Whitespace character is not restricted using Regex in KeyDown event in WPF

Question

I'm trying to restrict the first character other than English alphabet, but I'm still able to enter whitespace as first character which is not what I want. How can I prevent the first character from beeing a whitespace during the typing?

private void TbxCode_PreviewTextInput(object sender, TextCompositionEventArgs e)
{
    System.Text.RegularExpressions.Regex myRegex = new System.Text.RegularExpressions.Regex(@"^[a-zA-Z]$");

    if (tbxCode.Text.Length == 0)
    {
        if (!myRegex.IsMatch(e.Text))
        {
            e.Handled = true;
        }
    }
}

<Window x:Class="WpfApp"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:cgmsui="clr-namespace:ClassGetMSUI;assembly=ClassGetMSUI">

        <Grid>
            <TextBox Name="tbxCode"
                     Width="100"
                     Height="20"
                     PreviewTextInput="TbxCodeHoraire_PreviewTextInput" />
        </Grid>
</Window>

Note the issue is not the regex, `^[a-zA-Z]$` only matches a single ASCII letter. — Wiktor Stribiżew, Jul 21 '22 at 09:28

score 0 · Answer 1 · answered Jul 21 '22 at 11:08

0

if (tbxCode.Text.Length == 0) you're only handling the regex when there is nothing in the text box, you should change it to if (tbxCode.Text.Length != 0).

Also, your regex only allow a single character in the text box. If you want to validate only the fisrt character, it should be @"^[a-zA-Z]";

answered Jul 21 '22 at 11:08

Magnetron

7,495
1
25
41

I tried to change with your modifications but nothing happend. I still can add a whitespace as first character. – LaurentMts Jul 21 '22 at 11:32

score 0 · Answer 2 · answered Jul 21 '22 at 13:25

Here is my solution

private void TbxCode_PreviewKeyDown(object sender, KeyEventArgs e)
    {
        System.Text.RegularExpressions.Regex myRegex = new System.Text.RegularExpressions.Regex(@"^[a-zA-Z]$");
        if (e.Key == Key.Space)
        {
            if (tbxCode.Text.Length == 0)
            {
                if (!myRegex.IsMatch(e.Key.ToString()))
                {
                    e.Handled = true;
                }
            }
        }
    }

Thanks to this article : Why does PreviewTextInput not handle spaces?

Whitespace character is not restricted using Regex in KeyDown event in WPF

2 Answers2