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

namespace WpfApp1
{
    class PatternTextBox : TextBox
    {
        /// <summary>
        /// 正規表現を指定するプロパティ
        /// </summary>
        public static readonly DependencyProperty PatternProperty =
            DependencyProperty.Register("Pattern", typeof(string), typeof(PatternTextBox), new PropertyMetadata(OnInitalizeData));

        /// <summary>
        /// 正規表現を保持
        /// </summary>
        private Regex reg;

        /// <summary>
        /// パターン
        /// </summary>
        public string Pattern
        {
            get
            {
                return (string)GetValue(PatternProperty);
            }
            set
            {
                SetValue(PatternProperty, value);
            }
        }

        /// <summary>
        /// テキスト入力時にパターンにマッチするか確認
        /// </summary>
        /// <param name="e"></param>
        protected override void OnPreviewTextInput(TextCompositionEventArgs e)
        {
            //変更不可の場合は何もしない
            if (this.IsReadOnly)
            {
                return;
            }

            if (reg == null) return;
            if ((e.Text != "\r" && e.Text != "\t") && !reg.IsMatch(e.Text))
            {
                e.Handled = true;
                //正規表現でエラーになったときの処理
                //今回はTextを消してます
                Text = null;
            }
        }

        /// <summary>
        /// 初期化時にRegに反映
        /// </summary>
        /// <param name="obj"></param>
        /// <param name="e"></param>
        protected static void OnInitalizeData(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            var ctl = obj as PatternTextBox;
            if (ctl == null) return;
            if (ctl.Pattern != null)
            {
                ctl.reg = new Regex(ctl.Pattern);
            }
        }
    }
}

<Window x:Class="WpfApp1.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:WpfApp1"
        mc:Ignorable="d"
        Title="MainWindow" Height="250" Width="250">
    <Grid>
        <StackPanel VerticalAlignment="Center" HorizontalAlignment="Center">
            <Label Content="数字のみOKなTextBox"/>
            <local:PatternTextBox Pattern="^[0-9]*$" Width="150"/>
            <Label Content="大文字英語だけOKなTextBox"/>
            <local:PatternTextBox Pattern="^[A-Z]*$" Width="150"/>
        </StackPanel>
    </Grid>
</Window>