diff --git a/tools/style/check-header-guards.py b/tools/style/check-header-guards.py
index 644e0f2..1196660 100755
--- a/tools/style/check-header-guards.py
+++ b/tools/style/check-header-guards.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python3
 # Copyright 2016 The Fuchsia Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
@@ -34,13 +34,13 @@
     'ZIRCON_SYSTEM_PUBLIC',
     'ZIRCON_THIRD_PARTY_ULIB_MUSL_INCLUDE',
 ]
-sysroot_prefix = re.compile('^(' + string.join(SYSROOT_PREFIXES, '|') + ')_')
+sysroot_prefix = re.compile('^(' + '|'.join(SYSROOT_PREFIXES) + ')_')
 
 PUBLIC_PREFIXES = [
     'ZIRCON_SYSTEM_ULIB_.*_INCLUDE', 'GARNET_PUBLIC', 'PERIDOT_PUBLIC',
     'TOPAZ_PUBLIC', 'SDK'
 ]
-public_prefix = re.compile('^(' + string.join(PUBLIC_PREFIXES, '|') + ')_')
+public_prefix = re.compile('^(' + '|'.join(PUBLIC_PREFIXES) + ')_')
 
 all_header_guards = collections.defaultdict(list)
 
@@ -232,7 +232,7 @@
 
 def check_collisions():
   collisions = 0
-  for header_guard, paths in all_header_guards.iteritems():
+  for header_guard, paths in iter(all_header_guards.items()):
     if len(paths) == 1:
       continue
     collisions += 1