Issue #25953: re.sub() now raises an error for invalid numerical group

reference in replacement template even if the pattern is not found in
the string.  Error message for invalid group reference now includes the
group index and the position of the reference.
Based on patch by SilentGhost.

3 files changed, 38 insertions, 29 deletions

diff --git a/Lib/sre_parse.py b/Lib/sre_parse.py
index 3d38673c32..ab37fd3fe2 100644
--- a/Lib/sre_parse.py
+++ b/Lib/sre_parse.py
@@ -395,7 +395,7 @@ def _escape(source, escape, state):
                                        len(escape))
                 state.checklookbehindgroup(group, source)
                 return GROUPREF, group
-            raise source.error("invalid group reference", len(escape))
+            raise source.error("invalid group reference %d" % group, len(escape) - 1)
         if len(escape) == 2:
             if c in ASCIILETTERS:
                 raise source.error("bad escape %s" % escape, len(escape))
@@ -725,8 +725,8 @@ def _parse(source, state, verbose):
                             raise source.error("bad group number",
                                                len(condname) + 1)
                         if condgroup >= MAXGROUPS:
-                            raise source.error("invalid group reference",
-                                               len(condname) + 1)
+                            msg = "invalid group reference %d" % condgroup
+                            raise source.error(msg, len(condname) + 1)
                     state.checklookbehindgroup(condgroup, source)
                 elif char in FLAGS or char == "-":
                     # flags
@@ -883,7 +883,9 @@ def parse_template(source, pattern):
     literals = []
     literal = []
     lappend = literal.append
-    def addgroup(index):
+    def addgroup(index, pos):
+        if index > pattern.groups:
+            raise s.error("invalid group reference %d" % index, pos)
         if literal:
             literals.append(''.join(literal))
             del literal[:]
@@ -916,9 +918,9 @@ def parse_template(source, pattern):
                         raise s.error("bad character in group name %r" % name,
                                       len(name) + 1) from None
                     if index >= MAXGROUPS:
-                        raise s.error("invalid group reference",
+                        raise s.error("invalid group reference %d" % index,
                                       len(name) + 1)
-                addgroup(index)
+                addgroup(index, len(name) + 1)
             elif c == "0":
                 if s.next in OCTDIGITS:
                     this += sget()
@@ -939,7 +941,7 @@ def parse_template(source, pattern):
                                           'range 0-0o377' % this, len(this))
                         lappend(chr(c))
                 if not isoctal:
-                    addgroup(int(this[1:]))
+                    addgroup(int(this[1:]), len(this) - 1)
             else:
                 try:
                     this = chr(ESCAPES[this][1])
@@ -966,5 +968,5 @@ def expand_template(template, match):
         for index, group in groups:
             literals[index] = g(group) or empty
     except IndexError:
-        raise error("invalid group reference")
+        raise error("invalid group reference %d" % index)
     return empty.join(literals)
diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index 4cdc59127a..3bd6d7b461 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -5,7 +5,6 @@ import locale
 import re
 from re import Scanner
 import sre_compile
-import sre_constants
 import sys
 import string
 import traceback
@@ -186,18 +185,19 @@ class ReTests(unittest.TestCase):
                                 r'octal escape value \777 outside of '
                                 r'range 0-0o377', 0)
 
-        self.checkTemplateError('x', r'\1', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\8', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\9', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\11', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\18', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\1a', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\90', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\99', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\118', 'x', 'invalid group reference') # r'\11' + '8'
-        self.checkTemplateError('x', r'\11a', 'x', 'invalid group reference')
-        self.checkTemplateError('x', r'\181', 'x', 'invalid group reference') # r'\18' + '1'
-        self.checkTemplateError('x', r'\800', 'x', 'invalid group reference') # r'\80' + '0'
+        self.checkTemplateError('x', r'\1', 'x', 'invalid group reference 1', 1)
+        self.checkTemplateError('x', r'\8', 'x', 'invalid group reference 8', 1)
+        self.checkTemplateError('x', r'\9', 'x', 'invalid group reference 9', 1)
+        self.checkTemplateError('x', r'\11', 'x', 'invalid group reference 11', 1)
+        self.checkTemplateError('x', r'\18', 'x', 'invalid group reference 18', 1)
+        self.checkTemplateError('x', r'\1a', 'x', 'invalid group reference 1', 1)
+        self.checkTemplateError('x', r'\90', 'x', 'invalid group reference 90', 1)
+        self.checkTemplateError('x', r'\99', 'x', 'invalid group reference 99', 1)
+        self.checkTemplateError('x', r'\118', 'x', 'invalid group reference 11', 1)
+        self.checkTemplateError('x', r'\11a', 'x', 'invalid group reference 11', 1)
+        self.checkTemplateError('x', r'\181', 'x', 'invalid group reference 18', 1)
+        self.checkTemplateError('x', r'\800', 'x', 'invalid group reference 80', 1)
+        self.checkTemplateError('x', r'\8', '', 'invalid group reference 8', 1)
 
