keep _str_split

Author: yuanx749

pandas/core/strings/object_array.py

@@ -339,8 +339,14 @@ def _str_split(
             new_pat: str | re.Pattern
             if regex is True or isinstance(pat, re.Pattern):
                 new_pat = re.compile(pat)
-            else:
+            elif regex is False:
                 new_pat = pat
+            # regex is None so link to old behavior #43563
+            else:
+                if len(pat) == 1:
+                    new_pat = pat
+                else:
+                    new_pat = re.compile(pat)
 
             if isinstance(new_pat, re.Pattern):
                 if n is None or n == -1: