Substring with reverse index

How can I extract "456" from "xxx_456" where xxx is of indefinite length?

slice works just fine in IE and other browsers, it's part of the specification and it's the most efficient method too:

alert("xxx_456".slice(-3));
//-> 456

slice Method (String) - MSDN
slice - Mozilla Developer Center

var str = "xxx_456";
var str_sub = str.substr(str.lastIndexOf("_")+1);

If it is not always three digits at the end (and seperated by an underscore). If the end delimiter is not always an underscore, then you could use regex:

var pat = /([0-9]{1,})$/;
var m = str.match(pat);

The substring method allows you to specify start and end index:

var str = "xxx_456";
var subStr = str.substring(str.length - 3, str.length);

you can just split it up and get the last element

var string="xxx_456";
var a=string.split("_");
alert(a[1]); #or a.pop

alert("xxxxxxxxxxx_456".substr(-3))

caveat: according to mdc, not IE compatible

Simple regex for any number of digits at the end of a string:

'xxx_456'.match(/\d+$/)[0]; //456
'xxx_4567890'.match(/\d+$/)[0]; //4567890

or use split/pop indeed:

('yyy_xxx_45678901').split(/_/).pop(); //45678901

String.prototype.reverse( ) {
     return Array.prototype.slice.call(this)
          .reverse()
          .join()
          .replace(/,/g,'')
}

using a reverse string method

var str = "xxx_456"
str = str.reverse() // 654_xxx 
str = str.substring(0,3) // 654 
str = str.reverse() //456

if your reverse method returns the string then chain the methods for a cleaner solution.

A crazy regex approach

"xxx_456".match(/...$/)[0] //456

here is my custom function

function reverse_substring(str,from,to){
  var temp="";
  var i=0;
  var pos = 0;
  var append;      
  for(i=str.length-1;i>=0;i--){
    //alert("inside loop " + str[i]);
    if(pos == from){
		 append=true;
	}
	
	if(pos == to){
		 append=false;
		 break;
	}
	if(append){
		 temp = str[i] + temp;
	}
	pos++;
  }
  alert("bottom loop " + temp);
}

var str = "bala_123";
reverse_substring(str,0,3);

This function works for reverse index.

Also you can get the result by using substring and lastIndexOf -

alert("xxx_456".substring("xxx_456".lastIndexOf("_")+1));

Although this is an old question, to support answer by user187291

In case of fixed length of desired substring I would use substr() with negative argument for its short and readable syntax

"xxx_456".substr(-3)

For now it is compatible with common browsers and not yet strictly deprecated.