diff --git a/doc/manual/release-notes.xml b/doc/manual/release-notes.xml
index e32bc2da..53c8d4bf 100644
--- a/doc/manual/release-notes.xml
+++ b/doc/manual/release-notes.xml
@@ -55,7 +55,10 @@
 
   <listitem><para>TODO: new built-ins
   <function>builtins.attrNames</function>,
-  <function>builtins.filterSource</function>.</para></listitem>
+  <function>builtins.filterSource</function>,
+  <function>builtins.sub</function>,
+  <function>builtins.stringLength</function>,
+  <function>builtins.substring</function>.</para></listitem>
   
 
 </itemizedlist>
diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
index b80e2069..01806a6c 100644
--- a/src/libexpr/primops.cc
+++ b/src/libexpr/primops.cc
@@ -723,6 +723,14 @@ static Expr primAdd(EvalState & state, const ATermVector & args)
 }
 
 
+static Expr primSub(EvalState & state, const ATermVector & args)
+{
+    int i1 = evalInt(state, args[0]);
+    int i2 = evalInt(state, args[1]);
+    return makeInt(i1 - i2);
+}
+
+
 static Expr primLessThan(EvalState & state, const ATermVector & args)
 {
     int i1 = evalInt(state, args[0]);
@@ -779,6 +787,36 @@ static Expr primFilterSource(EvalState & state, const ATermVector & args)
 }
 
 
+/*************************************************************
+ * String manipulation
+ *************************************************************/
+
+
+/* `substr start len str' returns the substring of `str' starting at
+   character position `min(start, stringLength str)' inclusive and
+   ending at `min(start + len, stringLength str)'.  `start' must be
+   non-negative. */
+static Expr prim_substring(EvalState & state, const ATermVector & args)
+{
+    int start = evalInt(state, args[0]);
+    int len = evalInt(state, args[1]);
+    PathSet context;
+    string s = coerceToString(state, args[2], context);
+
+    if (start < 0) throw EvalError("negative start position in `substring'");
+
+    return makeStr(string(s, start, len), context);
+}
+
+
+static Expr prim_stringLength(EvalState & state, const ATermVector & args)
+{
+    PathSet context;
+    string s = coerceToString(state, args[0], context);
+    return makeInt(s.size());
+}
+
+
 void EvalState::addPrimOps()
 {
     addPrimOp("builtins", 0, primBuiltins);
@@ -813,10 +851,14 @@ void EvalState::addPrimOps()
     addPrimOp("removeAttrs", 2, primRemoveAttrs);
     addPrimOp("relativise", 2, primRelativise);
     addPrimOp("__add", 2, primAdd);
+    addPrimOp("__sub", 2, primSub);
     addPrimOp("__lessThan", 2, primLessThan);
     addPrimOp("__toFile", 2, primToFile);
     addPrimOp("__filterSource", 2, primFilterSource);
+
+    addPrimOp("__substring", 3, prim_substring);
+    addPrimOp("__stringLength", 1, prim_stringLength);
 }
 
- 
+
 }
diff --git a/tests/filter-source.nix.in b/tests/filter-source.nix.in
index 8493835c..6fd49479 100644
--- a/tests/filter-source.nix.in
+++ b/tests/filter-source.nix.in
@@ -4,6 +4,9 @@ derivation {
   builder = "@shell@";
   args = ["-e" "-x" (builtins.toFile "builder" "PATH=@testPath@; ln -s $input $out")];
   input =
-    let filter = path: type: type != "symlink" && baseNameOf (toString path) != "foo";
+    let filter = path: type:
+      type != "symlink"
+      && baseNameOf path != "foo"
+      && !((import ./lang/lib.nix).hasSuffix ".bak" (baseNameOf path));
     in builtins.filterSource filter ./test-tmp/filterin;
 }
diff --git a/tests/filter-source.sh b/tests/filter-source.sh
index 96ccaedc..73f35302 100644
--- a/tests/filter-source.sh
+++ b/tests/filter-source.sh
@@ -5,6 +5,9 @@ mkdir $TEST_ROOT/filterin
 mkdir $TEST_ROOT/filterin/foo
 touch $TEST_ROOT/filterin/foo/bar
 touch $TEST_ROOT/filterin/xyzzy
+touch $TEST_ROOT/filterin/b
+touch $TEST_ROOT/filterin/bak
+touch $TEST_ROOT/filterin/bla.c.bak
 ln -s xyzzy $TEST_ROOT/filterin/link
 
 $NIX_BIN_DIR/nix-build ./filter-source.nix -o $TEST_ROOT/filterout
@@ -12,4 +15,6 @@ $NIX_BIN_DIR/nix-build ./filter-source.nix -o $TEST_ROOT/filterout
 set -x
 test ! -e $TEST_ROOT/filterout/foo/bar
 test -e $TEST_ROOT/filterout/xyzzy
+test -e $TEST_ROOT/filterout/bak
+test ! -e $TEST_ROOT/filterout/bla.c.bak
 test ! -L $TEST_ROOT/filterout/link
diff --git a/tests/lang/eval-fail-substring.nix b/tests/lang/eval-fail-substring.nix
new file mode 100644
index 00000000..f37c2bc0
--- /dev/null
+++ b/tests/lang/eval-fail-substring.nix
@@ -0,0 +1 @@
+builtins.substring (builtins.sub 0 1) 1 "x"
diff --git a/tests/lang/eval-okay-substring.exp b/tests/lang/eval-okay-substring.exp
new file mode 100644
index 00000000..d6098476
--- /dev/null
+++ b/tests/lang/eval-okay-substring.exp
@@ -0,0 +1 @@
+Str("ooxfoobarybarzobaabb",[])
diff --git a/tests/lang/eval-okay-substring.nix b/tests/lang/eval-okay-substring.nix
new file mode 100644
index 00000000..184d7258
--- /dev/null
+++ b/tests/lang/eval-okay-substring.nix
@@ -0,0 +1,19 @@
+with builtins;
+
+let
+
+  s = "foobar";
+
+in
+
+substring 1 2 s
++ "x"
++ substring 0 (stringLength s) s
++ "y"
++ substring 3 100 s
++ "z"
++ substring 2 (sub (stringLength s) 3) s
++ "a"
++ substring 3 0 s
++ "b"
++ substring 3 1 s
diff --git a/tests/lang/lib.nix b/tests/lang/lib.nix
index f888453f..ec705299 100644
--- a/tests/lang/lib.nix
+++ b/tests/lang/lib.nix
@@ -1,18 +1,26 @@
+with builtins;
+
 rec {
 
   fold = op: nul: list:
     if list == []
     then nul
-    else op (builtins.head list) (fold op nul (builtins.tail list));
+    else op (head list) (fold op nul (tail list));
 
   concat =
     fold (x: y: x + y) "";
 
   flatten = x:
-    if builtins.isList x
+    if isList x
     then fold (x: y: (flatten x) ++ y) [] x
     else [x];
 
-  sum = fold (x: y: builtins.add x y) 0;
+  sum = fold (x: y: add x y) 0;
+
+  hasSuffix = ext: fileName:
+    let lenFileName = stringLength fileName;
+        lenExt = stringLength ext;
+    in !(lessThan lenFileName lenExt) &&
+       substring (sub lenFileName lenExt) lenFileName fileName == ext;
 
 }