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<?php preg_match_all('~[\\\\a-z0-9_]~', 'a\\\\b', $matches); print_r($matches); preg_match_all('~[a-z0-9_\\\\]~', 'a\\\\b', $matches); print_r($matches); preg_match_all('~[\\d-z]~', 'a\\\\b', $matches); print_r($matches); preg_match_all('~[\\\\d-z]~', 'a\\\\b', $matches); print_r($matches);
Output for 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.27, 8.2.0 - 8.2.17, 8.3.0 - 8.3.4
Array ( [0] => Array ( [0] => a [1] => \ [2] => \ [3] => b ) ) Array ( [0] => Array ( [0] => a [1] => \ [2] => \ [3] => b ) ) Warning: preg_match_all(): Compilation failed: invalid range in character class at offset 3 in /in/hBE88 on line 8 Array ( [0] => Array ( [0] => a [1] => \ [2] => \ [3] => b ) ) Array ( [0] => Array ( [0] => \ [1] => \ ) )
Output for 7.1.25 - 7.1.30, 7.2.0 - 7.2.33
Array ( [0] => Array ( [0] => a [1] => \ [2] => \ [3] => b ) ) Array ( [0] => Array ( [0] => a [1] => \ [2] => \ [3] => b ) ) Array ( [0] => Array ( ) ) Array ( [0] => Array ( [0] => \ [1] => \ ) )

preferences:
157.2 ms | 402 KiB | 164 Q