--TEST-- X (PCRE_EXTRA) modififer --FILE-- <?php var_dump(preg_match('/\y/', '\y')); var_dump(preg_match('/\y/X', '\y')); ?> --EXPECTF-- int(1) Warning: preg_match(): Compilation failed: unrecognized character follows \ at offset 1 in %spcre_extra.php on line 4 bool(false)