$re = '~\[table]\R*([^[]*(?:\[(?!/?table])[^[]*)*)\R*\[/table]~';
$str = '
[b]Check out this demo[/b]
¯\\_(ツ)_/¯
[table]
[**]header1[||]header2[||]header3[||]...[/**]
[*]child1.1[|]child1.2[|]child1.3[|]...
[*]child2.1[|]child2.2[|]child2.3[|]...
[*]child3.1[|]child3.2[|]child3.3[|]...
[*]...[|]...[|]...[|]...
[/table]
simple text
[table]
[**]a 1[||]and a 2[/**]
[*]A[|]B
[*]C[|]D
[/table]
[s]3, you\'re out[/s]
blah';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php