[nerv_]

var x = "abcdef";
var z = /.(?=(.))/g;
var a = [];

while(w = z.exec(x)) a.push(w[0] + w[1]);

alert("[" + a.join(",") + "]");

var x = "abcdef", z = [];
for(var i = 0; i < x.length - 1; i++) z.push(x.substring(i, i + 2));

alert("[" + z.join(",") + "]");