WordPress REGEXP for meta_query key

functions.php

function meta_query_key_compare( $query ) {

	$meta_query = $query->get('meta_query');

	if(empty($meta_query)) {
		return;
	}

	$marker = '__tmp_marker__';
	$rx = [];

	foreach($meta_query as $key => $meta_query_part) {
		if(isset($meta_query_part['key_compare']) && in_array( strtoupper($meta_query_part['key_compare']), ['REGEXP', 'RLIKE', 'LIKE']) && isset($meta_query_part['key'])) {
			$meta_query_part['key'] .= $marker . $key; //so we can find it later
			$query->query_vars['meta_query'][$key]['key'] = $meta_query_part['key'];
			$rx[$key] = $meta_query_part;
		}
	}

	foreach($meta_query as $key => $meta_query_part) {
		foreach ($meta_query_part as $key_i => $meta_i) {
			if (count($meta_i) >= 3) {
				if (isset($meta_i['key_compare']) && in_array( strtoupper($meta_i['key_compare']), [ 'REGEXP', 'RLIKE', 'LIKE']) && isset($meta_i['key'])) {
					$meta_i['key'] .= $marker . $key_i;
					$query->query_vars['meta_query'][$key][$key_i]['key'] = $meta_i['key'];
					$rx[$key][$key_i] = $meta_i;
				}
			}
		}
	}

	if(empty($rx)) {
		return;
	}


	add_filter('get_meta_sql', function($sql) use ($rx, $marker) {
		static $nr = 0;
		if(0 != $nr++) {
			return $sql;
		}

		foreach($rx as $key => $meta_query_part) { //replace the markers
			if( isset($meta_query_part['key'])) :
			$sql['where'] = str_replace(
				sprintf(
					".meta_key = '%s' ",
					$meta_query_part['key']
				),
				sprintf(
					".meta_key %s '%s' ",
					$meta_query_part['key_compare'],
					str_replace(
						$marker.$key,
						'',
						$meta_query_part['key']
					)
				), $sql['where']
			);
			endif;
		}

		foreach ($rx as $key => $meta_query_part) {
			if (!isset($meta_query_part['key'])) {
				foreach ($meta_query_part as $key_i => $part_i) {
					$sql['where'] = str_replace(
							sprintf(
									".meta_key = '%s' ", $part_i['key']
							), sprintf(
									".meta_key %s '%s' ", $part_i['key_compare'], str_replace(
											$marker . $key_i, '', $part_i['key']
									)
							), $sql['where']
					);
				}
			}
		}


		return $sql;
	});
}
add_action('pre_get_posts', 'meta_query_key_compare', 999);