simplify the code

Author: charlesdong1991

pandas/core/strings.py

@@ -551,12 +551,6 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
         raise TypeError("repl must be a string or callable")
 
     is_compiled_re = is_re(pat)
-    if not is_compiled_re and regex is None:
-        if len(pat) == 1 and re.findall(r"\W", pat):
-            regex = False
-            warnings.warn(f"{pat} is interpreted as a literal in default, "
-                          f"not regex. The default will change in the future",
-                          FutureWarning)
     if regex:
         if is_compiled_re:
             if (case is not None) or (flags != 0):
@@ -584,6 +578,12 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
         if callable(repl):
             raise ValueError("Cannot use a callable replacement when "
                              "regex=False")
+        # if regex is default None, and a single special character is given
+        # in pat, still take it as a literal, and raise the Future warning
+        if regex is None and len(pat) == 1 and re.findall(r"\W", pat):
+            warnings.warn(f"{pat} is interpreted as a literal in default, "
+                          f"not regex. The default will change in the future",
+                          FutureWarning)
         f = lambda x: x.replace(pat, repl, n)
 
     return _na_map(f, arr)