diff --git a/include/tvm/runtime/packed_func.h b/include/tvm/runtime/packed_func.h
index 5bf86d468bfdf..82b3dd4695415 100644
--- a/include/tvm/runtime/packed_func.h
+++ b/include/tvm/runtime/packed_func.h
@@ -69,19 +69,20 @@ namespace runtime {
 TVM_DLL std::string GetCustomTypeName(uint8_t type_code);
 
 /*!
-  * \brief Runtime utility for getting custom type code from name
-  * \param type_name Custom type name
-  * \return Custom type code
-  */
-TVM_DLL uint8_t GetCustomTypeCode(const std::string& type_name);
-
-  /*!
-   * \brief Runtime utility for checking whether custom type is registered
-   * \param type_code Custom type code
-   * \return Bool representing whether type is registered
-   */
+ * \brief Runtime utility for checking whether custom type is registered
+ * \param type_code Custom type code
+ * \return Bool representing whether type is registered
+ */
 TVM_DLL bool GetCustomTypeRegistered(uint8_t type_code);
 
+/*!
+ * \brief Runtime utility for parsing string of the form "custom[<typename>]"
+ * \param s String to parse
+ * \param scan pointer to parsing pointer, which is scanning across s
+ * \return type code of custom type parsed
+ */
+TVM_DLL uint8_t ParseCustomDatatype(const std::string& s, const char** scan);
+
 // forward declarations
 class TVMArgs;
 class TVMArgValue;
@@ -1025,22 +1026,7 @@ inline TVMType String2TVMType(std::string s) {
     t.lanes = 1;
     return t;
   } else if (s.substr(0, 6) == "custom") {
-    // TODO(gus) this should be separated out into its own parsing function and cleaned up, or
-    // replaced by a regex.
-    scan = s.c_str() + 6;
-    if (*scan != '[')
-      LOG(FATAL) << "expected opening brace after 'custom' type in" << s;
-    ++scan;
-    size_t custom_name_len = 0;
-    while (scan + custom_name_len <= s.c_str() + s.length() &&
-           *(scan + custom_name_len) != ']')
-      ++custom_name_len;
-    if (*(scan + custom_name_len) != ']')
-      LOG(FATAL) << "expected closing brace after 'custom' type in" << s;
-    scan += custom_name_len + 1;
-
-    auto type_name = s.substr(7, custom_name_len);
-    t.code = GetCustomTypeCode(type_name);
+    t.code = ParseCustomDatatype(s, &scan);
   } else {
     scan = s.c_str();
     LOG(FATAL) << "unknown type " << s;
diff --git a/src/runtime/c_runtime_api.cc b/src/runtime/c_runtime_api.cc
index b93e3881fcbc1..20793b4618b3a 100644
--- a/src/runtime/c_runtime_api.cc
+++ b/src/runtime/c_runtime_api.cc
@@ -63,6 +63,34 @@ bool GetCustomTypeRegistered(uint8_t type_code) {
   return (*f)(type_code).operator bool();
 }
 
+uint8_t ParseCustomDatatype(const std::string& s, const char** scan) {
+  CHECK(s.substr(0, 6) == "custom") << "Not a valid custom datatype string";
+
+  auto tmp = s.c_str();
+
+  CHECK(s.c_str() == tmp);
+  *scan = s.c_str() + 6;
+  CHECK(s.c_str() == tmp);
+  if (**scan != '[') LOG(FATAL) << "expected opening brace after 'custom' type in" << s;
+  CHECK(s.c_str() == tmp);
+  *scan += 1;
+  CHECK(s.c_str() == tmp);
+  size_t custom_name_len = 0;
+  CHECK(s.c_str() == tmp);
+  while (*scan + custom_name_len <= s.c_str() + s.length() && *(*scan + custom_name_len) != ']')
+    ++custom_name_len;
+  CHECK(s.c_str() == tmp);
+  if (*(*scan + custom_name_len) != ']')
+    LOG(FATAL) << "expected closing brace after 'custom' type in" << s;
+  CHECK(s.c_str() == tmp);
+  *scan += custom_name_len + 1;
+  CHECK(s.c_str() == tmp);
+
+  auto type_name = s.substr(7, custom_name_len);
+  CHECK(s.c_str() == tmp);
+  return GetCustomTypeCode(type_name);
+}
+
 class DeviceAPIManager {
  public:
   static const int kMaxDeviceAPI = 32;