-define(TABSTOPS,4).

expand_tabs(Str) ->
    expand_tabs("", Str).

expand_tabs(Accum, Str) ->
    Tabloc = string:chr(Str, $\t),
    case Tabloc of
        0 -> Accum ++ Str;
        _ -> UpToTab = string:substr(Str, 1, Tabloc - 1),
             AfterTab = string:substr(Str, Tabloc + 1),
             Extra = (Tabloc - 1) rem ?TABSTOPS,
             SpaceCount = case Extra of
                              0 -> ?TABSTOPS;
                              _ -> ?TABSTOPS - Extra
                          end,
             expand_tabs(Accum ++ UpToTab ++
               string:chars($ , SpaceCount), AfterTab)
    end.

1> cookbook:expand_tabs("12345\t678\t9").             
"12345   678 9"
2> cookbook:expand_tabs("this is\ta test.").          
"this is a test."
3> cookbook:expand_tabs("this is \ta test.").
"this is     a test."
4> cookbook:expand_tabs("this is  \ta test.").
"this is     a test."
5> cookbook:expand_tabs("this is   \ta test.").
"this is     a test."
6> cookbook:expand_tabs("this is    \ta test.").
"this is     a test."
7> cookbook:expand_tabs("this is     \ta test.").
"this is         a test."