That's the `\\ `.  A space being escaped, but the escape slash needs to be escaped itself because it's a php string. `\s` Does provide additional coverage for tabs and newlines, so I'm happy to make the change.

I assumed that the escape slash in `\s` would need to be escaped, leaving `\\s`, but that does not appear to be the case. Both `\s` and `\\s` look to come out to be the same.

    $str = "abc 123";
    var_dump(
        (bool)preg_match('/[\s]/', $str),
        (bool)preg_match('/[\\s]/', $str),
        (bool)preg_match('/[\\\\s]/', $str),
        '\s',
        '\\s',
        '\\\\s'
    );

output

    bool(true)
    bool(true)
    bool(false)  # expected
    string(2) "\s" # ok
    string(2) "\s" # also ok
    string(3) "\\s"