RegExp (preg) - how to split long string to length,without cutting words

Hi, I am a RegExp newbie, and need help with this: i have a long string (500 / 600 chars), and need to split it in lines. Each line has a maximum length (20), and words in the line shouldn't be cut, instead the line should end at the end of the word whose last char position is <= 20. the effect is like a left align in a word processor, where lines wrap at 20, and the words are not cut. example: string= "aaaaa bbbbb ccccc ddddd eeeee ffffffffff ggggg"

(char)            1111111111222222222233333333334444444
         1234567890123456789012345678901234567890123456

regexp should output: "aaaaa bbbbb ccccc ddddd eeeee ffffffffff ggggg" Is it a sensible thing to do this job with regExp or is it better to use the usual string functions? Thanx moondog