add validation for regex flags

Author: miya0001

src/DB_Command.php

@@ -820,6 +820,10 @@ public function search( $args, $assoc_args ) {
 
 		if ( ( $regex = \WP_CLI\Utils\get_flag_value( $assoc_args, 'regex', false ) ) ) {
 			$regex_flags = \WP_CLI\Utils\get_flag_value( $assoc_args, 'regex-flags', false );
+			// http://php.net/manual/en/reference.pcre.pattern.modifiers.php
+			if ( $regex_flags && ! preg_match( '/^(?!.*(.).*\1)[imsxeADSUXJu]+$/', $regex_flags ) ) {
+				WP_CLI::error( "Incorrect PCRE modifiers." );
+			}
 			$regex_delimiter = \WP_CLI\Utils\get_flag_value( $assoc_args, 'regex-delimiter', '/' );
 			if ( '' === $regex_delimiter ) {
 				$regex_delimiter = '/';