trouble with replace

[f pemberton]

I have a string (xdata) and theres a newline after every 17 characters
of the string. I was wondering how I can replace multiple substrings
multiple times within a string? To put it another way, this is what i
want to do.

Substring to find ("abcdef") replace it with ("highway")
search again, substring to find ("defgef") replace it with
("news").Search again and find ("effwer") replace it with
("monitor").Example string to search under is '''defgefabcdefyuuuu


  effwerbyuuuterrfr'''

I've tried using replace but its not working for me.
xdata.replace('abcdef', 'highway')
xdata.replace('defgef', 'news')
xdata.replace('effwer', 'monitor')
Basically I would want to get the output of   "newshighwayyuuuu


       monitorbyuuuterrfr"

Thanks, any help would be appreciated.