Fix the 'for' attribute check in presubmit.

chrome/browser/web_dev_style/html_checker.py

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 Presubmit for Chromium HTML resources. See chrome/browser/PRESUBMIT.py.
 """
 
 import regex_check
 
 
 class HtmlChecker(object):
   def __init__(self, input_api, output_api, file_filter=None):
     self.input_api = input_api
     self.output_api = output_api
     self.file_filter = file_filter
 
   def LabelCheck(self, line_number, line):
     return regex_check.RegexCheck(self.input_api.re, line_number, line,
-        "(for=)",
+        r"(?:^|\s)(for=)",
         "Avoid 'for' attribute on <label>. Place the input within the <label>, "
         "or use aria-labelledby for <select>.")
 
   def RunChecks(self):
     """Check for violations of the Chromium web development style guide. See
        http://chromium.org/developers/web-development-style-guide
     """
     results = []
 
     affected_files = self.input_api.change.AffectedFiles(
         file_filter=self.file_filter, include_deletes=False)
 
     for f in affected_files:
       errors = []
 
       for line_number, line in f.ChangedContents():
         error = self.LabelCheck(line_number, line)
         if error:
           errors.append(error)
 
       if errors:
         abs_local_path = f.AbsoluteLocalPath()
         file_indicator = 'Found HTML style issues in %s' % abs_local_path
         prompt_msg = file_indicator + '\n\n' + '\n'.join(errors) + '\n'
         results.append(self.output_api.PresubmitPromptWarning(prompt_msg))
 
     return results