DouKaku?

import std.stdio;
import std.regexp;
import std.string;

string[] divid(uint n, string s){
    string[] ret;
    for(uint i;; i++){
        auto pat = "^" ~ format("(.{%d,%d})", i, i + 1).repeat(n) ~ "$";
        if(auto m = search(s, pat)){
            for(uint j = 1; j <= n; j++){
                ret ~= m.match(j);  // ret ~= m[j];
            }
            break;
        }
    }
    return ret;
}

void main(){
    auto sample = "ゆめよりもはかなき世のなかをなげきわびつゝあかしくらすほどに四月十よひにもなりぬれば木のしたくらがりもてゆく";
    writefln(divid(4, sample).join("\n"));
    writefln("----");
    writefln(divid(5, sample).join("\n"));
    writefln("----");
    writefln(divid(6, sample).join("\n"));
}