This WPF small project will demonstrate how to create a TextBox that only accepts alpha characters and spaces. We’ve created a TextBox, given it a Name and created a PreviewTextInput event for it.

Below is the XAMLcode.

<Window x:Class="ValidatingAlphaCharaters.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:ValidatingAlphaCharaters"
        mc:Ignorable="d"
        WindowStartupLocation="CenterScreen"
        Title="ValidatingAlphaCharaters" Height="300" Width="400">
    <StackPanel Background="#A4C2F8C3">
        <TextBlock Margin="6,6,6,6" TextWrapping="Wrap">Limit what the user can type into a TextBox! In this example we will only allow
        <Run FontWeight="Bold">alphabetical characters</Run> to be typed in. That is a to z in both upper and lower case, as well as spaces.
        We will not allow the Tab key or the Return key (to create a new line). We do not allow any numbers or other special
        characters on the keyboard such as the following: ! @ # $ % ^ &amp; * ( ) _ +  or any others.</TextBlock>
        <TextBox Name="txtbxAlpha" Margin="6,6,6,6" Width="280" Height="52" TextWrapping="Wrap"
                  PreviewTextInput="TxtbxAlpha_PreviewTextInput"  MaxLength="500"></TextBox>
        <TextBlock Margin="6,6,6,6" TextWrapping="Wrap">However, the use can still paste non-alpha characters into the above text box.
        Below is the Regex in the code behind. </TextBlock>
        <TextBlock Margin="6,6,6,6"><Run FontSize="14" FontWeight="Bold">Regex(@"^[\p{Lu}\p{Ll}]+$")</Run></TextBlock>
    </StackPanel>
</Window>

Below is the C# code behind.

using System.Text.RegularExpressions;
using System.Windows;
using System.Windows.Input;

namespace ValidatingAlphaCharaters
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
        private void TxtbxAlpha_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            e.Handled = !IsTextAllowed(e.Text);
        }
        private static readonly Regex rgx = new Regex(@"^[\p{Lu}\p{Ll}]+$");
        private static bool IsTextAllowed(string text)
        {
            return rgx.IsMatch(text);
        }
    }
}

Below is a brief explanation of the regular expression (Regex) code.

• ^     the beginning of the line, followed by . . .
• [     a character class that contains . . .
• \p{Lu}     a lowercase Unicode letter, or . . .
• \p{Ll}     an uppercase Unicode letter . . .
• ]     the end of the character class . . .
• +     that’s found one or more times, followed by . . .
• $     the end of the line.

In this project we do not allow the user to enter periods.