BUG : Fix Series.str.contains with compiled regex on Arrow string

Author: Aniketsy

pandas/core/strings/accessor.py

@@ -1338,13 +1338,39 @@ def contains(
         4    False
         dtype: bool
         """
-        if regex and re.compile(pat).groups:
-            warnings.warn(
-                "This pattern is interpreted as a regular expression, and has "
-                "match groups. To actually get the groups, use str.extract.",
-                UserWarning,
-                stacklevel=find_stack_level(),
-            )
+        from pandas.core.dtypes.dtypes import ArrowDtype
+        import re
+
+        # --- Handle Arrow-backed string arrays with compiled regex patterns ---
+        # Arrow backend does not support compiled regex objects or Python regex flags.
+        # If a compiled regex is passed, only allow it if no flags are set.
+
+        if isinstance(self._data.dtype, ArrowDtype) and isinstance(pat, re.Pattern):
+            if flags != 0:
+                raise NotImplementedError(
+                    "Series.str.contains() with a compiled regex pattern and flag is "
+                    "not supported for Arrow-backed string arrays."
+               )
+            pat = pat.pattern
+            regex = True
+
+        if regex:
+            try:
+                _compiled = pat if isinstance(pat, re.Pattern) else re.compile(
+                    pat, flags=flags
+                )
+                if _compiled.groups:
+                    warnings.warn(
+                        "This pattern is interpreted as a regular expression, and has "
+                        "match groups. To actually get the groups, use str.extract.",
+                        UserWarning,
+                        stacklevel=find_stack_level(),
+                    )
+            except re.error as e:
+                raise ValueError(
+                    f"Invalid regex pattern passed to str.contains(): {e}"
+                ) from e
+
 
         result = self._data.array._str_contains(pat, case, flags, na, regex)
         return self._wrap_result(result, fill_value=na, returns_string=False)