diff --git a/arrays-strings/replace_char.ipynb b/arrays-strings/replace_char.ipynb
index e8b8605..d1d11cd 100644
--- a/arrays-strings/replace_char.ipynb
+++ b/arrays-strings/replace_char.ipynb
@@ -38,21 +38,51 @@
    "source": [
     "## Test Cases\n",
     "\n",
-    "* NULL\n",
+    "* NULL->NULL\n",
     "* ' ' -> '%20'\n",
     "* ' foo bar ' -> '%20foo%20bar%20'\n",
     "* 'foo' -> 'foo'"
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "collapsed": true
+   },
+   "outputs": [],
+   "source": [
+    "from nose.tools import assert_equal\n",
+    "\n",
+    "class Test(object):\n",
+    "    def test_replace_char(self, func):\n",
+    "        str0 = None\n",
+    "        str1 = bytearray(' ||')\n",
+    "        str2 = bytearray(' foo bar ||||||')\n",
+    "        str3 = bytearray('foo')\n",
+    "        encode_spaces(str0, 0)\n",
+    "        encode_spaces(str1, 1)\n",
+    "        encode_spaces(str2, 9)\n",
+    "        encode_spaces(str3, 3)\n",
+    "        assert_equal(str0, None)\n",
+    "        assert_equal(str1, '%20')\n",
+    "        assert_equal(str2, '%20foo%20bar%20')\n",
+    "        assert_equal(str3, 'foo')\n",
+    "\n",
+    "def run_tests(func):\n",
+    "    test = Test()\n",
+    "    test.test_replace_char(func)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {},
    "source": [
     "## Algorithm\n",
     "\n",
-    "![alt text](https://raw.githubusercontent.com/donnemartin/algorithms-data-structures/master/images/replace_string.jpg)\n",
+    "Since Python strings are immutable, we'll use a bytearray instead to exercise in-place string manipulation as you would get with a C string (which is null terminated, as seen in the diagram below).  For the python bytearray we will not use a null terminator.\n",
     "\n",
-    "Since Python strings are immutable, we'll use a bytearray instead to exercise in-place string manipulation as you would get with a C string (which is null terminated, as seen in the above diagram).  For the python bytearray we will not use a null terminator.\n",
+    "![alt text](https://raw.githubusercontent.com/donnemartin/algorithms-data-structures/master/images/replace_string.jpg)\n",
     "\n",
     "* Count the number of spaces in the bytearray\n",
     "* Determine the new bytearray length\n",
@@ -101,18 +131,7 @@
     "            string[new_length] = string[i]\n",
     "            new_length -= 1\n",
     "\n",
-    "str0 = None\n",
-    "str1 = bytearray(' ||')\n",
-    "str2 = bytearray(' foo bar ||||||')\n",
-    "str3 = bytearray('foo')\n",
-    "encode_spaces(str0, 0)\n",
-    "encode_spaces(str1, 1)\n",
-    "encode_spaces(str2, 9)\n",
-    "encode_spaces(str3, 3)\n",
-    "print(str0)\n",
-    "print(str1)\n",
-    "print(str2)\n",
-    "print(str3)"
+    "run_tests(encode_spaces)"
    ]
   },
   {
@@ -138,7 +157,10 @@
     "    return re.sub(' ', '%20', string)\n",
     "\n",
     "def encode_spaces_alt2(string):\n",
-    "    return string.replace(' ', '%20')"
+    "    return string.replace(' ', '%20')\n",
+    "\n",
+    "run_tests(encode_spaces_alt)\n",
+    "run_tests(encode_spaces_alt2)"
    ]
   }
  ],