bp_xprofile_filter_meta_query( string $q )

function bp_xprofile_filter_meta_query( $q ) {
	global $wpdb;

	$raw_q = $q;

	/*
	 * Replace quoted content with __QUOTE__ to avoid false positives.
	 * This regular expression will match nested quotes.
	 */
	$quoted_regex = "/'[^'\\\\]*(?:\\\\.[^'\\\\]*)*'/s";
	preg_match_all( $quoted_regex, $q, $quoted_matches );
	$q = preg_replace( $quoted_regex, '__QUOTE__', $q );

	// Get the first word of the command.
	preg_match( '/^(\S+)/', $q, $first_word_matches );

	if ( empty( $first_word_matches[0] ) ) {
		return $raw_q;
	}

	// Get the field type.
	preg_match( '/xprofile_(group|field|data)_id/', $q, $matches );

	if ( empty( $matches[0] ) || empty( $matches[1] ) ) {
		return $raw_q;
	}

	switch ( $first_word_matches[0] ) {

		/**
		 * SELECT:
		 * - replace 'xprofile_{fieldtype}_id' with 'object_id'
		 * - ensure that 'object_id' is aliased to 'xprofile_{fieldtype}_id',
		 *   because update_meta_cache() needs the column name to parse
		 *   the query results
		 * - append the 'object type' WHERE clause
		 */
		case 'SELECT' :
			$q = str_replace(
				array(
					$matches[0],
					'SELECT object_id',
					'WHERE ',
				),
				array(
					'object_id',
					'SELECT object_id AS ' . $matches[0],
					$wpdb->prepare( 'WHERE object_type = %s AND ', $matches[1] ),
				),
				$q
			);
			break;

		/**
		 * UPDATE and DELETE:
		 * - replace 'xprofile_{fieldtype}_id' with 'object_id'
		 * - append the 'object type' WHERE clause
		 */
		case 'UPDATE' :
		case 'DELETE' :
			$q = str_replace(
				array(
					$matches[0],
					'WHERE ',
				),
				array(
					'object_id',
					$wpdb->prepare( 'WHERE object_type = %s AND ', $matches[1] ),
				),
				$q
			);
			break;

		/**
		 * UPDATE and DELETE:
		 * - replace 'xprofile_{fieldtype}_id' with 'object_id'
		 * - ensure that the object_type field gets filled in
		 */
		case 'INSERT' :
			$q = str_replace(
				array(
					'`' . $matches[0] . '`',
					'VALUES (',
				),
				array(
					'`object_type`,`object_id`',
					$wpdb->prepare( "VALUES (%s,", $matches[1] ),
				),
				$q
			);
			break;
	}

	// Put quoted content back into the string.
	if ( ! empty( $quoted_matches[0] ) ) {
		for ( $i = 0; $i < count( $quoted_matches[0] ); $i++ ) {
			$quote_pos = strpos( $q, '__QUOTE__' );
			$q = substr_replace( $q, $quoted_matches[0][ $i ], $quote_pos, 9 );
		}
	}

	return $q;
}