fix IndexError in DateTimeParser.parse with malformed format strings

arrow/parser.py

@@ -422,12 +422,21 @@ 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(

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: