Forms TextBox + Stopping certain text

[nelsonbrodyk]

Hey All,
I am trying to make a textbox that will only allow alphanumeric values. I accomplished this by using the key pressed event, and validating the character against a regex and saying that the character is handled if the character does not match a regex.

This works great, but it breaks in one case: Pasting into the text box. How can I accomplish my goal of stopping invalid pasting into the textbox as well? I have been trying to use the text changed event, but by that point the text seems already set.

So is there a way to do this, or is there some way to check what the new text is before approving it?

Thanks!

[cloud255]

Hey you could try to use one of the key events like below:

if (e.Control && e.KeyCode == Keys.V)
            {
                textBox1.Text = originalText;
            }

this might affect some of your other events, cant tell for sure without seeing your code.

Good luck

[balabaster]

As it would happen, I just had to do this yesterday - are you doing this in a windows application or a web application? In a windows application, you do the following:

Private Sub VarNameKeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtVarName.KeyPress
        If Not Asc(e.KeyChar) = Keys.Back Then Return 'If you stop them using delete/backspace, this is really annoying, so best let those through...
 
        Dim Pattern = "[_a-zA-Z]" 'Pattern of allowable chars
        If Not Regex.IsMatch(e.KeyChar, Pattern) Then
            e.Handled = True 'Ignore it
        End If
 
End Sub

You can get more complex than this too by checking the current length of the text, say for instance, when they're entering a postal code in Canada which requires the following pattern [A-Z][0-9][A-Z]\s?[0-9][A-Z][0-9]

By checking the current length, you could change the pattern of allowable chars, if the length is 0, only allow alphas, if the length is 1 only allow digits etc. If the length is 2, only allow alphas, automatically add a space after the entered char and then only allow numerics again.

I used this method to only allow validly formatted variable names into a text box for an expression builder I'm writing (make sure to import System.Text.RegularExpressions):

Private Sub VarNameKeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtVarName.KeyPress
 
    Dim txtObj As TextBox = CType(Sender, Textbox) 'Get the textbox
 
    If Asc(e.KeyChar) = Keys.Back Then Return 'Allow the user to delete
 
    Dim Pattern As String = Nothing
    If (txtObj.Text.Length = 0) Then
        Pattern = "[_a-zA-Z]" 'The first char must be an _ or an alpha
    Else
        Pattern = "[_a-zA-Z0-9]" 'The remaining chars can be _ or alphanumeric
    End If
 
    If Not Regex.IsMatch(e.KeyChar, Pattern) Then e.Handled = True 'Ignore It
 
End Sub

[nelsonbrodyk]

As it would happen, I just had to do this yesterday - are you doing this in a windows application or a web application? In a windows application, you do the following:

Private Sub VarNameKeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtVarName.KeyPress
        If Not Asc(e.KeyChar) = Keys.Back Then Return 'If you stop them using delete/backspace, this is really annoying, so best let those through...
 
        Dim Pattern = "[_a-zA-Z]" 'Pattern of allowable chars
        If Not Regex.IsMatch(e.KeyChar, Pattern) Then
            e.Handled = True 'Ignore it
        End If
 
End Sub

You can get more complex than this too by checking the current length of the text, say for instance, when they're entering a postal code in Canada which requires the following pattern [A-Z][0-9][A-Z]\s?[0-9][A-Z][0-9]

By checking the current length, you could change the pattern of allowable chars, if the length is 0, only allow alphas, if the length is 1 only allow digits etc. If the length is 2, only allow alphas, automatically add a space after the entered char and then only allow numerics again.

I used this method to only allow validly formatted variable names into a text box for an expression builder I'm writing (make sure to import System.Text.RegularExpressions):

Private Sub VarNameKeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtVarName.KeyPress
 
    Dim txtObj As TextBox = CType(Sender, Textbox) 'Get the textbox
 
    If Asc(e.KeyChar) = Keys.Back Then Return 'Allow the user to delete
 
    Dim Pattern As String = Nothing
    If (txtObj.Text.Length = 0) Then
        Pattern = "[_a-zA-Z]" 'The first char must be an _ or an alpha
    Else
        Pattern = "[_a-zA-Z0-9]" 'The remaining chars can be _ or alphanumeric
    End If
 
    If Not Regex.IsMatch(e.KeyChar, Pattern) Then e.Handled = True 'Ignore It
 
End Sub

This is what I have done and working. The problem is, if you copy and paste invalid data into the text box, it allows it, and it breaks the validation. I'm trying to figure out how to stop that scenario.

