Fix ClassicIdentifierSuggestionGenerator to reject all-numeric slugs

app/services/project_identifiers/classic_identifier_suggestion_generator.rb

@@ -67,8 +67,12 @@ module ProjectIdentifiers
 
     private
 
+    BLANK_SLUG_SUBSTITUTIONS = { "." => "dot", "!" => "bang" }.freeze
+
     def slugify(name)
-      name.to_url.first(Projects::Identifier::CLASSIC_IDENTIFIER_MAX_LENGTH).presence
+      slug = name.to_url.first(Projects::Identifier::CLASSIC_IDENTIFIER_MAX_LENGTH).presence
+      slug ||= BLANK_SLUG_SUBSTITUTIONS[name]
+      slug if slug && Projects::Identifier::CLASSIC_IDENTIFIER_FORMAT.match?(slug)
     end
 
     def fallback_base

spec/services/project_identifiers/classic_identifier_suggestion_generator_spec.rb

@@ -63,6 +63,14 @@ RSpec.describe ProjectIdentifiers::ClassicIdentifierSuggestionGenerator do
       it "falls back to a randomised project-XXXXXX identifier" do
         expect(described_class.new.suggest_identifier("!!!")).to match(/\Aproject-[a-z0-9]{5}\z/)
       end
+
+      it "maps '.' to 'dot'" do
+        expect(described_class.new.suggest_identifier(".")).to eq("dot")
+      end
+
+      it "maps '!' to 'bang'" do
+        expect(described_class.new.suggest_identifier("!")).to eq("bang")
+      end
     end
 
     context "when the name is all-numeric" do