Fix #61072: inconsistent fullmatch results with regex alternation

pandas/core/strings/accessor.py

@@ -1462,15 +1462,57 @@ def fullmatch(self, pat, case: bool = True, flags: int = 0, na=lib.no_default):
             matches the regular expression.
         extract : Extract matched groups.
 
-        Examples
-        --------
-        >>> ser = pd.Series(["cat", "duck", "dove"])
-        >>> ser.str.fullmatch(r"d.+")
-        0   False
-        1    True
-        2    True
-        dtype: bool
+        Notes
+        -----
+        This method enforces consistent behavior between Python's string dtype
+        and PyArrow-backed string arrays when using regular expressions
+        containing alternation (|). For regex patterns with alternation operators,
+        the method ensures proper grouping by wrapping the pattern in parentheses
+        when using PyArrow-backed string arrays.
         """
+        is_pyarrow = False
+        arr = self._data.array
+        arr_type = type(arr).__name__
+        is_pyarrow = arr_type == "ArrowStringArray"
+        if not is_pyarrow:
+            is_pyarrow = "Arrow" in arr_type
+            if not is_pyarrow and hasattr(arr, "dtype"):
+                dtype_str = str(arr.dtype)
+                is_pyarrow = (
+                    "pyarrow" in dtype_str.lower() or "arrow" in dtype_str.lower()
+                )
+        if is_pyarrow and "|" in pat:
+
+            def _is_fully_wrapped(pattern):
+                if not (pattern.startswith("(") and pattern.endswith(")")):
+                    return False
+                inner = pattern[1:-1]
+                level = 0
+                escape = False
+                in_char_class = False
+                for char in inner:
+                    if escape:
+                        escape = False
+                        continue
+                    if char == "\\":
+                        escape = True
+                    elif not in_char_class and char == "[":
+                        in_char_class = True
+                    elif in_char_class and char == "]":
+                        in_char_class = False
+                    elif not in_char_class:
+                        if char == "(":
+                            level += 1
+                        elif char == ")":
+                            if level == 0:
+                                return False
+                            level -= 1
+                return level == 0
+
+            if not (
+                pat.startswith("(") and pat.endswith(")") and _is_fully_wrapped(pat)
+            ):
+                pat = f"({pat})"
         result = self._data.array._str_fullmatch(pat, case=case, flags=flags, na=na)
         return self._wrap_result(result, fill_value=na, returns_string=False)
 

pandas/tests/strings/test_pyarrow_format_behavior.py

@@ -0,0 +1,36 @@
+import pytest
+
+from pandas import Series
+
+
+@pytest.mark.parametrize("dtype", [str])
+def test_string_array(dtype):
+    test_series = Series(["asdf", "as"], dtype=dtype)
+    regex = r"((as)|(as))"
+    regex2 = r"(as)|(as)"
+    assert list(test_series.str.fullmatch(regex)) == [False, True]
+    assert list(test_series.str.fullmatch(regex2)) == [False, True]
+
+
+@pytest.mark.parametrize(
+    "data, pattern, expected",
+    [
+        (["cat", "duck", "dove"], r"d.+", [False, True, True]),
+    ],
+)
+def test_string_match(data, pattern, expected):
+    ser = Series(data)
+    assert list(ser.str.fullmatch(pattern)) == expected
+
+
+@pytest.mark.parametrize("dtype", [str])
+@pytest.mark.parametrize(
+    "pattern, expected",
+    [
+        (r"(foo)|((as)(df)?)", [True, True, True]),
+        ("foo|as", [False, True, True]),
+    ],
+)
+def test_string_alternation_patterns(dtype, pattern, expected):
+    ser = Series(["asdf", "foo", "as"], dtype=dtype)
+    assert list(ser.str.fullmatch(pattern)) == expected