Another day, another script added to my library…. I had a credit card text field in one of my Sencha Touch projects that needed to be “dash separated” while the person is typing. Below is my solution to this particular minor challenge. Please remove any other validation in your project as this already features:

  • Removal of all undesired alpha characters and symbols
  • Limited to a maximum of 16 numbers
  • Plays nice when you backspace from the end

Note that you shouldn’t try to correct a number in the middle of the string – if you delete a number the entire string gets re-written for the current string of numbers. Also, the first thing that may come to mind is the HTML 5 “pattern” attribute which accepts a regular expression with which to validate the text field value, however, that would validate as you type, not format the string in the desired manner while the typing is happening. Sencha has its own way of validating text fields which is essentially the same thing but the issue is that it also doesn’t do on-the-fly string formatting.

So here is the function itself, which works perfectly in a normal web-browser (yes, a caveat for android webkit follows):

    function do_ccFormat(str){
	var nums = str.substr(0,19).replace(/[^\d]/gi,'');
	var r = nums.match(/(\d){4}/g);
	if (r){	
	    var i=0, nStr = '';
	    for (;i

However, in Android 2.3.6's webkit there is some odd behavior that is a pattern that I'm sure others somewhere have come up against. So the pattern is this: first assume that you are doing some string manipulation on the keyup event, waiting for some criteria to be met at every key stroke before manipulating the string. In the case of my CC formatter its the 5th character that triggers the string re-write. So lets say you type 5 characters such as "12345". When "5" is typed the entire string is replaced with a new one that contains a new character/delimiter - like so: "1234-5". The very next character that you type will not go at the end where the cursor is but before the last character that was entered. So if we enter "6" the string ends up like this: "1234-65". Further, if you hit the backspace key you backup not from the cursor position (which is at the end of the string) but from where the last character was erroneously entered! Weird stuff! This gets worse with every new delimiter that is added to the string.

Upon inspection and some thought, I realize that replacing the value with a formatted value as you type is something that the Android 2.x webkit can't handle when it has to then figure out where the cursor should go. It seems that programatically setting the value does not update the cursor position for the field - it stays where it is. If you pay close attention when all this happens you'll notice that the cursor will quickly jump around.

I then hit upon the idea of blurring the field (thinking to therefore dump whatever erroneous machinations may be at play) and then quickly give focus back to it knowing that the act of giving focus to a field sets the cursor to the end. This works as long as you space out the blur/focus methods with a setTimeout. I spaced them out by 100 milliseconds hoping that the soft keyboard wouldn't flash. This appeared to work well Android 2.3.6 (a Samsung Galaxy S2). The keyboard would just sit there apparently none-the-wiser.

While this did seem to work it turned out to be unpredictable. In Android 2.2 the keyboard would go away and not reappear, sometimes that would happen in 2.3.6, and it does go away in Android 3.

Its unfortunate that this doesn't work in pre-ICS Android - it is what it is. Below is how I implemented it in Sencha Touch 1 - as you can see I apply it on keyup for iOS and ICS for as-you-type formatting. For pre-ICS Android I limit the keyed cc number length to a max of 16 characters and apply CC formater when the CC field is blurred (a length of 19 is needed otherwise since we are adding 3 additional characters in the form of the "-" delimiter).

I should end by saying that this obviously does not include a MOD10 check. I'll toss in my MOD10 checker at a later date as a separate article - it will include the ability to check not only entire cc numbers - which all of the readily available MOD10 checkers do - but it will also allow you to check a CC number for its **type** based on the first four digits. Until then, here's Credit Card String Formatter sample implementation:

    maxLength:'19',//allow the 3 dashes needed to format the string to be included
            if ( || ( && > 3)){
            } else { // assume old android
            if ( && < 3){