git add whatsnew and change docstring

Author: charlesdong1991

doc/source/whatsnew/v0.24.0.rst

@@ -1667,6 +1667,7 @@ Strings
 - Bug in :meth:`Index.str.split` was not nan-safe (:issue:`23677`).
 - Bug :func:`Series.str.contains` not respecting the ``na`` argument for a ``Categorical`` dtype ``Series`` (:issue:`22158`)
 - Bug in :meth:`Index.str.cat` when the result contained only ``NaN`` (:issue:`24044`)
+- Bug in :func:`Series.str.replace` not applying regex in patterns of length 1 (:issue:`24804`)
 
 Interval
 ^^^^^^^^

pandas/core/strings.py

@@ -456,9 +456,10 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
     flags : int, default 0 (no flags)
         - re module flags, e.g. re.IGNORECASE
         - Cannot be set if `pat` is a compiled regex
-    regex : boolean, default True
+    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.
         - Cannot be set to False if `pat` is a compiled regex or `repl` is
           a callable.