diff --git a/CHANGES.md b/CHANGES.md
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,6 +1,11 @@
 Changes
 =======
 
+0.3.1
+-----
+
+Add `comap`
+
 0.3.0.3
 -------
 
diff --git a/Text/Regex/Applicative.hs b/Text/Regex/Applicative.hs
--- a/Text/Regex/Applicative.hs
+++ b/Text/Regex/Applicative.hs
@@ -20,6 +20,7 @@
     , reFoldl
     , Greediness(..)
     , few
+    , comap
     , withMatched
     , match
     , (=~)
diff --git a/Text/Regex/Applicative/Interface.hs b/Text/Regex/Applicative/Interface.hs
--- a/Text/Regex/Applicative/Interface.hs
+++ b/Text/Regex/Applicative/Interface.hs
@@ -29,6 +29,21 @@
 instance (char ~ Char, string ~ String) => IsString (RE char string) where
     fromString = string
 
+-- | 'RE' is a profunctor. This is its contravariant map.
+--
+-- (A dependency on the @profunctors@ package doesn't seem justified.)
+comap :: (s2 -> s1) -> RE s1 a -> RE s2 a
+comap f re =
+  case re of
+    Eps -> Eps
+    Symbol t p    -> Fmap f $ Symbol t (p . f)
+    Alt r1 r2     -> Alt (comap f r1) (comap f r2)
+    App r1 r2     -> App (comap f r1) (comap f r2)
+    Fmap g r      -> Fmap g (comap f r)
+    Fail          -> Fail
+    Rep gr fn a r -> Rep gr fn a (comap f r)
+    Void r        -> Void (comap f r)
+
 -- | Match and return a single symbol which satisfies the predicate
 psym :: (s -> Bool) -> RE s s
 psym p = Symbol (error "Not numbered symbol") p
diff --git a/regex-applicative.cabal b/regex-applicative.cabal
--- a/regex-applicative.cabal
+++ b/regex-applicative.cabal
@@ -1,86 +1,36 @@
--- regex-applicative.cabal auto-generated by cabal init. For
--- additional options, see
--- http://www.haskell.org/cabal/release/cabal-latest/doc/users-guide/authors.html#pkg-descr.
--- The name of the package.
 Name:                regex-applicative
-
--- The package version. See the Haskell package versioning policy
--- (http://www.haskell.org/haskellwiki/Package_versioning_policy) for
--- standards guiding when and how versions should be incremented.
-
--- DO NOT FORGET TO UPDATE THE GIT TAG BELOW!!!
-Version:             0.3.0.3
-
--- A short (one-line) description of the package.
+Version:             0.3.1
 Synopsis:            Regex-based parsing with applicative interface
-
--- A longer description of the package.
 Description:         
     regex-applicative is a Haskell library for parsing using regular expressions.
     Parsers can be built using Applicative interface.
-
--- URL for the project homepage or repository.
 Homepage:            https://github.com/feuerbach/regex-applicative
-
--- The license under which the package is released.
 License:             MIT
-
--- The file containing the license text.
 License-file:        LICENSE
-
--- The package author(s).
 Author:              Roman Cheplyaka
-
--- An email address to which users can send suggestions, bug reports,
--- and patches.
 Maintainer:          Roman Cheplyaka <roma@ro-che.info>
-
--- A copyright notice.
--- Copyright:           
-
 Category:            Text
-
 Build-type:          Simple
-
--- Extra files to be distributed with the package, such as examples or
--- a README.
 Extra-source-files:  README.md CREDITS.md CHANGES.md
-
--- Constraint on the version of Cabal needed to build this package.
 Cabal-version:       >=1.10
 
 Source-repository head
   type:     git
   location: git://github.com/feuerbach/regex-applicative.git
 
-Source-repository this
-  type:     git
-  location: git://github.com/feuerbach/regex-applicative.git
-  tag:      v0.3.0.3
-
 Library
   Default-language:    Haskell2010
-  -- Packages needed in order to build this package.
   Build-depends:       base < 5,
                        containers,
                        transformers
-
-
-  -- Modules exported by the library.
   Exposed-modules:     Text.Regex.Applicative
                        Text.Regex.Applicative.Object
                        Text.Regex.Applicative.Common
                        Text.Regex.Applicative.Reference
-  
-  -- Modules not exported by this package.
   Other-modules:       Text.Regex.Applicative.Interface
                        Text.Regex.Applicative.Types
                        Text.Regex.Applicative.Compile
                        Text.Regex.Applicative.StateQueue
-  
-  -- Extra tools (e.g. alex, hsc2hs, ...) needed to build the source.
-  -- Build-tools:         
-  
   GHC-Options:     -O2
                    -Wall
                    -fno-warn-name-shadowing