str1="2017-10-15 this is happy day...">>> re.sub("(\d{4})-(\d{2})-(\d{2})",r"\2/\3/\1",str1)##'10/15/2017 this is happy day...'>>> re.sub("(?P
     
      \d{4})-(?P
      
       \d{2})-(?P
       
        \d{2})",r"\g
        
         /\g
         
          /\g
          
           ",str1)#'10/15/2017 this is happy day...'