Replace regex with Bundler::LockfileParser

changelog/change_replace_regex_with_BundlerLockfileParser.md

@@ -0,0 +1 @@
+* [#12180](https://github.com/rubocop/rubocop/pull/12180): Replace regex with `Bundler::LockfileParser`. ([@amomchilov][])

lib/rubocop/config.rb

@@ -302,16 +302,22 @@ def target_rails_version_from_bundler_lock_file
 
     # @return [Float, nil] The Rails version as a `major.minor` Float.
     def read_rails_version_from_bundler_lock_file
-      lock_file_path = bundler_lock_file_path
-      return nil unless lock_file_path
-
-      File.foreach(lock_file_path) do |line|
-        # If Rails (or one of its frameworks) is in Gemfile.lock or gems.lock, there should be
-        # a line like:
-        #         railties (X.X.X)
-        result = line.match(/^\s+railties\s+\((\d+\.\d+)/)
-        return result.captures.first.to_f if result
-      end
+      return nil unless gem_versions_in_target
+
+      # Look for `railties` instead of `rails`, to support apps that only use a subset of `rails`
+      # See https://github.com/rubocop/rubocop/pull/11289
+      rails_version_in_target = gem_versions_in_target['railties']
+      return nil unless rails_version_in_target
+
+      gem_version_to_major_minor_float(rails_version_in_target)
+    end
+
+    # @param [Gem::Version] gem_version an object like `Gem::Version.new("7.1.2.3")`
+    # @return [Float] The major and minor version, like `7.1`
+    def gem_version_to_major_minor_float(gem_version)
+      segments = gem_version.canonical_segments
+      # segments.fetch(0).to_f + (segments.fetch(1, 0.0).to_f / 10)
+      Float("#{segments.fetch(0)}.#{segments.fetch(1, 0)}")
     end
 
     # @returns [Hash{String => Gem::Version}] The locked gem versions, keyed by the gems' names.