diff --git a/Main.hs b/Main.hs
--- a/Main.hs
+++ b/Main.hs
@@ -2,6 +2,7 @@
 import Regex.Genex
 import System.IO
 import System.Environment
+import Data.Char (isDigit)
 
 defaultRegex :: String
 defaultRegex = "a(b|c)d{2,3}e*"
@@ -15,7 +16,16 @@
             prog <- getProgName
             if prog == "<interactive>" then run defaultRegex else do
                 fail $ "Usage: " ++ prog ++ " regex [regex...]"
-        rx -> genexPrint rx
+        rx | all isPure rx -> mapM_ ((putStr "0 " >>) . print) (genexPure rx)
+           | otherwise     -> genexPrint rx
+    where
+    isPure [] = True
+    isPure ('\\':'\\':cs) = isPure cs
+    isPure ('\\':'b':_) = False
+    isPure ('\\':c:cs)
+        | isDigit c = False
+        | otherwise = isPure cs
+    isPure (_:cs) = isPure cs
 
 run :: String -> IO ()
 run regex = genexPrint [regex]
diff --git a/regex-genex.cabal b/regex-genex.cabal
--- a/regex-genex.cabal
+++ b/regex-genex.cabal
@@ -1,9 +1,9 @@
 Name            : regex-genex
-Version         : 0.5.0
+Version         : 0.5.1
 license         : OtherLicense
 license-file    : LICENSE
 cabal-version   : >= 1.6
-copyright       : 2011 Audrey Tang
+copyright       : 2011-2012 Audrey Tang
 maintainer      : Audrey Tang <audreyt@audreyt.org>
 category        : Text, Regex
 stability       : experimental