I needed to sort a file according to the number of words in each line. Using the :s command, I inserted the number of words each line contains, at the beginning of the line. To simplify sorting (using the excellent VisSort), instead of inserting a number, I put the corresponding character (number plus 64, so a line with one word will start with 'A'). Here is the expression I used: :%s/^.*/\=nr2char(strlen((substitute(substitute(submatch(0), "\\S\\+", "x", "g"), "\\s","","g")))+64) . " " . submatch(0) Explanation, from the inside out: part1 = substitute(submatch(0), "\\S\\+", "x", "g")