From b3bee7194a153863abeacc2132e3d1f43c6db7fa Mon Sep 17 00:00:00 2001
From: Amandeep vishwkarma <amandeepvishwkarma07@gmail.com>
Date: Tue, 19 May 2026 22:48:27 +0530
Subject: [PATCH] fix: handle IndexError on malformed format strings in parser

- Catch IndexError during token group extraction and raise ParserMatchError instead.
- Add test case for malformed format strings lacking expected regex named groups.
- Fixes #1191
---
 arrow/parser.py      | 17 +++++++++++------
 tests/test_parser.py |  9 +++++++++
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/arrow/parser.py b/arrow/parser.py
index fc3774b09..43ca13eb3 100644
--- a/arrow/parser.py
+++ b/arrow/parser.py
@@ -422,12 +422,17 @@ def parse(
         parts: _Parts = {}
         for token in fmt_tokens:
             value: Union[Tuple[str, str, str], str]
-            if token == "Do":
-                value = match.group("value")
-            elif token == "W":
-                value = (match.group("year"), match.group("week"), match.group("day"))
-            else:
-                value = match.group(token)
+            try:
+                if token == "Do":
+                    value = match.group("value")
+                elif token == "W":
+                    value = (match.group("year"), match.group("week"), match.group("day"))
+                else:
+                    value = match.group(token)
+            except IndexError:
+                raise ParserMatchError(
+                    f"Failed to match {fmt!r} when parsing {datetime_string!r}."
+                )
 
             if value is None:
                 raise ParserMatchError(
diff --git a/tests/test_parser.py b/tests/test_parser.py
index 7038d880f..4b41a7111 100644
--- a/tests/test_parser.py
+++ b/tests/test_parser.py
@@ -779,6 +779,15 @@ def test_parse_normalize_whitespace(self):
         with pytest.raises(ParserError):
             self.parser.parse("  \n Jun   1\t 2005\n ", "MMM D YYYY")
 
+    def test_parse_malformed_fmt_no_indexerror(self):
+        """Malformed format strings should raise ParserMatchError, not IndexError."""
+        with pytest.raises(ParserMatchError):
+            self.parser.parse(
+                "foo",
+                "[-FFFFFFFFFFFFFFFFFFFFF-[-FFFFFFFFFFFF"
+                "[ |||||||| 7   v.dG(dG\\][3zasks  &",
+            )
+
 
 @pytest.mark.usefixtures("dt_parser_regex")
 class TestDateTimeParserRegex: