diff --git a/src/AbsSyn.hs b/src/AbsSyn.hs
index 3eb8f6b3..7aca5849 100644
--- a/src/AbsSyn.hs
+++ b/src/AbsSyn.hs
@@ -14,7 +14,7 @@ module AbsSyn (
   wrapperName,
   Scanner(..),
   RECtx(..),
-  RExp(..),
+  RExp(..), nullable,
   DFA(..), State(..), SNum, StartCode, Accept(..),
   RightContext(..), showRCtx, strtype,
   encodeStartCodes, extractActions,
@@ -188,7 +188,7 @@ usesPreds dfa
 -- Regular expressions
 
 -- `RExp' provides an abstract syntax for regular expressions.  `Eps' will
--- match empty strings; `Ch p' matches strings containinng a single character
+-- match empty strings; `Ch p' matches strings containing a single character
 -- `c' if `p c' is true; `re1 :%% re2' matches a string if `re1' matches one of
 -- its prefixes and `re2' matches the rest; `re1 :| re2' matches a string if
 -- `re1' or `re2' matches it; `Star re', `Plus re' and `Ques re' can be
@@ -196,13 +196,13 @@ usesPreds dfa
 -- for a formal definition of the semantics of these operators.
 
 data RExp
-  = Eps
-  | Ch CharSet
-  | RExp :%% RExp
-  | RExp :| RExp
-  | Star RExp
-  | Plus RExp
-  | Ques RExp
+  = Eps            -- ^ Empty.
+  | Ch CharSet     -- ^ Singleton.
+  | RExp :%% RExp  -- ^ Sequence.
+  | RExp :| RExp   -- ^ Alternative.
+  | Star RExp      -- ^ Zero or more repetitions.
+  | Plus RExp      -- ^ One  or more repetitions.
+  | Ques RExp      -- ^ Zero or one  repetitions.
 
 instance Show RExp where
   showsPrec _ Eps = showString "()"
@@ -213,6 +213,17 @@ instance Show RExp where
   showsPrec _ (Plus r) = shows r . ('+':)
   showsPrec _ (Ques r) = shows r . ('?':)
 
+-- | A regular expression is nullable if it matches the empty string.
+nullable :: RExp -> Bool
+nullable Eps       = True
+nullable Ch{}      = False
+nullable (l :%% r) = nullable l && nullable r
+nullable (l :|  r) = nullable l || nullable r
+nullable Star{}    = True
+nullable (Plus r)  = nullable r
+nullable Ques{}    = True
+
+
 {------------------------------------------------------------------------------
                           Abstract Regular Expression
 ------------------------------------------------------------------------------}
diff --git a/src/Main.hs b/src/Main.hs
index 1f578733..e35c9992 100644
--- a/src/Main.hs
+++ b/src/Main.hs
@@ -18,7 +18,7 @@ import Info
 import Map ( Map )
 import qualified Map hiding ( Map )
 import Output
-import ParseMonad ( runP )
+import ParseMonad ( runP, Warning(..) )
 import Parser
 import Scan
 import Util ( hline )
@@ -114,11 +114,20 @@ parseScript file prg =
         Left (Nothing, err) ->
                 die (file ++ ": " ++ err ++ "\n")
 
-        Right script@(_, _, scanner, _) -> do
+        Right (warnings, script@(_, _, scanner, _)) -> do
           -- issue 46: give proper error when lexer definition is empty
           when (null $ scannerTokens scanner) $
             dieAlex $ file ++ " contains no lexer rules\n"
+          -- issue 71: warn about nullable regular expressions
+          mapM_ printWarning warnings
           return script
+  where
+  printWarning (WarnNullableRExp (AlexPn _ line col) msg) =
+    hPutStrLn stderr $ concat
+      [ "Warning: "
+      , file , ":", show line , ":" , show col , ": "
+      , msg
+      ]
 
 alex :: [CLIFlags]
      -> FilePath
diff --git a/src/ParseMonad.hs b/src/ParseMonad.hs
index 5f85cc6b..7272226d 100644
--- a/src/ParseMonad.hs
+++ b/src/ParseMonad.hs
@@ -1,5 +1,5 @@
 -- -----------------------------------------------------------------------------
--- 
+--
 -- ParseMonad.hs, part of Alex
 --
 -- (c) Simon Marlow 2003
@@ -9,7 +9,7 @@
 module ParseMonad (
         AlexInput, alexInputPrevChar, alexGetChar, alexGetByte,
         AlexPosn(..), alexStartPos,
- 
+        Warning(..), warnIfNullable,
         P, runP, StartCode, failP, lookupSMac, lookupRMac, newSMac, newRMac,
         setStartCode, getStartCode, getInput, setInput,
  ) where
@@ -23,7 +23,7 @@ import UTF8
 #if __GLASGOW_HASKELL__ < 710
 import Control.Applicative ( Applicative(..) )
 #endif
-import Control.Monad ( liftM, ap )
+import Control.Monad ( liftM, ap, when )
 import Data.Word (Word8)
 -- -----------------------------------------------------------------------------
 -- The input type
@@ -49,7 +49,7 @@ alexGetChar (_, _ ,_ : _, _) = undefined -- hide compiler warning
 alexGetByte :: AlexInput -> Maybe (Byte,AlexInput)
 alexGetByte (p,c,(b:bs),s) = Just (b,(p,c,bs,s))
 alexGetByte (_,_,[],[]) = Nothing
-alexGetByte (p,_,[],(c:s))  = let p' = alexMove p c 
+alexGetByte (p,_,[],(c:s))  = let p' = alexMove p c
                                   (b:bs) = UTF8.encode c
                               in p' `seq`  Just (b, (p', c, bs, s))
 
@@ -57,7 +57,7 @@ alexGetByte (p,_,[],(c:s))  = let p' = alexMove p c
 -- Token positions
 
 -- `Posn' records the location of a token in the input text.  It has three
--- fields: the address (number of chacaters preceding the token), line number
+-- fields: the address (number of charaters preceding the token), line number
 -- and column of a token within the file. `start_pos' gives the position of the
 -- start of the file and `eof_pos' a standard encoding for the end of file.
 -- `move_pos' calculates the new position after traversing a given character,
@@ -77,15 +77,22 @@ alexMove (AlexPn a l c) _    = AlexPn (a+1)  l     (c+1)
 -- -----------------------------------------------------------------------------
 -- Alex lexing/parsing monad
 
+data Warning
+  = WarnNullableRExp
+    { _warnPos  :: AlexPosn  -- ^ The position of the code following the regex.
+    , _warnText :: String    -- ^ Warning text.
+    }
+
 type ParseError = (Maybe AlexPosn, String)
 type StartCode = Int
 
-data PState = PState {
-                smac_env  :: Map String CharSet,
-                rmac_env  :: Map String RExp,
-                startcode :: Int,
-                input     :: AlexInput
-             }
+data PState = PState
+  { warnings  :: [Warning]           -- ^ Stack of warnings, top = last warning.
+  , smac_env  :: Map String CharSet
+  , rmac_env  :: Map String RExp
+  , startcode :: Int
+  , input     :: AlexInput
+  }
 
 newtype P a = P { unP :: PState -> Either ParseError (PState,a) }
 
@@ -102,15 +109,27 @@ instance Monad P where
                         Right (env',ok) -> unP (k ok) env'
  return = pure
 
-runP :: String -> (Map String CharSet, Map String RExp) 
-        -> P a -> Either ParseError a
-runP str (senv,renv) (P p) 
+-- | Run the parser on given input.
+runP :: String
+          -- ^ Input string.
+     -> (Map String CharSet, Map String RExp)
+          -- ^ Character set and regex definitions.
+     -> P a
+          -- ^ Parsing computation.
+     -> Either ParseError ([Warning], a)
+          -- ^ List of warnings in first-to-last order, result.
+runP str (senv,renv) (P p)
   = case p initial_state of
         Left err -> Left err
-        Right (_,a) -> Right a
- where initial_state = 
-          PState{ smac_env=senv, rmac_env=renv,
-             startcode = 0, input=(alexStartPos,'\n',[],str) }
+        Right (s, a) -> Right (reverse (warnings s), a)
+  where
+  initial_state = PState
+    { warnings  = []
+    , smac_env  = senv
+    , rmac_env  = renv
+    , startcode = 0
+    , input     = (alexStartPos, '\n', [], str)
+    }
 
 failP :: String -> P a
 failP str = P $ \PState{ input = (p,_,_,_) } -> Left (Just p,str)
@@ -121,24 +140,24 @@ failP str = P $ \PState{ input = (p,_,_,_) } -> Left (Just p,str)
 
