bpo-30349: Raise FutureWarning for nested sets and set operations

Doc/library/re.rst

@@ -200,6 +200,20 @@ The special characters are:
      place it at the beginning of the set.  For example, both ``[()[\]{}]`` and
      ``[]()[{}]`` will both match a parenthesis.
 
+   * Support of nested sets and set operations as in `Unicode Technical
+     Standard #18`_ might be added in the future.  This would change the
+     syntax, so to facilitate this change a :exc:`FutureWarning` will be raised
+     in ambiguous cases for the time being.
+     That include sets starting with a literal ``'['`` or containing literal
+     character sequences ``'--'``, ``'&&'``, ``'~~'``, and ``'||'``.  To
+     avoid a warning escape them with a backslash.
+
+   .. _Unicode Technical Standard #18: https://unicode.org/reports/tr18/
+
+   .. versionchanged:: 3.7
+      :exc:`FutureWarning` is raised if a character set contains constructs
+      that will change semantically in the future.
+
 ``|``
    ``A|B``, where *A* and *B* can be arbitrary REs, creates a regular expression that
    will match either *A* or *B*.  An arbitrary number of REs can be separated by the
@@ -829,7 +843,7 @@ form.
 
       >>> legal_chars = string.ascii_lowercase + string.digits + "!#$%&'*+-.^_`|~:"
       >>> print('[%s]+' % re.escape(legal_chars))
-      [abcdefghijklmnopqrstuvwxyz0123456789!\#\$%&'\*\+\-\.\^_`\|~:]+
+      [abcdefghijklmnopqrstuvwxyz0123456789!\#\$%\&'\*\+\-\.\^_`\|\~:]+
 
       >>> operators = ['+', '-', '*', '/', '**']
       >>> print('|'.join(map(re.escape, sorted(operators, reverse=True))))

Doc/tools/susp-ignored.csv

@@ -300,7 +300,7 @@ whatsnew/3.2,,:gz,">>> with tarfile.open(name='myarchive.tar.gz', mode='w:gz') a
 whatsnew/3.2,,:location,zope9-location = ${zope9:location}
 whatsnew/3.2,,:prefix,zope-conf = ${custom:prefix}/etc/zope.conf
 library/re,,`,!#$%&'*+-.^_`|~:
-library/re,,`,!\#\$%&'\*\+\-\.\^_`\|~:
+library/re,,`,!\#\$%\&'\*\+\-\.\^_`\|\~:
 library/tarfile,,:xz,'x:xz'
 library/xml.etree.elementtree,,:sometag,prefix:sometag
 library/xml.etree.elementtree,,:fictional,"<actors xmlns:fictional=""http://characters.example.com"""

Doc/whatsnew/3.7.rst

@@ -700,6 +700,17 @@ Changes in the Python API
   argument ``os.scandir`` instead of ``os.listdir`` when listing the direcory
   is failed.
 
+* Support of nested sets and set operations in regular expressions as in
+  `Unicode Technical Standard #18`_ might be added in the future.  This would
+  change the syntax, so to facilitate this change a :exc:`FutureWarning` will
+  be raised in ambiguous cases for the time being.
+  That include sets starting with a literal ``'['`` or containing literal
+  character sequences ``'--'``, ``'&&'``, ``'~~'``, and ``'||'``.  To
+  avoid a warning escape them with a backslash.
+  (Contributed by Serhiy Storchaka in :issue:`30349`.)
+
+.. _Unicode Technical Standard #18: https://unicode.org/reports/tr18/
+
 
 Changes in the C API
 --------------------

Lib/email/_header_value_parser.py

@@ -1354,15 +1354,14 @@ def __str__(self):
 
 _wsp_splitter = re.compile(r'([{}]+)'.format(''.join(WSP))).split
 _non_atom_end_matcher = re.compile(r"[^{}]+".format(
-    ''.join(ATOM_ENDS).replace('\\','\\\\').replace(']',r'\]'))).match
+    re.escape(''.join(ATOM_ENDS)))).match
 _non_printable_finder = re.compile(r"[\x00-\x20\x7F]").findall
 _non_token_end_matcher = re.compile(r"[^{}]+".format(
-    ''.join(TOKEN_ENDS).replace('\\','\\\\').replace(']',r'\]'))).match
+    re.escape(''.join(TOKEN_ENDS)))).match
 _non_attribute_end_matcher = re.compile(r"[^{}]+".format(
-    ''.join(ATTRIBUTE_ENDS).replace('\\','\\\\').replace(']',r'\]'))).match
+    re.escape(''.join(ATTRIBUTE_ENDS)))).match
 _non_extended_attribute_end_matcher = re.compile(r"[^{}]+".format(
-    ''.join(EXTENDED_ATTRIBUTE_ENDS).replace(
-                                    '\\','\\\\').replace(']',r'\]'))).match
+    re.escape(''.join(EXTENDED_ATTRIBUTE_ENDS)))).match
 
 def _validate_xtext(xtext):
     """If input token contains ASCII non-printables, register a defect."""

