FIX: Complete underscore support in topic tag filter regex (#37552)

Adds support of `tags:hello_world,hello_not_world`

lib/topics_filter.rb

@@ -901,7 +901,7 @@ class TopicsFilter
       break if key_prefix && key_prefix != "-"
 
       value.scan(
-        /\A(?<tag_names>([\p{N}\p{L}\-_]+)(?<delimiter>[,+])?([\p{N}\p{L}\-]+)?(\k<delimiter>[\p{N}\p{L}\-]+)*)\z/,
+        /\A(?<tag_names>([\p{N}\p{L}\-_]+)(?<delimiter>[,+])?([\p{N}\p{L}\-_]+)?(\k<delimiter>[\p{N}\p{L}\-_]+)*)\z/,
       ) do |tag_names, delimiter|
         match_all =
           if delimiter == ","

spec/lib/topics_filter_spec.rb

@@ -1492,6 +1492,31 @@ RSpec.describe TopicsFilter do
           ).to contain_exactly(topic_with_tag.id, topic_with_tag_and_tag2.id)
         end
       end
+
+      describe "when query string contains multiple tags with underscores" do
+        before do
+          tag.update!(name: "tag_one")
+          tag2.update!(name: "tag_two")
+        end
+
+        it "should return topics when filtering with comma-separated underscore tags" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("tags:tag_one,tag_two")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_tag.id, topic_with_tag_and_tag2.id, topic_with_tag2.id)
+        end
+
+        it "should return topics when filtering with plus-separated underscore tags" do
+          expect(
+            TopicsFilter
+              .new(guardian: Guardian.new)
+              .filter_from_query_string("tags:tag_one+tag_two")
+              .pluck(:id),
+          ).to contain_exactly(topic_with_tag_and_tag2.id)
+        end
+      end
     end
 
     describe "when filtering by tag_groups" do