code change based on tom review

Author: charlesdong1991

pandas/core/strings.py

@@ -459,7 +459,10 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
     regex : boolean, default None
         - If True, assumes the passed-in pattern is a regular expression.
         - If False, treats the pattern as a literal string
-        - If pat is single special character, default regex is False.
+        - If `pat` is a single character and `regex` is not specified, `pat`
+          is interpreted as a string literal. If `pat` is also a regular
+          expression symbol, a warning is issued that in the future `pat`
+          will be interpreted as a regex, rather than a literal.
         - Cannot be set to False if `pat` is a compiled regex or `repl` is
           a callable.
 
@@ -580,7 +583,7 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
                              "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):
+        if regex is None and len(pat) == 1 and re.escape(pat) != pat:
             warnings.warn("'{}' is interpreted as a literal in ".format(pat) +
                           "default, not regex. It will change in the future.",
                           FutureWarning)