Add CustomFilterKeyword#to_regex method (#28893)

spec/models/custom_filter_keyword_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe CustomFilterKeyword do
+  describe '#to_regex' do
+    context 'when whole_word is true' do
+      it 'builds a regex with boundaries and the keyword' do
+        keyword = described_class.new(whole_word: true, keyword: 'test')
+
+        expect(keyword.to_regex).to eq(/(?mix:\b#{Regexp.escape(keyword.keyword)}\b)/)
+      end
+
+      it 'builds a regex with starting boundary and the keyword when end with non-word' do
+        keyword = described_class.new(whole_word: true, keyword: 'test#')
+
+        expect(keyword.to_regex).to eq(/(?mix:\btest\#)/)
+      end
+
+      it 'builds a regex with end boundary and the keyword when start with non-word' do
+        keyword = described_class.new(whole_word: true, keyword: '#test')
+
+        expect(keyword.to_regex).to eq(/(?mix:\#test\b)/)
+      end
+    end
+
+    context 'when whole_word is false' do
+      it 'builds a regex with the keyword' do
+        keyword = described_class.new(whole_word: false, keyword: 'test')
+
+        expect(keyword.to_regex).to eq(/test/i)
+      end
+    end
+  end
+end