gh-109747: Improve errors for unsupported look-behind patterns (GH-109859)

Now re.error is raised instead of OverflowError or RuntimeError for
too large width of look-behind pattern.

The limit is increased to 2**32-1 (was 2**31-1).

1 files changed, 23 insertions, 0 deletions

diff --git a/Lib/test/test_re.py b/Lib/test/test_re.py
index 301d4a5..1eca22f 100644
--- a/Lib/test/test_re.py
+++ b/Lib/test/test_re.py
@@ -1861,6 +1861,29 @@ class ReTests(unittest.TestCase):
         self.assertRaises(OverflowError, re.compile, r".{%d,}?" % 2**128)
         self.assertRaises(OverflowError, re.compile, r".{%d,%d}" % (2**129, 2**128))
 
+    def test_look_behind_overflow(self):
+        string = "x" * 2_500_000
+        p1 = r"(?<=((.{%d}){%d}){%d})"
+        p2 = r"(?<!((.{%d}){%d}){%d})"
+        # Test that the templates are valid and look-behind with width 2**21
+        # (larger than sys.maxunicode) are supported.
+        self.assertEqual(re.search(p1 % (2**7, 2**7, 2**7), string).span(),
+                         (2**21, 2**21))
+        self.assertEqual(re.search(p2 % (2**7, 2**7, 2**7), string).span(),
+                         (0, 0))
+        # Test that 2**22 is accepted as a repetition number and look-behind
+        # width.
+        re.compile(p1 % (2**22, 1, 1))
+        re.compile(p1 % (1, 2**22, 1))
+        re.compile(p1 % (1, 1, 2**22))
+        re.compile(p2 % (2**22, 1, 1))
+        re.compile(p2 % (1, 2**22, 1))
+        re.compile(p2 % (1, 1, 2**22))
+        # But 2**66 is too large for look-behind width.
+        errmsg = "looks too much behind"
+        self.assertRaisesRegex(re.error, errmsg, re.compile, p1 % (2**22, 2**22, 2**22))
+        self.assertRaisesRegex(re.error, errmsg, re.compile, p2 % (2**22, 2**22, 2**22))
+
     def test_backref_group_name_in_exception(self):
         # Issue 17341: Poor error message when compiling invalid regex
         self.checkPatternError('(?P=<foo>)',