diff --git a/lib/mocha/backtrace_filter.rb b/lib/mocha/backtrace_filter.rb
index 69215e7..9edad0a 100644
--- a/lib/mocha/backtrace_filter.rb
+++ b/lib/mocha/backtrace_filter.rb
@@ -2,14 +2,14 @@ module Mocha
 
   class BacktraceFilter
   
-    LIB_DIRECTORY = File.expand_path(File.join(File.dirname(__FILE__), "..")) + File::SEPARATOR
+    LIB_DIRECTORY = File.dirname(File.dirname(__FILE__)) + File::SEPARATOR
 
     def initialize(lib_directory = LIB_DIRECTORY)
-      @lib_directory = lib_directory
+      @lib_directory = Regexp.new('^' + Regexp.escape(lib_directory))
     end
 
     def filtered(backtrace)
-      backtrace.reject { |location| Regexp.new(@lib_directory).match(File.expand_path(location)) }
+      backtrace.reject { |location| location =~ @lib_directory }
     end
 
   end