Newline is matched only when adding a bracket to s in (s)*R in Regex in Ahk?

  str .= (RegExReplace("`r`n0AAA", "`am)s*R",   "1ZZZ")) . "`r`n" ; this one fail to replace, why?
  str .= (RegExReplace("`r`n0AAA", "`am)(s)*R", "2ZZZ")) . "`r`n" ; I have to add a bracket to (s) ?

Full code:

#Requires AutoHotkey v1.1.33+
#Persistent
#SingleInstance force

^t::
  idx := 0
  str := ""
  ; s* vs (s)*
  str .= (RegExReplace("`r`n0AAA", "`am)s*R",   "1ZZZ")) . "`r`n" ; this one fail to replace, why?
  str .= (RegExReplace("`r`n0AAA", "`am)(s)*R", "2ZZZ")) . "`r`n" ; I have to add a bracket to (s) ?
  MsgBox, % str
return

/* 
Output:
---------------------------

0AAA
2ZZZ0AAA
*/

^+1:: Reload