Underscore.js mixin that provides a 'printf'-like functionality

_formatting.js

_.mixin({
  format: function(string) {
  	var i = 1, a, f = string, o = [], m, p, c, x, s = '';
  	while (f) {
  		if (m = /^[^\x25]+/.exec(f)) {
  			o.push(m[0]);
  		}
  		else if (m = /^\x25{2}/.exec(f)) {
  			o.push('%');
  		}
  		else if (m = /^\x25(?:(\d+)\$)?(\+)?(0|'[^$])?(-)?(\d+)?(?:\.(\d+))?([b-fosuxX])/.exec(f)) {
  			
  			if (((a = arguments[m[1] || i++]) == null) || (a == undefined)) {
  				throw('Too few arguments.');
  			};
  			
  			if (/[^s]/.test(m[7]) && (typeof(a) != 'number')) {
  				throw('Expecting number but found ' + typeof(a));
  			};
  			
  			switch (m[7]) {
  				case 'b': a = a.toString(2); break;
  				case 'c': a = String.fromCharCode(a); break;
  				case 'd': a = parseInt(a); break;
  				case 'e': a = m[6] ? a.toExponential(m[6]) : a.toExponential(); break;
  				case 'f': a = m[6] ? parseFloat(a).toFixed(m[6]) : parseFloat(a); break;
  				case 'o': a = a.toString(8); break;
  				case 's': a = ((a = String(a)) && m[6] ? a.substring(0, m[6]) : a); break;
  				case 'u': a = Math.abs(a); break;
  				case 'x': a = a.toString(16); break;
  				case 'X': a = a.toString(16).toUpperCase(); break;
  			};
  			a = (/[def]/.test(m[7]) && m[2] && a >= 0 ? '+'+ a : a);
  			c = m[3] ? m[3] == '0' ? '0' : m[3].charAt(1) : ' ';
  			x = m[5] - String(a).length - s.length;
  			p = m[5] ? string.str_repeat(c, x) : '';
  			o.push(s + (m[4] ? a + p : p + a));
  		}
  		else { throw("Error"); };
  		
  		f = f.substring(m[0].length);
  	};
  	return o.join('');

    function str_repeat(i, m) {
    	for (var o = []; m > 0; o[--m] = i);
    	return o.join('');
    };
  }
});