Prevents error when search query ends with empty parenthesis

app/models/search_builder.rb

@@ -6,7 +6,7 @@ class SearchBuilder < Blacklight::SearchBuilder
   include BlacklightAdvancedSearch::AdvancedSearchBuilder
   include BlacklightHelper
 
-  self.default_processor_chain += %i[cleanup_boolean_operators add_advanced_search_to_solr
+  self.default_processor_chain += %i[parslet_trick cleanup_boolean_operators add_advanced_search_to_solr
                                      cjk_mm wildcard_char_strip excessive_paging_error
                                      only_home_facets left_anchor_escape_whitespace
                                      series_title_results pul_holdings html_facets
@@ -17,6 +17,16 @@ def cleanup_boolean_operators(solr_parameters)
     solr_parameters[:q] = cleaned_query(solr_parameters[:q])
   end
 
+  # Blacklight uses Parslet https://rubygems.org/gems/parslet/versions/2.0.0 to parse the user query
+  # and unfortunately Parslet gets confused when the user's query ends with "()". Here we tweak the
+  # query to prevent the error and let the query to be parsed as if the ending "()" was not present.
+  # Notice that we must update the value in `blacklight_params[:q]`
+  def parslet_trick(_solr_parameters)
+    return unless blacklight_params[:q].is_a?(String)
+    return unless blacklight_params[:q].strip.end_with?("()")
+    blacklight_params[:q] = blacklight_params[:q].strip.gsub("()", "")
+  end
+
   # Only search for coin records when querying with the numismatics advanced search
   def numismatics_advanced(solr_parameters)
     return unless blacklight_params[:advanced_type] == 'numismatics'

spec/models/search_builder_spec.rb

@@ -43,5 +43,11 @@
       search_builder.blacklight_params[:search_field] = 'advanced'
       expect(search_builder.excessive_paging?).to be false
     end
+
+    it 'handles query ending with empty parenthesis' do
+      search_builder.blacklight_params[:q] = 'hello world ()'
+      search_builder.parslet_trick({})
+      expect(search_builder.blacklight_params[:q].end_with?("()")).to be false
+    end
   end
 end