Exploring Textbox Behavior
The textbox is one of those great controls that is so simple, you sort of take it for granted that it will just always be there and always work the way you expect it to. Exactly what should you expect? Here's how an HTML textbox control behaves by default:

Keypress	Result
A letter, number, space, or other character key	A character appears in the textbox.
Backspace	Removes the character before the cursor.
Enter	Submits a form.
Delete	Removes the character after the cursor.
Home	Moves the cursor to the front of the textbox.
End	Moves the cursor to the end of the textbox.
Cursor Arrow Keys	Moves the cursor around in the textbox.
Ctrl + C	Copies the selected text to the clipboard.
Ctrl + X	Removes the highlighted text from the textbox and places it on the clipboard.
Ctrl + V	Inserts text on the clipboard into the textbox at the cursor position.
Ctrl + Z	Undoes the last change to the textbox's contents.

Wow! The textbox sure does a lot, doesn't it? The difficulty in creating a masked input control for the textbox is, of course, that you have to take into account all of these different expected behaviors. You'll want to intercept some of the behaviors, but not others. The trick is to address each of these behaviors separately, so that the final data masking doesn't have any logical holes that allow users to enter invalid data (and you know that if there is a hole, some user, somewhere, will definitely find it).

In this article, you'll create a textbox into which users can enter only numbers. To ensure its suitability for Web applications, the control must work in Netscape Navigator 4.7x, Internet Explorer 5.0+, and Netscape 6.0+.

Introduction

Blocking Keystrokes

Introduction	Exploring Textbox Behavior	Blocking Keystrokes
Dealing with Special Keys	Dealing with Cut, Copy, Paste, and Undo	Don't Forget Paste Validation
Polishing the Mask