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Blocking paste

main difference between the two is that

allowChars()

is looking for the

validchars

attribute and

returns

true

only if

sChar

is contained in

sValidChars

. Once again, keep in mind that, if the Ctrl key

is down, the user is performing some sort of function on the text box, so the method has to return

true

The method is even used in a similar manner, by returning a value to the

onkeypress

event handler:

<input type=”text” validchars=”0123456789”

onkeypress=”return TextUtil.allowChars(this, event)” />

<textarea rows=”10” cols=”25” validchars=”0123456789”

onkeypress=”return TextUtil.allowChars(this, event)” />

The previous text boxes only allow numerals to be entered (no spaces, letters, or anything else). This

functionality can also be used in a number of creative ways:

<input type=”text” validchars=”0123456789”

onkeypress=”return FormUtil.allow(this, event)” />

<input type=”text” validchars=”YN”

onkeypress=”return FormUtil.allow(this, event)” />

Don’t forget the paste

One aspect of text validation that most developers forget about is that the user can paste a value into the

text box. In the

blockCars()

and

allowChars()

methods, it is assumed that the user is typing in the

characters one by one, so they check each character as it comes in. When a user pastes a value, an entire

string is being placed in the text box. You have two ways to deal with validating pasted values: Either

don’t allow pasting or validate the text box when it loses focus.

Blocking paste

Blocking the user ’s capability to paste is very easy to accomplish, but you must cover all bases. A user

can paste in two ways: by clicking Paste on the text box context menu (when right-clicking on it) or by

holding the Ctrl key and pressing V.

In Internet Explorer, the solution is very simple because there is a

paste

event. If the

onpaste

event

handler prevents the default behavior, no paste works no matter how the user tries to do it:

<input type=”text” onkeypress=”return allow(this, event) “ validchars=”0123456789”

onpaste=”return false” />

For other browsers, the process is a little more involved. The first thing to do is block the context menu,

which can be accomplished by returning

false

from the

oncontextmenu

event handler.

<input type=”text” onkeypress=”return allow(this, event) “ validchars=”0123456789”

onpaste=”return false” oncontextmenu=”return false” />

Next, you need to block pasting when the user presses Ctrl and V. The part that makes this easy is that

pressing Ctrl and V causes the

keypress

event to fire, so its possible to use the

allowChars()

and

blockChars()

methods with some modifications:

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Forms and Data Integrity

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