sort dictionary by key length

Possible Duplicate:
Dictionary sorting by key length

I need to use dictionary for "search and replace". And I want that first it use longest keys.

So that

text = 'xxxx'
dict = {'xxx' : '3','xx' : '2'} 
for key in dict:
    text = text.replace(key, dict[key])

should return "3x", not "22" as it is now.

Something like

for key in sorted(dict, ???key=lambda key: len(mydict[key])):

Just can't get what is inside.
Is it possible to do in one string?