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diff --git a/src/Text/Pandoc/Filter/Path.hs b/src/Text/Pandoc/Filter/Path.hs
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+++ b/src/Text/Pandoc/Filter/Path.hs
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+{-
+Copyright (C) 2006-2018 John MacFarlane <jgm@berkeley.edu>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program; if not, write to the Free Software
+Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
+-}
+
+{- |
+   Module      : Text.Pandoc.Filter.Path
+   Copyright   : Copyright (C) 2006-2018 John MacFarlane
+   License     : GNU GPL, version 2 or above
+
+   Maintainer  : John MacFarlane <jgm@berkeley@edu>
+   Stability   : alpha
+   Portability : portable
+
+Expand paths of filters, searching the data directory.
+-}
+module Text.Pandoc.Filter.Path
+  ( expandFilterPath
+  ) where
+
+import Text.Pandoc.Class (PandocMonad, fileExists, getUserDataDir)
+import System.FilePath ((</>), isRelative)
+
+  -- First we check to see if a filter is found.  If not, and if it's
+  -- not an absolute path, we check to see whether it's in `userdir/filters`.
+  -- If not, we leave it unchanged.
+expandFilterPath :: PandocMonad m => FilePath -> m FilePath
+expandFilterPath fp = do
+  mbDatadir <- getUserDataDir
+  fpExists <- fileExists fp
+  if fpExists
+     then return fp
+     else case mbDatadir of
+               Just datadir | isRelative fp -> do
+                 let filterPath = datadir </> "filters" </> fp
+                 filterPathExists <- fileExists filterPath
+                 if filterPathExists
+                    then return filterPath
+                    else return fp
+               _ -> return fp