let this regexp be stricter

The purpose of this regexp is to detect strings that
are valid identifiers to be used as method names.
The anchor $ allows a trailing newline, and both ^
and $ may be bypassed because Ruby regexps are
multiline always. The anchors \A and \z are the
proper ones to ensure string boundaries.

activemodel/lib/active_model/attribute_methods.rb

@@ -56,7 +56,7 @@ module ActiveModel
   module AttributeMethods
     extend ActiveSupport::Concern
 
-    COMPILABLE_REGEXP = /^[a-zA-Z_]\w*[!?=]?$/
+    COMPILABLE_REGEXP = /\A[a-zA-Z_]\w*[!?=]?\z/
 
     included do
       class_attribute :attribute_method_matchers, :instance_writer => false