replaceVarsUnsafe.js

var replaceVarsUnsafe = function(vars, str, sep) {
	sep = sep||'%';
	for (var i in vars) str = str.replace(new RegExp('('+sep+i+sep+')+', 'g'), vars[i]);
	return str;
};

var s = "/home/%user%/somedir/%dir1%/foo/%dir2%/%dir1%/file.%ext%";

r = replaceVarsUnsafe({
	user: "alice",
	dir1: "aaaaaaa",
	dir2: "bbbbbbb",
	ext: "png"
}, s);

console.log(r);