[balabaster]

As it would happen, I just had to do this yesterday - are you doing this in a windows application or a web application? In a windows application, you do the following:

Private Sub VarNameKeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtVarName.KeyPress
        If Not Asc(e.KeyChar) = Keys.Back Then Return 'If you stop them using delete/backspace, this is really annoying, so best let those through...
 
        Dim Pattern = "[_a-zA-Z]" 'Pattern of allowable chars
        If Not Regex.IsMatch(e.KeyChar, Pattern) Then
            e.Handled = True 'Ignore it
        End If
 
End Sub

You can get more complex than this too by checking the current length of the text, say for instance, when they're entering a postal code in Canada which requires the following pattern [A-Z][0-9][A-Z]\s?[0-9][A-Z][0-9]

By checking the current length, you could change the pattern of allowable chars, if the length is 0, only allow alphas, if the length is 1 only allow digits etc. If the length is 2, only allow alphas, automatically add a space after the entered char and then only allow numerics again.

I used this method to only allow validly formatted variable names into a text box for an expression builder I'm writing (make sure to import System.Text.RegularExpressions):

Private Sub VarNameKeyPress(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles txtVarName.KeyPress
 
    Dim txtObj As TextBox = CType(Sender, Textbox) 'Get the textbox
 
    If Asc(e.KeyChar) = Keys.Back Then Return 'Allow the user to delete
 
    Dim Pattern As String = Nothing
    If (txtObj.Text.Length = 0) Then
        Pattern = "[_a-zA-Z]" 'The first char must be an _ or an alpha
    Else
        Pattern = "[_a-zA-Z0-9]" 'The remaining chars can be _ or alphanumeric
    End If
 
    If Not Regex.IsMatch(e.KeyChar, Pattern) Then e.Handled = True 'Ignore It
 
End Sub

In a web application, you obviously have to do this client side... beware though, a user can bypass this by turning off javascript, so be aware, that it must be validated on the postback by server side code by checking the whole string for validity (if that's even a word)...

As it happens, the javascript code isn't a world different than the windows application code:

<script type="text/javascript">
  var tb = document.getElementById("TextBoxElementID");
 
  tb.keydown = function(event)
  {
    var evt = event || window.event;
    var char = String.fromCharCode(evt.keyCode);
    var allowChars;
    if(this.text.length = 0)
    {
      allowChars = "[_a-zA-Z]";
    }
    else
    {
      allowChars = "[_a-zA-Z0-9]";
    }
    var re = new RegExp(allowChars);
    if(!re.test(char))
    {
      evt.returnValue = null;
    }
}
</script>

[balabaster]

This is what I have done and working. The problem is, if you copy and paste invalid data into the text box, it allows it, and it breaks the validation. I'm trying to figure out how to stop that scenario.

Hmm, I hadn't thought of that... what I would probably do in that case is set the ShortcutsEnabled property of the textbox to false.

I'm not sure what version of .NET this property was included, it may not be there in earlier versions than 2008. However, you can neatly sidestep the situation by replacing the context menu for the textbox object with a blank context menu, thus removing the right click ability to paste via the context menu, and catch the key combinations for CTRL+V or SHIFT+INS and cancel them rendering them useless and hence turning off the ability to paste into the textbox.

[nelsonbrodyk]

Hmm, I hadn't thought of that... what I would probably do in that case is set the ShortcutsEnabled property of the textbox to false.

I'm not sure what version of .NET this property was included, it may not be there in earlier versions than 2008. However, you can neatly sidestep the situation by replacing the context menu for the textbox object with a blank context menu, thus removing the right click ability to paste via the context menu, and catch the key combinations for CTRL+V or SHIFT+INS and cancel them rendering them useless and hence turning off the ability to paste into the textbox.

Here is how I was able to handle it. Just got it working:

If the regex doesn't match, then set the text to the original text.

If the regex matches, allow it, and save the original text as the current text. Basically, you just keep the previous text as an instance variable, and revert back when you need to.

[balabaster]

Here is how I was able to handle it. Just got it working:

If the regex doesn't match, then set the text to the original text.

If the regex matches, allow it, and save the original text as the current text. Basically, you just keep the previous text as an instance variable, and revert back when you need to.

Yep, I guess that would do the job too... the marvelous thing about programming is there are many ways to skin a cat.