diff --git a/library/init/data/string/basic.lean b/library/init/data/string/basic.lean
index 188c94e150..b36ceea747 100644
--- a/library/init/data/string/basic.lean
+++ b/library/init/data/string/basic.lean
@@ -131,6 +131,16 @@ partial def posOfAux (s : String) (c : Char) (stopPos : Pos) : Pos → Pos
 @[inline] def posOf (s : String) (c : Char) : Pos :=
 posOfAux s c s.bsize 0
 
+partial def revPosOfAux (s : String) (c : Char) : Pos → Option Pos
+| pos :=
+ if s.get pos == c then some pos
+ else if pos == 0 then none
+ else revPosOfAux (s.prev pos)
+
+def revPosOf (s : String) (c : Char) : Option Pos :=
+if s.bsize == 0 then none
+else revPosOfAux s c (s.prev s.bsize)
+
 private def utf8ExtractAux₂ : List Char → Pos → Pos → List Char
 | []      _ _ := []
 | (c::cs) i e := if i = e then [] else c :: utf8ExtractAux₂ cs (i + csize c) e
diff --git a/library/init/system/filepath.lean b/library/init/system/filepath.lean
index 7abc4d8b1b..483cd3a3a3 100644
--- a/library/init/system/filepath.lean
+++ b/library/init/system/filepath.lean
@@ -5,7 +5,7 @@ Authors: Leonardo de Moura
 -/
 prelude
 import init.system.platform
-import init.data.char.basic
+import init.data.string.basic
 
 namespace System
 namespace FilePath
@@ -27,5 +27,10 @@ if isWindows then ';' else ':'
 def extSeparator : Char :=
 '.'
 
+def dirName (fname : String) : String :=
+match fname.revPosOf pathSeparator with
+| none => "."
+| some pos => { Substring . str := fname, startPos := 0, stopPos := pos }.toString
+
 end FilePath
 end System