diff --git a/tests/test-fix-topology.t b/tests/test-fix-topology.t
--- a/tests/test-fix-topology.t
+++ b/tests/test-fix-topology.t
@@ -1,3 +1,19 @@
+A script that implements uppercasing all letters in a file.
+
+  $ UPPERCASEPY="$TESTTMP/uppercase.py"
+  $ cat > $UPPERCASEPY <<EOF
+  > import sys
+  > from mercurial.utils.procutil import setbinary
+  > setbinary(sys.stdin)
+  > setbinary(sys.stdout)
+  > sys.stdout.write(sys.stdin.read().upper())
+  > EOF
+  $ TESTLINES="foo\nbar\nbaz\n"
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY
+  FOO
+  BAR
+  BAZ
+
 Tests for the fix extension's behavior around non-trivial history topologies.
 Looks for correct incremental fixing and reproduction of parent/child
 relationships. We indicate fixed file content by uppercasing it.
@@ -6,7 +22,7 @@
   > [extensions]
   > fix =
   > [fix]
-  > uppercase-whole-file:command=sed -e 's/.*/\U&/'
+  > uppercase-whole-file:command=$PYTHON $UPPERCASEPY
   > uppercase-whole-file:fileset=set:**
   > EOF
 
diff --git a/tests/test-fix.t b/tests/test-fix.t
--- a/tests/test-fix.t
+++ b/tests/test-fix.t
@@ -1,3 +1,58 @@
+A script that implements uppercasing of specific lines in a file. This
+approximates the behavior of code formatters well enough for our tests.
+
+  $ UPPERCASEPY="$TESTTMP/uppercase.py"
+  $ cat > $UPPERCASEPY <<EOF
+  > import sys
+  > from mercurial.utils.procutil import setbinary
+  > setbinary(sys.stdin)
+  > setbinary(sys.stdout)
+  > lines = set()
+  > for arg in sys.argv[1:]:
+  >   if arg == 'all':
+  >     sys.stdout.write(sys.stdin.read().upper())
+  >     sys.exit(0)
+  >   else:
+  >     first, last = arg.split('-')
+  >     lines.update(range(int(first), int(last) + 1))
+  > for i, line in enumerate(sys.stdin.readlines()):
+  >   if i + 1 in lines:
+  >     sys.stdout.write(line.upper())
+  >   else:
+  >     sys.stdout.write(line)
+  > EOF
+  $ TESTLINES="foo\nbar\nbaz\nqux\n"
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY
+  foo
+  bar
+  baz
+  qux
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY all
+  FOO
+  BAR
+  BAZ
+  QUX
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY 1-1
+  FOO
+  bar
+  baz
+  qux
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY 1-2
+  FOO
+  BAR
+  baz
+  qux
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY 2-3
+  foo
+  BAR
+  BAZ
+  qux
+  $ printf $TESTLINES | $PYTHON $UPPERCASEPY 2-2 4-4
+  foo
+  BAR
+  baz
+  QUX
+
 Set up the config with two simple fixers: one that fixes specific line ranges,
 and one that always fixes the whole file. They both "fix" files by converting
 letters to uppercase. They use different file extensions, so each test case can
@@ -10,10 +65,10 @@
   > evolution.createmarkers=True
   > evolution.allowunstable=True
   > [fix]
-  > uppercase-whole-file:command=sed -e 's/.*/\U&/'
+  > uppercase-whole-file:command=$PYTHON $UPPERCASEPY all
   > uppercase-whole-file:fileset=set:**.whole
-  > uppercase-changed-lines:command=sed
-  > uppercase-changed-lines:linerange=-e '{first},{last} s/.*/\U&/'
+  > uppercase-changed-lines:command=$PYTHON $UPPERCASEPY
+  > uppercase-changed-lines:linerange={first}-{last}
   > uppercase-changed-lines:fileset=set:**.changed
   > EOF
 
@@ -878,7 +933,7 @@
   $ hg commit -Aqm "foo"
   $ printf "Foo\nbar\nBaz\n" > foo.changed
   $ hg --debug fix --working-dir
-  subprocess: sed -e '1,1 s/.*/\U&/' -e '3,3 s/.*/\U&/'
+  subprocess: * $TESTTMP/uppercase.py 1-1 3-3 (glob)
 
   $ cd ..