function string_format(f, ...a) {
    if (!f.match(/^(?:(?:(?:[^{}]|(?:\{\{)|(?:\}\}))+)|(?:\{[0-9]+\}))+$/)) {
        throw new Error("Invalid format string.");
    }
    return f.replace(/((?:[^{}]|(?:\{\{)|(?:\}\}))+)|(?:\{([0-9]+)\})/g, (m, s, i) => {
        if (s) {
            return s.replace(/(?:{{)|(?:}})/g, m => m[0]);
        } else {
            if (i >= a.length) {
                throw new Error("Argument index is out of range in format.");
            }
            return a[i];
        }
    });
}