The field content is always treated like a string even if its a number.

Therefors you can use string function to check the length of the number, ie, that it's six digits long and you can check that the sub-string, ie, the first character is one of the required numbers.

You need a combination of:

var checkStr = new String(document.getElementById('fieldID').value);

checkStr.length == 6;

checkStr.charAt(checkStr.0) == 1;

and so forth.