clang-format-diff.py: fix -regex/-iregex matching

While debating the finer points of file extension matching, we somehow missed
the bigger problem that the current code will match anything starting with the
default or user-specified pattern (e.g. lit.site.cfg.in).

Fix this by doing what find(1) does, implicitly wrapping the pattern with ^$.

git-svn-id: svn://svn.chromium.org/llvm-project/cfe/trunk/tools/clang-format@197608 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
diff --git a/clang-format-diff.py b/clang-format-diff.py
index 5cece8d..fee16d0 100755
--- a/clang-format-diff.py
+++ b/clang-format-diff.py
@@ -43,7 +43,7 @@
                       help='apply edits to files instead of displaying a diff')
   parser.add_argument('-p', metavar='NUM', default=0,
                       help='strip the smallest prefix containing P slashes')
-  parser.add_argument('-regex', metavar='PATTERN', default='',
+  parser.add_argument('-regex', metavar='PATTERN', default=None,
                       help='custom pattern selecting file paths to reformat '
                       '(case sensitive, override -iregex)')
   parser.add_argument('-iregex', metavar='PATTERN', default=
@@ -66,11 +66,11 @@
     if filename == None:
       continue
 
-    if args.regex != '':
-      if not re.match(args.regex, filename):
+    if args.regex is not None:
+      if not re.match('^%s$' % args.regex, filename):
         continue
     else:
-      if not re.match(args.iregex, filename, re.IGNORECASE):
+      if not re.match('^%s$' % args.iregex, filename, re.IGNORECASE):
         continue
 
     match = re.search('^@@.*\+(\d+)(,(\d+))?', line)