WPF

private bool IsDecimal(string text)
        {
            bool blnValidDecimal = false;
            foreach (char c in text)
            {
                if (Char.IsNumber(c))
                {
                    blnValidDecimal = true;
                }
                if (char.IsDigit(c))
                {
                    blnValidDecimal = true;
                }
            }
            return blnValidDecimal;
        }

        private void txtAvgGrade_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            e.Handled = !IsDecimal(e.Text);
        }

private static bool IsEmailAllowed(string text)
        {
            Regex regEMail = new Regex(@"^[a-zA-Z][\w\.-]{2,28}[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$");
            return !regEMail.IsMatch(text);
        }

        private void txtEmail_PreviewTextInput(object sender, TextCompositionEventArgs e)
        {
            e.Handled = !IsEmailAllowed(e.Text);            
        } 

private void txtEmail_Validating(object sender, CancelEventArgs e)
        {
            System.Text.RegularExpressions.Regex rEMail = new System.Text.RegularExpressions.Regex(@"^[a-zA-Z][\w\.-]{2,28}[a-zA-Z0-9]@[a-zA-Z0-9][\w\.-]*[a-zA-Z0-9]\.[a-zA-Z][a-zA-Z\.]*[a-zA-Z]$");
            if (txtEmail.Text.Length > 0)
            {
                if (!rEMail.IsMatch(txtEmail.Text))
                {
                    MessageBox.Show("E-Mail expected", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    txtEmail.SelectAll();
                    e.Cancel = true;
                }
            }
        }