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).

Lib/re/_compiler.py

@@ -147,6 +147,8 @@ def _compile(code, pattern, flags):
                 emit(0) # look ahead
             else:
                 lo, hi = av[1].getwidth()
+                if lo > MAXCODE:
+                    raise error("looks too much behind")
                 if lo != hi:
                     raise error("look-behind requires fixed-width pattern")
                 emit(lo) # look behind
@@ -547,7 +549,7 @@ def _compile_info(code, pattern, flags):
     else:
         emit(MAXCODE)
         prefix = prefix[:MAXCODE]
-    emit(min(hi, MAXCODE))
+    emit(hi)
     # add literal prefix
     if prefix:
         emit(len(prefix)) # length

Lib/re/_parser.py

@@ -67,6 +67,10 @@
 TYPE_FLAGS = SRE_FLAG_ASCII | SRE_FLAG_LOCALE | SRE_FLAG_UNICODE
 GLOBAL_FLAGS = SRE_FLAG_DEBUG
 
+# Maximal value returned by SubPattern.getwidth().
+# Must be larger than MAXREPEAT, MAXCODE and sys.maxsize.
+MAXWIDTH = 1 << 64
+
 class State:
     # keeps track of state for parsing
     def __init__(self):
@@ -177,7 +181,7 @@ def getwidth(self):
         lo = hi = 0
         for op, av in self.data:
             if op is BRANCH:
-                i = MAXREPEAT - 1
+                i = MAXWIDTH
                 j = 0
                 for av in av[1]:
                     l, h = av.getwidth()
@@ -196,6 +200,9 @@ def getwidth(self):
             elif op in _REPEATCODES:
                 i, j = av[2].getwidth()
                 lo = lo + i * av[0]
+                if av[1] == MAXREPEAT and j:
+                    hi = MAXWIDTH
+                else:
                     hi = hi + j * av[1]
             elif op in _UNITCODES:
                 lo = lo + 1
@@ -216,7 +223,7 @@ def getwidth(self):
                 hi = hi + j
             elif op is SUCCESS:
                 break
-        self.width = min(lo, MAXREPEAT - 1), min(hi, MAXREPEAT)
+        self.width = min(lo, MAXWIDTH), min(hi, MAXWIDTH)
         return self.width
 
 class Tokenizer:

Lib/test/test_re.py

@@ -1861,6 +1861,29 @@ def test_repeat_minmax_overflow(self):
         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>)',

Misc/NEWS.d/next/Library/2023-09-25-20-05-41.gh-issue-109747._cRJH8.rst

@@ -0,0 +1,3 @@
+Improve errors for unsupported look-behind patterns. Now re.error is raised
+instead of OverflowError or RuntimeError for too large width of look-behind
+pattern.

Modules/_sre/sre.c

@@ -2070,8 +2070,6 @@ _validate_inner(SRE_CODE *code, SRE_CODE *end, Py_ssize_t groups)
             GET_SKIP;
             GET_ARG; /* 0 for lookahead, width for lookbehind */
             code--; /* Back up over arg to simplify math below */
-            if (arg & 0x80000000)
-                FAIL; /* Width too large */
             /* Stop 1 before the end; we check the SUCCESS below */
             if (_validate_inner(code+1, code+skip-2, groups))
                 FAIL;

Modules/_sre/sre_lib.h

@@ -591,8 +591,8 @@ SRE(match)(SRE_STATE* state, const SRE_CODE* pattern, int toplevel)
         /* optimization info block */
         /* <INFO> <1=skip> <2=flags> <3=min> ... */
         if (pattern[3] && (uintptr_t)(end - ptr) < pattern[3]) {
-            TRACE(("reject (got %zd chars, need %zd)\n",
+            TRACE(("reject (got %tu chars, need %zu)\n",
-                   end - ptr, (Py_ssize_t) pattern[3]));
+                   end - ptr, (size_t) pattern[3]));
             RETURN_FAILURE;
         }
         pattern += pattern[1] + 1;
@@ -1509,7 +1509,7 @@ SRE(match)(SRE_STATE* state, const SRE_CODE* pattern, int toplevel)
             /* <ASSERT> <skip> <back> <pattern> */
             TRACE(("|%p|%p|ASSERT %d\n", pattern,
                    ptr, pattern[1]));
-            if (ptr - (SRE_CHAR *)state->beginning < (Py_ssize_t)pattern[1])
+            if ((uintptr_t)(ptr - (SRE_CHAR *)state->beginning) < pattern[1])
                 RETURN_FAILURE;
             state->ptr = ptr - pattern[1];
             DO_JUMP0(JUMP_ASSERT, jump_assert, pattern+2);
@@ -1522,7 +1522,7 @@ SRE(match)(SRE_STATE* state, const SRE_CODE* pattern, int toplevel)
             /* <ASSERT_NOT> <skip> <back> <pattern> */
             TRACE(("|%p|%p|ASSERT_NOT %d\n", pattern,
                    ptr, pattern[1]));
-            if (ptr - (SRE_CHAR *)state->beginning >= (Py_ssize_t)pattern[1]) {
+            if ((uintptr_t)(ptr - (SRE_CHAR *)state->beginning) >= pattern[1]) {
                 state->ptr = ptr - pattern[1];
                 LASTMARK_SAVE();
                 if (state->repeat)
@@ -1658,9 +1658,9 @@ SRE(search)(SRE_STATE* state, SRE_CODE* pattern)
 
         flags = pattern[2];
 
-        if (pattern[3] && end - ptr < (Py_ssize_t)pattern[3]) {
+        if (pattern[3] && (uintptr_t)(end - ptr) < pattern[3]) {
-            TRACE(("reject (got %u chars, need %u)\n",
+            TRACE(("reject (got %tu chars, need %zu)\n",
-                   (unsigned int)(end - ptr), pattern[3]));
+                   end - ptr, (size_t) pattern[3]));
             return 0;
         }
         if (pattern[3] > 1) {