[alekslitvinenk]

Можно поступить так:

function convert(piece:String, pos:uint, _source:String):String{
	var rez:String = "<->";
	switch(piece){
		case "shi":
			rez = "ши";
		break;
 
		case "a":
			rez = "а";
		break;
 
		case "wa":
			rez = "ва";
		break;
 
		case "se":
			rez = "се";
		break;
	}
	return rez;
}
 
function translit(str:String):String{
	str = str.replace(new RegExp(/shi|a|wa|se/g), convert);
	return str;
}
 
trace(translit("shiwa"));