Description:
------------
When matching in UTF8 mode ('u' flag), the `preg_match` function checks the validity of the entire input string on every call.

PCRE2 has a flag to disable this behavior: `PCRE2_NO_UTF_CHECK`, but PHP does not expose it.

This means that when matching a lot of times against the same (potentially long) input string (for example for lexical analysis), a lot of unnecessary computations are performed, leading to catastrophic performance (`O(n²)` instead of expected `O(n)`).





Test script:
---------------
// see online at https://3v4l.org/Duv7g
$input_size = 1e4;
$str = str_repeat('a', $input_size);

$start = microtime(true);
$pos = 0;
while(preg_match('/\G\w/', $str, $m, 0, $pos)) ++$pos;
$end = microtime(true);

echo '>>> NO u flag: ', number_format(($end - $start)*1000, 6), 'ms', PHP_EOL;

$str = str_repeat('e', $input_size);

$start = microtime(true);
$pos = 0;
while(preg_match('/\G\w/u', $str, $m, 0, $pos)) ++$pos;
$end = microtime(true);

echo '>>> WITH u flag: ', number_format(($end - $start)*1000, 6), 'ms', PHP_EOL;

Expected result:
----------------
I expect the two loops to take roughly the same amount of time, with the second one being slower by a very short margin.
The performance should be `O(n)` for the two.

Actual result:
--------------
With $input_size === 1e4:
>>> NO u flag:     9.632111ms
>>> WITH u flag: 109.670877ms

With $input_size === 1e5:
>>> NO u flag:       96.043110ms
>>> WITH u flag: 10,151.215076ms

With $input_size === 2*1e5:
>>> NO u flag:      188.354015ms
>>> WITH u flag: 40,387.295008ms

Looks like we have the expected `O(n)` in first case,
and `O(n²)` with the /u flag.