Fix (Inflector::Methods#underscore): small regression

Author: Thornolf

activesupport/lib/active_support/inflector/methods.rb

@@ -97,7 +97,7 @@ def underscore(camel_cased_word)
       return camel_cased_word.to_s unless /[A-Z-]|::/.match?(camel_cased_word)
       word = camel_cased_word.to_s.gsub("::", "/")
       word.gsub!(inflections.acronyms_underscore_regex) { "#{$1 && '_' }#{$2.downcase}" }
-      word.gsub!(/([A-Z\d]+)(?=[A-Z][a-z])|([a-z\d])(?=[A-Z])/) { ($1 || $2) << "_" }
+      word.gsub!(/([A-Z]+)(?=[A-Z][a-z])|([a-z\d])(?=[A-Z])/) { ($1 || $2) << "_" }
       word.tr!("-", "_")
       word.downcase!
       word

activesupport/test/inflector_test_cases.rb

@@ -121,6 +121,7 @@ module InflectorTestCases
     "ApplicationController" => "application_controller",
     "Area51Controller"      => "area51_controller",
     "AppCDir"               => "app_c_dir",
+    "Accountsv2N2Test"      => "accountsv2_n2_test",
   }
 
   UnderscoreToLowerCamel = {