diff --git a/sample/calc-ja.y b/sample/calc-ja.y
index 05f2137a..89c237de 100644
--- a/sample/calc-ja.y
+++ b/sample/calc-ja.y
@@ -26,7 +26,7 @@ end
 ---- header
 #
 ---- inner
-  
+
   def evaluate(str)
     @tokens = []
     until str.empty?
diff --git a/sample/calc.y b/sample/calc.y
index 8135731d..8b13131a 100644
--- a/sample/calc.y
+++ b/sample/calc.y
@@ -24,7 +24,7 @@ end
 ---- header
 #
 ---- inner
-  
+
   def parse(str)
     @q = []
     until str.empty?
diff --git a/sample/hash.y b/sample/hash.y
index 85348e4e..7cb6739d 100644
--- a/sample/hash.y
+++ b/sample/hash.y
@@ -7,7 +7,7 @@ class HashParser
 rule
   hash    : '{' contents '}'   { val[1] }
           | '{' '}'            { Hash.new }
-           
+
                   # Racc can handle string over 2 bytes.
   contents: IDENT '=>' IDENT              { {val[0] => val[2]} }
           | contents ',' IDENT '=>' IDENT { val[0][val[2]] = val[4]; val[0] }
diff --git a/sample/syntax.y b/sample/syntax.y
index dd4c6699..ca4cdf2d 100644
--- a/sample/syntax.y
+++ b/sample/syntax.y
@@ -28,7 +28,7 @@ rule
               print 'cba'
             }
         | cont
-  
+
   cont  : A c2 B c2 C
 
   c2    : C C C C C
diff --git a/test/case.rb b/test/case.rb
index f4240bfd..d917f3a4 100644
--- a/test/case.rb
+++ b/test/case.rb
@@ -90,8 +90,7 @@ def assert_output_unchanged(asset)
       actual   = File.read("#{@TAB_DIR}/#{file}")
       result   = (strip_version(expected) == strip_version(actual))
 
-      assert(result, "Output of test/assets/#{file}.y differed from " \
-        "expectation. Try compiling it and diff with test/regress/#{file}.")
+      assert(result, proc {`diff -u #{REGRESS_DIR}/#{file} #{@TAB_DIR}/#{file}`})
     end
 
     def racc(*arg, **opt)