_wp_can_use_pcre_u() │ WP 4.2.2

Returns whether PCRE/u (PCRE_UTF8 modifier) is available for use.

Internal function — this function is designed to be used by the kernel itself. It is not recommended to use this function in your code.

No Hooks.

Returns

null. Nothing (null).

Usage

_wp_can_use_pcre_u( $set );
$set(true|false)
Deprecated. This argument is no longer used for testing purposes.
Default: null

Changelog

Since 4.2.2 Introduced.
Since 6.9.0 Deprecated the $set argument.

_wp_can_use_pcre_u() code WP 7.0

wp-includes/compat.php 
function _wp_can_use_pcre_u( $set = null ) {
	static $utf8_pcre = null;

	if ( isset( $set ) ) {
		_deprecated_argument( __FUNCTION__, '6.9.0' );
	}

	if ( isset( $utf8_pcre ) ) {
		return $utf8_pcre;
	}

	$utf8_pcre = true;
	set_error_handler(
		function ( $errno, $errstr ) use ( &$utf8_pcre ) {
			if ( str_starts_with( $errstr, 'preg_match():' ) ) {
				$utf8_pcre = false;
				return true;
			}

			return false;
		},
		E_WARNING
	);

	/*
	 * Attempt to compile a PCRE pattern with the PCRE_UTF8 flag. For
	 * systems lacking Unicode support this will trigger a warning
	 * during compilation, which the error handler will intercept.
	 */
	preg_match( '//u', '' );
	restore_error_handler();

	return $utf8_pcre;
}