Complex Regular Expression for a form validation

I have this form that validates with jquery & regular expressions...

...
//some code

var characterReg = /^([a-zA-Z0-9]{2,50})$/; //only letters and numbers

if ( !characterReg.test(inputValName) ){

			result.text('that\'s not a valid name!');

...
//more code

Now, I need a very specific validation of a field called 'NIF', wich is the identifier of a tax-paying company in my country. And I need some help to make a regular expression to use like I did in the above code.

So I found an old javascript code online that validates 'NIF':

function IsValidNIF(nif)
{
    var c;
    var checkDigit = 0;
    //Check if is not null, is numeric and if has 9 numbers
    if(nif != null && IsNumeric(nif) && nif.length == 9)
    {
        //Get the first number of NIF
        c = nif.charAt(0);
        //Check firt number is (1, 2, 5, 6, 8 or 9)
        if(c == ‘1' || c == ‘2' || c == ‘5' || c == ‘6' || c == ‘8' || c == ‘9')
        {
            //Perform CheckDigit calculations
            checkDigit = c * 9;
            var i = 0;
            for(i = 2; i <= 8; i++)
            {
                checkDigit += nif.charAt(i-1) * (10-i);
            }
            checkDigit = 11 - (checkDigit % 11);
            //if checkDigit is higher than ten set it to zero
            if(checkDigit >= 10)
                checkDigit = 0;
            //Compare checkDigit with the last number of NIF
            //If equal the NIF is Valid.            
            if(checkDigit == nif.charAt(8))
                return true;            
        }        
    }
    return false;
}

function IsNumeric(ObjVal)
{
return /^\d+$/.test(ObjVal);
}

So, is it possible to make a regular expression from this?

The rules are:

-must be only numbers
-must be 9 characters(numbers) long
-the first number must be 1 or 2 or 5 or 6 or 8 or 9
-the last number performs some validation(%module) that you should understand viewing the above code

Thank you.