[ALUG] text wrapping regex

Hello ALUG, I wonder if anyone can help with a regular expression (for text wrapping)? I need to match all the characters from either the beginning of the line or the last match (in global mode) up to the last space before the Xth character. I can do the first space after the Xth character: $ echo "A string with quite a lot of words and spaces in it." | sed "s/\(.\{,X\}\) /\1\n/g" Any ideas? Richard