Description:
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preg_split(pattern, subject) will fail if I use a pattern with extended (/x) modifier with escaped sequences (like \r or \n), but if I double escape it, it'll work. Seems a bug for me.

If I do: /\r?\n/    over "abc\r\n123" it will split to ["abc", "123"].
If I do: /\r?\n/x   over "abc\r\n123" it will will fail¹.
If I do: /\\r?\\n/x over "abc\r\n123" it will split to ["abc", "123"].

If you repair, I need to escape twice when I use /x modifier. It doesn't occur if I use preg_match(pattern, subject).

PHP 4.x it'll works correctly.

¹ Compilation failed: nothing to repeat at offset 1.

Test script:
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$split_normal             = "/\r?\n/";
$split_extended_escaped   = "/\r?\n/x";
$split_extended_reescaped = "/\\r?\\n/x";
$split_text               = "abc\r\n123";

var_dump(preg_split($split_normal,             $split_text)); // OK
var_dump(preg_split($split_extended_escaped,   $split_text)); // Wrongly FAIL
var_dump(preg_split($split_extended_reescaped, $split_text)); // Wrongly OK