#10713: Improve documentation for \b and \B and add a few tests. Initial...

#10713: Improve documentation for \b and \B and add a few tests.  Initial patch and tests by Martin Pool.

Doc/library/re.rst

@@ -325,14 +325,19 @@ the second character.  For example, ``\$`` matches the character ``'$'``.
    Matches the empty string, but only at the beginning or end of a word.  A word is
    defined as a sequence of alphanumeric or underscore characters, so the end of a
    word is indicated by whitespace or a non-alphanumeric, non-underscore character.
-   Note that  ``\b`` is defined as the boundary between ``\w`` and ``\W``, so the
-   precise set of characters deemed to be alphanumeric depends on the values of the
-   ``UNICODE`` and ``LOCALE`` flags.  Inside a character range, ``\b`` represents
-   the backspace character, for compatibility with Python's string literals.
+   Note that formally, ``\b`` is defined as the boundary between a ``\w`` and
+   a ``\W`` character (or vice versa), or between ``\w`` and the beginning/end
+   of the string, so the precise set of characters deemed to be alphanumeric
+   depends on the values of the ``UNICODE`` and ``LOCALE`` flags.
+   For example, ``r'\bfoo\b'`` matches ``'foo'``, ``'foo.'``, ``'(foo)'``,
+   ``'bar foo baz'`` but not ``'foobar'`` or ``'foo3'``.
+   Inside a character range, ``\b`` represents the backspace character, for compatibility with Python's string literals.
 
 ``\B``
    Matches the empty string, but only when it is *not* at the beginning or end of a
-   word.  This is just the opposite of ``\b``, so is also subject to the settings
+   word.  This means that ``r'py\B'`` matches ``'python'``, ``'py3'``, ``'py2'``,
+   but not ``'py'``, ``'py.'``, or ``'py!'``.
+   ``\B`` is just the opposite of ``\b``, so is also subject to the settings
    of ``LOCALE`` and ``UNICODE``.
 
 ``\d``

Lib/test/test_re.py

@@ -373,6 +373,32 @@ class ReTests(unittest.TestCase):
         self.assertEqual(re.search(r"\d\D\w\W\s\S",
                                    "1aa! a", re.UNICODE).group(0), "1aa! a")
 
+    def test_string_boundaries(self):
+        # See http://bugs.python.org/issue10713
+        self.assertEqual(re.search(r"\b(abc)\b", "abc").group(1),
+                         "abc")
+        # There's a word boundary at the start of a string.
+        self.assertTrue(re.match(r"\b", "abc"))
+        # A non-empty string includes a non-boundary zero-length match.
+        self.assertTrue(re.search(r"\B", "abc"))
+        # There is no non-boundary match at the start of a string.
+        self.assertFalse(re.match(r"\B", "abc"))
+        # However, an empty string contains no word boundaries, and also no
+        # non-boundaries.
+        self.assertEqual(re.search(r"\B", ""), None)
+        # This one is questionable and different from the perlre behaviour,
+        # but describes current behavior.
+        self.assertEqual(re.search(r"\b", ""), None)
+        # A single word-character string has two boundaries, but no
+        # non-boundary gaps.
+        self.assertEqual(len(re.findall(r"\b", "a")), 2)
+        self.assertEqual(len(re.findall(r"\B", "a")), 0)
+        # If there are no words, there are no boundaries
+        self.assertEqual(len(re.findall(r"\b", " ")), 0)
+        self.assertEqual(len(re.findall(r"\b", "   ")), 0)
+        # Can match around the whitespace.
+        self.assertEqual(len(re.findall(r"\B", " ")), 2)
+
     def test_bigcharset(self):
         self.assertEqual(re.match(u"([\u2222\u2223])",
                                   u"\u2222").group(1), u"\u2222")