 lookupSMac :: (AlexPosn,String) -> P CharSet
 lookupSMac (posn,smac)
- = P $ \s@PState{ smac_env = senv } -> 
+ = P $ \s@PState{ smac_env = senv } ->
        case Map.lookup smac senv of
         Just ok -> Right (s,ok)
         Nothing -> Left (Just posn, "unknown set macro: $" ++ smac)
 
 lookupRMac :: String -> P RExp
-lookupRMac rmac 
- = P $ \s@PState{ rmac_env = renv } -> 
+lookupRMac rmac
+ = P $ \s@PState{ rmac_env = renv } ->
        case Map.lookup rmac renv of
         Just ok -> Right (s,ok)
         Nothing -> Left (Nothing, "unknown regex macro: %" ++ rmac)
 
 newSMac :: String -> CharSet -> P ()
-newSMac smac set 
+newSMac smac set
   = P $ \s -> Right (s{smac_env = Map.insert smac set (smac_env s)}, ())
 
 newRMac :: String -> RExp -> P ()
-newRMac rmac rexp 
+newRMac rmac rexp
   = P $ \s -> Right (s{rmac_env = Map.insert rmac rexp (rmac_env s)}, ())
 
 setStartCode :: StartCode -> P ()
@@ -152,3 +171,21 @@ getInput = P $ \s -> Right (s, input s)
 
 setInput :: AlexInput -> P ()
 setInput inp = P $ \s -> Right (s{ input = inp }, ())
+
+-- | Add a warning if given regular expression is nullable
+--   unless the user wrote the regex 'Eps'.
+warnIfNullable
+  :: RExp       -- ^ Regular expression.
+  -> AlexPosn   -- ^ Position associated to regular expression.
+  -> P ()
+-- If the user wrote @()@, they wanted to match the empty sequence!
+-- Thus, skip the warning then.
+warnIfNullable Eps _ = return ()
+warnIfNullable r pos = when (nullable r) $ P $ \ s ->
+  Right (s{ warnings = WarnNullableRExp pos w : warnings s}, ())
+  where
+  w = unwords
+      [ "Regular expression"
+      , show r
+      , "matches the empty string."
+      ]
diff --git a/src/Parser.y b/src/Parser.y
index 0747cb1f..bf627fa8 100644
--- a/src/Parser.y
+++ b/src/Parser.y
@@ -109,16 +109,23 @@ tokendefs :: { [RECtx] }
 	| {- empty -}			{ [] }
 
 tokendef :: { [RECtx] }
-	: startcodes rule		{ [ replaceCodes $1 $2 ] }
+	: startcodes rule		{ [ replaceCodes $1 (snd $2) ] }
 	| startcodes '{' rules '}'	{ map (replaceCodes $1) $3 }
-	| rule				{ [ $1 ] }
-
-rule    :: { RECtx }
-	: context rhs			{ let (l,e,r) = $1 in
-					  RECtx [] l e r $2 }
+	| rule				{% do
+                                           let (pos, res@(RECtx _ _ e _ _)) = $1
+                                           warnIfNullable e pos
+                                           return [ res ]
+                                        }
+
+rule    :: { (AlexPosn, RECtx) }
+        : context rhs                   { let
+                                            (l, e, r)   = $1
+                                            (pos, code) = $2
+                                          in (pos, RECtx [] l e r code)
+                                        }
 
 rules	:: { [RECtx] }
-	: rule rules			{ $1 : $2 }
+	: rule rules			{ snd $1 : $2 }
 	| {- empty -}			{ [] }
 
 startcodes :: { [(String,StartCode)] }
@@ -132,9 +139,9 @@ startcode :: { String }
 	: ZERO 				{ "0" }
 	| ID	 			{ $1 }
 
-rhs	:: { Maybe Code }
-	: CODE 				{ case $1 of T _ (CodeT code) -> Just code }
-	| ';'	 			{ Nothing }
+rhs	:: { (AlexPosn, Maybe Code) }
+	: CODE 				{ case $1 of T pos (CodeT code) -> (pos, Just code) }
+	| ';'				{ (tokPosn $1, Nothing) }
 
 context :: { Maybe CharSet, RExp, RightContext RExp }
 	: left_ctx rexp right_ctx	{ (Just $1,$2,$3) }