Fix

Author: yuanx749

pandas/core/arrays/arrow/array.py

@@ -2579,7 +2579,7 @@ def _str_split(
             n = None
         if pat is None:
             split_func = pc.utf8_split_whitespace
-        elif regex:
+        elif regex or (regex is None and len(pat) != 1):
             split_func = functools.partial(pc.split_pattern_regex, pattern=pat)
         else:
             split_func = functools.partial(pc.split_pattern, pattern=pat)