Use UTF-8 mode on regex.

Author: gitlost

features/db-ack.feature

@@ -570,11 +570,11 @@ Feature: Ack through the database
     When I run `wp db ack "かきくけこ" --before_context=2 --after_context=1`
     Then STDOUT should contain:
       """
-      :えおかきくけこさし
+      :えおかきくけこさ
       """
     And STDOUT should not contain:
       """
-      えおかきくけこさしす
+      えおかきくけこさし
       """
 
     When I run `wp db ack "かきくけこ"`

src/DB_Command.php

@@ -755,7 +755,7 @@ public function ack( $args, $assoc_args ) {
 
 		$esc_like_search = self::esc_like( $search );
 		$safe_search = preg_quote( $search, '#' );
-		$search_regex = '#(.{0,' . $before_context . '})(' . $safe_search .')(.{0,' . $after_context . '})#i';
+		$search_regex = '#(.{0,' . $before_context . '})(' . $safe_search .')(.{0,' . $after_context . '})#ui';
 
 		$tables = WP_CLI\Utils\wp_get_table_names( $args, $assoc_args );