diff --git a/NOTICE b/NOTICE
index df3bf3d..1df1c74 100644
--- a/NOTICE
+++ b/NOTICE
@@ -782,6 +782,41 @@ Dependency: pretty-1.1.3.6
   
   -----------------------------------------------------------------------------
 
+================================================================================
+Dependency: primitive-0.7.0.1
+================================================================================
+
+  Copyright (c) 2008-2009, Roman Leshchinskiy
+  All rights reserved.
+  
+  Redistribution and use in source and binary forms, with or without
+  modification, are permitted provided that the following conditions are met:
+  
+  - Redistributions of source code must retain the above copyright notice,
+  this list of conditions and the following disclaimer.
+   
+  - Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+   
+  - Neither name of the University nor the names of its contributors may be
+  used to endorse or promote products derived from this software without
+  specific prior written permission. 
+  
+  THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY COURT OF THE UNIVERSITY OF
+  GLASGOW AND THE CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+  INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+  FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+  UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW OR THE CONTRIBUTORS BE LIABLE
+  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+  OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
+  DAMAGE.
+  
+
 ================================================================================
 Dependency: process-1.6.9.0
 ================================================================================
@@ -1006,3 +1041,38 @@ Dependency: unix-2.7.2.2
   OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
   DAMAGE.
 
+================================================================================
+Dependency: vector-0.12.1.2
+================================================================================
+
+  Copyright (c) 2008-2012, Roman Leshchinskiy
+  All rights reserved.
+  
+  Redistribution and use in source and binary forms, with or without
+  modification, are permitted provided that the following conditions are met:
+  
+  - Redistributions of source code must retain the above copyright notice,
+  this list of conditions and the following disclaimer.
+   
+  - Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+   
+  - Neither name of the University nor the names of its contributors may be
+  used to endorse or promote products derived from this software without
+  specific prior written permission. 
+  
+  THIS SOFTWARE IS PROVIDED BY THE UNIVERSITY COURT OF THE UNIVERSITY OF
+  GLASGOW AND THE CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
+  INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND
+  FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+  UNIVERSITY COURT OF THE UNIVERSITY OF GLASGOW OR THE CONTRIBUTORS BE LIABLE
+  FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+  DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+  SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+  CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+  LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+  OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH
+  DAMAGE.
+  
+
diff --git a/src/Language/SystemVerilog/Parser.hs b/src/Language/SystemVerilog/Parser.hs
index 50c0208..5d60a94 100644
--- a/src/Language/SystemVerilog/Parser.hs
+++ b/src/Language/SystemVerilog/Parser.hs
@@ -36,8 +36,7 @@ parseFile' includePaths env skipPreprocessor path = do
     let runner = if skipPreprocessor then annotate else preprocess
     preResult <- liftIO $ runner includePaths env path
     (contents, env') <- liftEither preResult
-    result <- liftIO $ uncurry lexStr $ unzip contents
-    tokens <- liftEither result
+    tokens <- liftEither $ uncurry lexStr $ unzip contents
     let position =
             if null tokens
                 then Position path 1 1
diff --git a/src/Language/SystemVerilog/Parser/Lex.x b/src/Language/SystemVerilog/Parser/Lex.x
index 8917a77..167e99a 100644
--- a/src/Language/SystemVerilog/Parser/Lex.x
+++ b/src/Language/SystemVerilog/Parser/Lex.x
@@ -10,9 +10,6 @@
  - `begin_keywords` and `end_keywords` lexer directives are handled here.
  -}
 
--- This pragma gets rid of a warning caused by alex 3.2.5.
-{-# OPTIONS_GHC -fno-warn-unused-imports #-}
-
 module Language.SystemVerilog.Parser.Lex
     ( lexStr
     ) where
@@ -20,12 +17,13 @@ module Language.SystemVerilog.Parser.Lex
 import Control.Monad.Except
 import qualified Data.Map.Strict as Map
 import qualified Data.Set as Set
+import qualified Data.Vector as Vector
 
 import Language.SystemVerilog.Parser.Keywords (specMap)
 import Language.SystemVerilog.Parser.Tokens
 }
 
-%wrapper "monadUserState"
+%wrapper "posn"
 
 -- Numbers
 
@@ -472,28 +470,14 @@ tokens :-
     .                  { tok Unknown }
 
 {
--- our custom lexer state
-data AlexUserState = LS
-    { lsToks      :: [Token] -- tokens read so far, *in reverse order* for efficiency
-    , lsPositions :: [Position] -- character positions
-    } deriving (Eq, Show)
-
--- this initial user state does not contain the initial token positions; alex
--- requires that this be defined; we override it before we begin the actual
--- lexing procedure
-alexInitUserState :: AlexUserState
-alexInitUserState = LS [] []
-
 -- lexer entrypoint
-lexStr :: String -> [Position] -> IO (Either String [Token])
-lexStr chars positions = do
-    let setEnv = modify $ \s -> s { lsPositions = positions }
-    let result = runAlex chars $ setEnv >> alexMonadScan >> get
-    return $ case result of
-        Left msg -> Left msg
-        Right finalState ->
-            runExcept $ postProcess [] tokens
-            where tokens = reverse $ lsToks finalState
+lexStr :: String -> [Position] -> Either String [Token]
+lexStr chars positions =
+    runExcept $ postProcess [] tokens
+    where
+        tokensRaw = alexScanTokens chars
+        positionsVec = Vector.fromList positions
+        tokens = map (\tkf -> tkf positionsVec) tokensRaw
 
 -- process begin/end keywords directives
 postProcess :: [Set.Set TokenName] -> [Token] -> Except String [Token]
@@ -526,25 +510,8 @@ postProcess stack (t : ts) = do
                 then Token Id_simple ('_' : str) pos
                 else t
 
--- invoked by alexMonadScan
-alexEOF :: Alex ()
-alexEOF = return ()
-
--- get the current user state
-get :: Alex AlexUserState
-get = Alex $ \s -> Right (s, alex_ust s)
-
--- apply a transformation to the current user state
-modify :: (AlexUserState -> AlexUserState) -> Alex ()
-modify f = Alex func
-    where func s = Right (s { alex_ust = new }, ())
-            where new = f (alex_ust s)
-
-tok :: TokenName -> AlexInput -> Int -> Alex ()
-tok tokId (AlexPn pos _ _, _, _, input) len = do
-    let tokStr = take len input
-    tokPos <- get >>= return . (!! pos) . lsPositions
-    let t = Token tokId tokStr tokPos
-    modify $ \s -> s { lsToks = t : (lsToks s) }
-    alexMonadScan
+tok :: TokenName -> AlexPosn -> String -> Vector.Vector Position -> Token
+tok tokId (AlexPn charPos _ _) tokStr positions =
+    Token tokId tokStr tokPos
+    where tokPos = positions Vector.! charPos
 }
diff --git a/sv2v.cabal b/sv2v.cabal
index 51e97bb..d3751c8 100644
--- a/sv2v.cabal
+++ b/sv2v.cabal
@@ -32,6 +32,7 @@ executable sv2v
     , githash
     , hashable
     , mtl
+    , vector
   other-modules:
     -- SystemVerilog modules
     Language.SystemVerilog