sprintf() implementation in JavaScript

sprintf.js

function sprintf() {
    var i = 0, a, f = arguments[i++], 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] ? str_repeat(c, x) : '';
            o.push(s + (m[4] ? a + p : p + a));
        }
        else {
            throw('Huh ?!');
        }
        f = f.substring(m[0].length);
    }
    return o.join('');
}