#pattern fails for Unicode points

Let's say you want to include some Unicode character points in your pattern.

$form['my_element'] = array(
  '#title' => t('My cool field'),
  '#type' => 'text',
  '#pattern' => '[\u00C0-\u00D6]',
);

If you go use the form and type in "ÇÁ" and submit the form. The JS validation passes so the submission goes through but then on the server side, a PHP warning gets raised from FormElement::validatePattern() and the validation fails there.

preg_match(): Compilation failed: PCRE does not support \L, \l, \N{name}, \U, or \u at offset NN in Drupal\Core\Render\Element\FormElement::validatePattern() (line 143 of core/lib/Drupal/Core/Render/Element/FormElement.php).

See #4:
\Drupal\Core\Render\Element\FormElement::validatePattern() seems to indicate that it should be okay:

// The pattern must match the entire string and should have the same
// behavior as the RegExp object in ECMA 262.
Both ECMA 262 and PCRE allow you craft a regex to target specific Unicode items, but they differ (\uhhhh and \x{hhhh} respectively), so if you use a #pattern that passes for one of them, it will fail for the other.