[3.6] bpo-30298: Weaken the condition of deprecation warnings for inline modifiers. (GH-1490) (#1525)

Now allowed several subsequential inline modifiers at the start of the
pattern (e.g. '(?i)(?s)...').  In verbose mode whitespaces and comments
now are allowed before and between inline modifiers (e.g.
'(?x) (?i) (?s)...')..
(cherry picked from commit 305ccbe27ea5ba82fd2d8c32ec739f980e524330)
diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index 6aa49c3..e1c7496 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -412,7 +412,7 @@
     sourcematch = source.match
     start = source.tell()
     while True:
-        itemsappend(_parse(source, state, verbose))
+        itemsappend(_parse(source, state, verbose, not nested and not items))
         if not sourcematch("|"):
             break
 
@@ -466,7 +466,7 @@
     subpattern.append((GROUPREF_EXISTS, (condgroup, item_yes, item_no)))
     return subpattern
 
-def _parse(source, state, verbose):
+def _parse(source, state, verbose, first=False):
     # parse a simple pattern
     subpattern = SubPattern(state)
 
@@ -730,10 +730,9 @@
                     state.checklookbehindgroup(condgroup, source)
                 elif char in FLAGS or char == "-":
                     # flags
-                    pos = source.pos
                     flags = _parse_flags(source, state, char)
                     if flags is None:  # global flags
-                        if pos != 3:  # "(?x"
+                        if not first or subpattern:
                             import warnings
                             warnings.warn(
                                 'Flags not at the start of the expression %s%s' % (
@@ -742,6 +741,8 @@
                                 ),
                                 DeprecationWarning, stacklevel=7
                             )
+                        if (state.flags & SRE_FLAG_VERBOSE) and not verbose:
+                            raise Verbose
                         continue
                     add_flags, del_flags = flags
                     group = None
@@ -795,9 +796,6 @@
                 msg = "unknown flag" if char.isalpha() else "missing -, : or )"
                 raise source.error(msg, len(char))
     if char == ")":
-        if ((add_flags & SRE_FLAG_VERBOSE) and
-            not (state.flags & SRE_FLAG_VERBOSE)):
-            raise Verbose
         state.flags |= add_flags
         return None
     if add_flags & GLOBAL_FLAGS:
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index b945cf0..c52417b 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -1303,32 +1303,43 @@
         upper_char = '\u1ea0' # Latin Capital Letter A with Dot Below
         lower_char = '\u1ea1' # Latin Small Letter A with Dot Below
 
-        p = re.compile(upper_char, re.I | re.U)
-        q = p.match(lower_char)
+        p = re.compile('.' + upper_char, re.I | re.S)
+        q = p.match('\n' + lower_char)
         self.assertTrue(q)
 
-        p = re.compile(lower_char, re.I | re.U)
-        q = p.match(upper_char)
+        p = re.compile('.' + lower_char, re.I | re.S)
+        q = p.match('\n' + upper_char)
         self.assertTrue(q)
 
-        p = re.compile('(?i)' + upper_char, re.U)
-        q = p.match(lower_char)
+        p = re.compile('(?i).' + upper_char, re.S)
+        q = p.match('\n' + lower_char)
         self.assertTrue(q)
 
-        p = re.compile('(?i)' + lower_char, re.U)
-        q = p.match(upper_char)
+        p = re.compile('(?i).' + lower_char, re.S)
+        q = p.match('\n' + upper_char)
         self.assertTrue(q)
 
-        p = re.compile('(?iu)' + upper_char)
-        q = p.match(lower_char)
+        p = re.compile('(?is).' + upper_char)
+        q = p.match('\n' + lower_char)
         self.assertTrue(q)
 
-        p = re.compile('(?iu)' + lower_char)
-        q = p.match(upper_char)
+        p = re.compile('(?is).' + lower_char)
+        q = p.match('\n' + upper_char)
         self.assertTrue(q)
 
-        self.assertTrue(re.match('(?ixu) ' + upper_char, lower_char))
-        self.assertTrue(re.match('(?ixu) ' + lower_char, upper_char))
+        p = re.compile('(?s)(?i).' + upper_char)
+        q = p.match('\n' + lower_char)
+        self.assertTrue(q)
+
+        p = re.compile('(?s)(?i).' + lower_char)
+        q = p.match('\n' + upper_char)
+        self.assertTrue(q)
+
+        self.assertTrue(re.match('(?ix) ' + upper_char, lower_char))
+        self.assertTrue(re.match('(?ix) ' + lower_char, upper_char))
+        self.assertTrue(re.match(' (?i) ' + upper_char, lower_char, re.X))
+        self.assertTrue(re.match('(?x) (?i) ' + upper_char, lower_char))
+        self.assertTrue(re.match(' (?x) (?i) ' + upper_char, lower_char, re.X))
 
         p = upper_char + '(?i)'
         with self.assertWarns(DeprecationWarning) as warns:
@@ -1346,6 +1357,26 @@
             'Flags not at the start of the expression %s (truncated)' % p[:20]
         )
 
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(re.match('(?s).(?i)' + upper_char, '\n' + lower_char))
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(re.match('(?i) ' + upper_char + ' (?x)', lower_char))
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(re.match(' (?x) (?i) ' + upper_char, lower_char))
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(re.match('^(?i)' + upper_char, lower_char))
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(re.match('$|(?i)' + upper_char, lower_char))
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(re.match('(?:(?i)' + upper_char + ')', lower_char))
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(re.fullmatch('(^)?(?(1)(?i)' + upper_char + ')',
+                                         lower_char))
+        with self.assertWarns(DeprecationWarning):
+            self.assertTrue(re.fullmatch('($)?(?(1)|(?i)' + upper_char + ')',
+                                         lower_char))
+
+
     def test_dollar_matches_twice(self):
         "$ matches the end of string, and just before the terminating \n"
         pattern = re.compile('$')
diff --git a/Misc/NEWS b/Misc/NEWS
index 64974ef..e7090fc 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -36,6 +36,12 @@
 Library
 -------
 
+- bpo-30298: Weaken the condition of deprecation warnings for inline modifiers.
+  Now allowed several subsequential inline modifiers at the start of the
+  pattern (e.g. ``'(?i)(?s)...'``).  In verbose mode whitespaces and comments
+  now are allowed before and between inline modifiers (e.g.
+  ``'(?x) (?i) (?s)...'``).
+
 - bpo-29990: Fix range checking in GB18030 decoder.  Original patch by Ma Lin.
 
 - Revert bpo-26293 for zipfile breakage. See also bpo-29094.