Right, I was hurrying to put it up and leave.   Forgot to point out it only works for normal input files where each line ends in a newline.

Actually, your algorithm solves the problem without abandoning regexen.   Unless we're missing something, Larry was thinking along the lines of bart's logic above, that you have to know the new length of the line up to this point.   But you are just depending on the text in front of the present tab(s) to be aligned either with the beginning of the line or the previous tab stop, so there is no need to consider the entire preceding line segment:

$ perl -we'$_="\t\tYou\tare\tmaking\t\ttoo\tmany\tassumptions...\t\t";

  s/(^|[^\t]+)(\t+)/$1." " x (length($2) * 8 - (length($1) & 7))/ge;
print'
                You     are     making          too     many    assump
+tions...          $ 



( s/(.*?)(\t+)/...
[download]

is simpler but presumably less efficient (which was your initial concern).   However, note that it will work correctly on a multiline string:

$ perl -we'$_="\t\tYou\tare\tmaking\t\ttoo\n\tmany\tassumptions...\t";

  s/(.*?)(\t+)/$1." "x(length($2)*8-(length($1)&7))/ge;
print'
                You     are     making          too
        many    assumptions...  $
[download]

  p