (8) [9] {0}';

$result = preg_replace($re, '', $str);

echo $result;

Output

[123]{45}(67)(8)[9]{0}

If you want to remove all other values:

(?:\[[^][]*]|\([^()]*\)|{[^{}]*})(*SKIP)(*F)|.

Regex demo