Lib/re.py

@@ -251,8 +251,9 @@ def template(pattern, flags=0):
 # SPECIAL_CHARS
 # closing ')', '}' and ']'
 # '-' (a range in character set)
+# '&', '~', (extended character set operations)
 # '#' (comment) and WHITESPACE (ignored) in verbose mode
-_special_chars_map = {i: '\\' + chr(i) for i in b'()[]{}?*+-|^$\\.# \t\n\r\v\f'}
+_special_chars_map = {i: '\\' + chr(i) for i in b'()[]{}?*+-|^$\\.&~# \t\n\r\v\f'}
 
 def escape(pattern):
     """

Lib/sre_parse.py

@@ -517,6 +517,12 @@ def _parse(source, state, verbose, nested, first=False):
             setappend = set.append
 ##          if sourcematch(":"):
 ##              pass # handle character classes
+            if source.next == '[':
+                import warnings
+                warnings.warn(
+                    'Possible nested set at position %d' % source.tell(),
+                    FutureWarning, stacklevel=nested + 6
+                )
             negate = sourcematch("^")
             # check remaining characters
             while True:
@@ -529,6 +535,17 @@ def _parse(source, state, verbose, nested, first=False):
                 elif this[0] == "\\":
                     code1 = _class_escape(source, this)
                 else:
+                    if set and this in '-&~|' and source.next == this:
+                        import warnings
+                        warnings.warn(
+                            'Possible set %s at position %d' % (
+                                'difference' if this == '-' else
+                                'intersection' if this == '&' else
+                                'symmetric difference' if this == '~' else
+                                'union',
+                                source.tell() - 1),
+                            FutureWarning, stacklevel=nested + 6
+                        )
                     code1 = LITERAL, _ord(this)
                 if sourcematch("-"):
                     # potential range
@@ -545,6 +562,13 @@ def _parse(source, state, verbose, nested, first=False):
                     if that[0] == "\\":
                         code2 = _class_escape(source, that)
                     else:
+                        if that == '-':
+                            import warnings
+                            warnings.warn(
+                                'Possible set difference at position %d' % (
+                                    source.tell() - 2),
+                                FutureWarning, stacklevel=nested + 6
+                            )
                         code2 = LITERAL, _ord(that)
                     if code1[0] != LITERAL or code2[0] != LITERAL:
                         msg = "bad character range %s-%s" % (this, that)

Lib/test/test_re.py

@@ -914,6 +914,51 @@ def test_not_literal(self):
         self.assertEqual(re.search(r"\s([^a])", " b").group(1), "b")
         self.assertEqual(re.search(r"\s([^a]*)", " bb").group(1), "bb")
 
+    def test_possible_set_operations(self):
+        s = bytes(range(128)).decode()
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[0-9--1]')
+        self.assertEqual(p.findall(s), list('-./0123456789'))
+        self.assertEqual(re.findall(r'[--1]', s), list('-./01'))
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[%--1]')
+        self.assertEqual(p.findall(s), list("%&'()*+,-1"))
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[%--]')
+        self.assertEqual(p.findall(s), list("%&'()*+,-"))
+
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[0-9&&1]')
+        self.assertEqual(p.findall(s), list('&0123456789'))
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[\d&&1]')
+        self.assertEqual(p.findall(s), list('&0123456789'))
+        self.assertEqual(re.findall(r'[&&1]', s), list('&1'))
+
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[0-9||a]')
+        self.assertEqual(p.findall(s), list('0123456789a|'))
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[\d||a]')
+        self.assertEqual(p.findall(s), list('0123456789a|'))
+        self.assertEqual(re.findall(r'[||1]', s), list('1|'))
+
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[0-9~~1]')
+        self.assertEqual(p.findall(s), list('0123456789~'))
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[\d~~1]')
+        self.assertEqual(p.findall(s), list('0123456789~'))
+        self.assertEqual(re.findall(r'[~~1]', s), list('1~'))
+
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[[0-9]|]')
+        self.assertEqual(p.findall(s), list('0123456789[]'))
+
+        with self.assertWarns(FutureWarning):
+            p = re.compile(r'[[:digit:]|]')
+        self.assertEqual(p.findall(s), list(':[]dgit'))
+
     def test_search_coverage(self):
         self.assertEqual(re.search(r"\s(b)", " b").group(1), "b")
         self.assertEqual(re.search(r"a\s", "a ").group(0), "a ")
@@ -932,7 +977,7 @@ def assertMatch(self, pattern, text, match=None, span=None,
         self.assertEqual(m.group(), match)
         self.assertEqual(m.span(), span)
 
-    LITERAL_CHARS = string.ascii_letters + string.digits + '!"%&\',/:;<=>@_`~'
+    LITERAL_CHARS = string.ascii_letters + string.digits + '!"%\',/:;<=>@_`'
 
     def test_re_escape(self):
         p = ''.join(chr(i) for i in range(256))

Misc/NEWS.d/next/Library/2017-10-05-12-45-29.bpo-30349.6zKJsF.rst

@@ -0,0 +1,3 @@
+FutureWarning is now emitted if a regular expression contains character set
+constructs that will change semantically in the future (nested sets and set
+operations).