Some special characters are not restricted using Regex in KeyDown event in WPF

Question

I'm trying to restrict all characters other than English alphabets, but I'm still able to enter some naughty characters which is not good! How can I prevent that. These naughty characters not caught by my regex are - _ + = ? < > '.

private void AlphaOnlyTextBox_OnKeyDown(object sender, KeyEventArgs e)
{    
    var restrictedChars = new Regex(@"[^a-zA-Z\s]");

    var match = restrictedChars.Match(e.Key.ToString());

    // Check for a naughty character in the KeyDown event.
    if (match.Success)
    {
        // Stop the character from being entered into the control since it is illegal.
        e.Handled = true;
    }
}

<Window x:Class="WpfApplication4.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <Grid>

        <TextBox Height="21"
                 Width="77" 
                 MaxLength="2"
                 KeyDown="AlphaOnlyTextBox_OnKeyDown"
                                           >
        </TextBox>
    </Grid>
</Window>

The regex seems to catch those (as tested on regexr.com). Are you sure the problem isn't elsewhere? — rtmh, Jul 11 '16 at 18:04
Yes, the problem [must be somewhere else](http://regexstorm.net/tester?p=%5b%5ea-zA-Z%5cs%5d&i=-+_+%2b+%3d+%3f+%3c+%3e+%27). — Wiktor Stribiżew, Jul 11 '16 at 18:05
@rtmh You can build the question easily in WPF/C#. It doesn't work for some odd reason! — Vahid, Jul 11 '16 at 18:07
@rtmh One suggestion! Maybe it has something to do with pressing the Shift button! Because we need to press that to enter +? — Vahid, Jul 11 '16 at 18:11
I wouldn't say we can build it *easily*. As for that issue, look into the specifics of the KeyEventArgs::Key functions, does it return what you expect? — rtmh, Jul 11 '16 at 18:13
Indeed the Key object seems to represent only a single key so you are right about the area of your problem. I am unfamiliar with the environment you are using, but I suspect there is a method for character input checking, if not as a feature of the API, then as a best practice that you may be able to find on SO. — rtmh, Jul 11 '16 at 18:18
Honestly I was half way through an answer but couldn't figure out the MSDN documentation in time. These pages might be helpful, however: [first](https://msdn.microsoft.com/en-us/library/ms171549(v=vs.110).aspx), [second](https://msdn.microsoft.com/en-us/library/system.windows.forms.control.keypress(v=vs.110).aspx), [third](https://msdn.microsoft.com/en-us/library/system.windows.forms.control.keydown(v=vs.110).aspx) — rtmh, Jul 11 '16 at 18:32
I am not sure, but perhaps, [this one](http://stackoverflow.com/questions/1458748/wpf-onkeydown-not-being-called-for-space-key-in-control-derived-from-wpf-text) is related. — Wiktor Stribiżew, Jul 11 '16 at 18:32
`@"^[a-zA-Z]+$"` should work. It assumes the Latin Alphabet. Taken from: http://stackoverflow.com/questions/1181419/verifying-that-a-string-contains-only-letters-in-c-sharp — Adam, Jul 11 '16 at 18:36

score 2 · Answer 1 · answered Jul 11 '16 at 18:35

2

Try this expression:

var restrictedChars = new Regex(@"[^(\W_0-9)]+");

It will exclude everything but large and small letter characters (not depending on a spceific language).

Hope it helps!

answered Jul 11 '16 at 18:35

Itay Podhajcer

2,616
2
9
14

Thanks but it excludes everything! Even the normal alphabets! – Vahid Jul 11 '16 at 18:40
Make sure you are using an upper case '**W**' in the expression, because a lower case '**w**' might cause the problem you have with the expression in my answer. – Itay Podhajcer Jul 11 '16 at 18:52
Thanks Italy, it seems the problem is somewhere else, see the answer below. – Vahid Jul 11 '16 at 18:53

Vahid · Accepted Answer · 2016-07-11T19:03:06.173

After so much head-scratching I realized that for some reasons unknown to me, KeyDown event doesn't capture certain characters, but PreviewTextInput does!

    private void UIElement_OnPreviewTextInput(object sender, TextCompositionEventArgs e)
    {
        var restrictedChars = new Regex(@"[^a-zA-Z\s]");

        var match = restrictedChars.Match(e.Text);

        // Check for a naughty character in the KeyDown event.
        if (match.Success)
        {
            // Stop the character from being entered into the control since it is illegal.
            e.Handled = true;
        }
    }

    <TextBox Height="21"
             Width="77" 
             MaxLength="2"
             PreviewTextInput="UIElement_OnPreviewTextInput"
                                       >
    </TextBox>

If you also want to disable Space button:

    <TextBox Height="21"
             Width="77" 
             MaxLength="2"
             PreviewTextInput="UIElement_OnPreviewTextInput"
             PreviewKeyDown="UIElement_OnKeyDown"
                                       >
    </TextBox>

    private void UIElement_OnKeyDown(object sender, KeyEventArgs e)
    {
        if (e.Key == Key.Space)
        {
            e.Handled = true;
        }
    }

I knew there must have been something better, Good find. :) – rtmh Jul 11 '16 at 18:54 — rtmh, Jul 11 '16 at 18:54

Some special characters are not restricted using Regex in KeyDown event in WPF

2 Answers2

Linked