use $1 as index of an array in replace function

Question

var str = "1-2";
var arr = ["", "a", "b"];

I would like to replace 1 with "a", and 2 with "b", here is my code. But it didn't work.Pls help me out.

str = str.replace('(\d)-', arr["$1"]+"-");
str = str.replace('-(\d)', "-"+arr["$1"]);

Answer 1

Use an anonymous function:

var arr = [ /* ... */ ];

str = str.replace(/\d/, function(match) {
   return arr[match];
});

Note, that this will only replace one occurence (commenters were faster that me editing; you might want to use the g modifier for the regex in order to repeat replacing until the regex doesn't match).

The anonymous function's arguments are the full matched string and capture groups (if any).

Answer 2

var str = "1-2";
var arr = ["", "a", "b"];
str = str.replace(/(\d)-/, function (mathchedText,$1,offset,str) {
    return arr[$1] + "-"
});

str = str.replace(/-(\d)/,function (mathchedText,$1,offset,str) {
    return "-" + arr[$1]
});

document.write(str);

Here's the working code: http://jsfiddle.net/HsQC7/