do replacement evenly

[oyster]

I have some strings, and I want to write them into a text files, one
string one line
but there is a requirement: every line has a max length of a certain
number(for example, 10), so I have to replace extra SPACE*3 with
SPACE*2, at the same time, I want to make the string looks good, so,
for "I am123456line123456three"(to show the SPACE clearly, I type it
with a number), the first time, I replace the first SPACE, and get "I
am23456line123456three", then I must replace at the second SPACE
block, so I get  "I am23456line23456three", and so on, if no SPACE*3
is found, I have to aString.replace(SPACE*2, SPACE).
I hope I have stated my case clear.

Then the question is, is there a nice solution?

thanx