diff -r fecf9e6d7630 Lib/doctest.py
--- a/Lib/doctest.py	Fri Apr 22 00:17:46 2011 +0100
+++ b/Lib/doctest.py	Fri Apr 22 15:24:56 2011 -0400
@@ -96,7 +96,8 @@
 import sys, traceback, inspect, linecache, os, re
 import unittest, difflib, pdb, tempfile
 import warnings
-from io import StringIO
+import tokenize
+from io import StringIO, BytesIO
 from collections import namedtuple
 
 TestResults = namedtuple('TestResults', 'failed attempted')
@@ -653,15 +654,10 @@
 
         return source, options, want, exc_msg
 
-    # This regular expression looks for option directives in the
-    # source code of an example.  Option directives are comments
-    # starting with "doctest:".  Warning: this may give false
-    # positives for string-literals that contain the string
-    # "#doctest:".  Eliminating these false positives would require
-    # actually parsing the string; but we limit them by ignoring any
-    # line containing "#doctest:" that is *followed* by a quote mark.
-    _OPTION_DIRECTIVE_RE = re.compile(r'#\s*doctest:\s*([^\n\'"]*)$',
-                                      re.MULTILINE)
+    # This regular expression checks if a comment is an option
+    # directive.  Option directives are comments
+    # starting with "doctest:".
+    _OPTION_DIRECTIVE_RE = re.compile(r'^#\s*doctest:\s*([^\'"]*)$')
 
     def _find_options(self, source, name, lineno):
         """
@@ -673,7 +669,13 @@
         """
         options = {}
         # (note: with the current regexp, this will match at most once:)
-        for m in self._OPTION_DIRECTIVE_RE.finditer(source):
+        for token in tokenize.tokenize(BytesIO(source.encode('utf-8')).readline):
+            if token.type != tokenize.COMMENT:
+                continue
+            m = self._OPTION_DIRECTIVE_RE.match(token.string)
+            if m is None:
+                continue
+            
             option_strings = m.group(1).replace(',', ' ').split()
             for option in option_strings:
                 if (option[0] not in '+-' or
diff -r fecf9e6d7630 Lib/test/test_doctest.py
--- a/Lib/test/test_doctest.py	Fri Apr 22 00:17:46 2011 +0100
+++ b/Lib/test/test_doctest.py	Fri Apr 22 15:24:56 2011 -0400
@@ -1487,6 +1487,28 @@
         [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
     TestResults(failed=2, attempted=3)
 
+It used to be the case that option directives were parsed out of the source
+text using a regular expression, which resulted in problems.
+That is now corrected, and only real comments count as directives.
+
+In older versions of doctest, the below would fail, because
+DONT_ACCEPT_BLANKLINE would be set, even though it wasn't a real directive.
+
+    >>> def printsomething(f): print('')
+    >>> @printsomething
+    ... def foo():
+    ...     '''This is a pointless function with a comment that isn't a comment
+    ...     
+    ...     Example of a doctest containing a doctest! This is probably
+    ...     dangerous.
+    ...     
+    ...         >>> print('<BLANKLINE>') # doctest: +DONT_ACCEPT_BLANKLINE
+    ...         <BLANKLINE>
+    ...     
+    ...     '''
+    ... 
+    <BLANKLINE>
+
 Multiple options may be modified by a single option directive.  They
 may be separated by whitespace, commas, or both: