--TEST-- preg_replace() --FILE-- <?php var_dump(preg_replace('{{\D+}}', 'x', '{abcd}')); var_dump(preg_replace('{{\D+}}', 'ddd', 'abcd')); var_dump(preg_replace('/(ab)(c)(d)(e)(f)(g)(h)(i)(j)(k)/', 'a${1}2$103', 'zabcdefghijkl')); var_dump(preg_replace_callback('//e', '', '')); var_dump(preg_replace_callback('//e', 'strtolower', '')); ?> --EXPECTF-- string(1) "x" string(4) "abcd" string(8) "zaab2k3l" Warning: preg_replace_callback(): Requires argument 2, '', to be a valid callback in %spreg_replace.php on line 8 string(0) "" Warning: preg_replace_callback(): Modifier /e cannot be used with replacement callback in %spreg_replace.php on line 10 NULL