         # in python2.3 (etc), these loop endlessly in sre_parser.py
         self.assertEqual(re.sub('(((((((((((x)))))))))))', r'\11', 'x'), 'x')
@@ -271,9 +271,9 @@ class ReTests(unittest.TestCase):
         self.checkTemplateError('(?P<a>x)', r'\g<1a1>', 'xx',
                                 "bad character in group name '1a1'", 3)
         self.checkTemplateError('(?P<a>x)', r'\g<2>', 'xx',
-                                'invalid group reference')
+                                'invalid group reference 2', 3)
         self.checkTemplateError('(?P<a>x)', r'\2', 'xx',
-                                'invalid group reference')
+                                'invalid group reference 2', 1)
         with self.assertRaisesRegex(IndexError, "unknown group name 'ab'"):
             re.sub('(?P<a>x)', r'\g<ab>', 'xx')
         self.assertEqual(re.sub('(?P<a>x)|(?P<b>y)', r'\g<b>', 'xx'), '')
@@ -558,10 +558,11 @@ class ReTests(unittest.TestCase):
                                'two branches', 10)
 
     def test_re_groupref_overflow(self):
-        self.checkTemplateError('()', r'\g<%s>' % sre_constants.MAXGROUPS, 'xx',
-                                'invalid group reference', 3)
-        self.checkPatternError(r'(?P<a>)(?(%d))' % sre_constants.MAXGROUPS,
-                               'invalid group reference', 10)
+        from sre_constants import MAXGROUPS
+        self.checkTemplateError('()', r'\g<%s>' % MAXGROUPS, 'xx',
+                                'invalid group reference %d' % MAXGROUPS, 3)
+        self.checkPatternError(r'(?P<a>)(?(%d))' % MAXGROUPS,
+                               'invalid group reference %d' % MAXGROUPS, 10)
 
     def test_re_groupref(self):
         self.assertEqual(re.match(r'^(\|)?([^()]+)\1$', '|a|').groups(),
@@ -1007,7 +1008,7 @@ class ReTests(unittest.TestCase):
         self.checkPatternError(r"\567",
                                r'octal escape value \567 outside of '
                                r'range 0-0o377', 0)
-        self.checkPatternError(r"\911", 'invalid group reference', 0)
+        self.checkPatternError(r"\911", 'invalid group reference 91', 1)
         self.checkPatternError(r"\x1", r'incomplete escape \x1', 0)
         self.checkPatternError(r"\x1z", r'incomplete escape \x1', 0)
         self.checkPatternError(r"\u123", r'incomplete escape \u123', 0)
@@ -1061,7 +1062,7 @@ class ReTests(unittest.TestCase):
         self.checkPatternError(br"\567",
                                r'octal escape value \567 outside of '
                                r'range 0-0o377', 0)
-        self.checkPatternError(br"\911", 'invalid group reference', 0)
+        self.checkPatternError(br"\911", 'invalid group reference 91', 1)
         self.checkPatternError(br"\x1", r'incomplete escape \x1', 0)
         self.checkPatternError(br"\x1z", r'incomplete escape \x1', 0)
 
diff --git a/Misc/NEWS b/Misc/NEWS
index cec6306f68..fb14c00400 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -16,6 +16,12 @@ Core and Builtins
 - Issue #23782: Fixed possible memory leak in _PyTraceback_Add() and exception
   loss in PyTraceBack_Here().
 
+- Issue #25953: re.sub() now raises an error for invalid numerical group
+  reference in replacement template even if the pattern is not found in
+  the string.  Error message for invalid group reference now includes the
+  group index and the position of the reference.
+  Based on patch by SilentGhost.
+
 - Issue #28183: Optimize and cleanup dict iteration.
 
 - Issue #26081: Added C implementation of asyncio.Future.