Validations in WPF

Validating textboxes in WPF. The validtion is checked at LostFocus event of TextBox

public void ValidateTextbox(ref TextBox ctrltxt, string p)
{
int count = 0;
string errormessage=string.Empty;
BindingExpression expression = ctrltxt.GetBindingExpression(TextBox.TextProperty);
expression.UpdateSource();
if (true) // check validation condition or reg expression here
{
ValidationError error = new ValidationError(new ExceptionValidationRule(), expression, errormessage, null);
Validation.MarkInvalid(expression, error);
}
}

XAML code for the TEXTBOX is as below

<TextBox Name="FieldName" HorizontalAlignment="Stretch"
VerticalAlignment="Stretch"
Text="{Binding Path=DisplayData, ValidatesOnDataErrors=True ,NotifyOnValidationError=True , UpdateSourceTrigger=LostFocus, ValidatesOnExceptions=True}"
Grid.Column="1"
Margin="4,4,4,4" Validation.ErrorTemplate="{StaticResource InputErrorTemplate}" >
</TextBox>

InputErrorTemplate is as below


<ControlTemplate x:Key="InputErrorTemplate">  
    <DockPanel>  
        <Border Name="validationBorder" BorderBrush="Red" BorderThickness="2" Padding="1" CornerRadius="3">  
            <Border.Resources>  
                <Storyboard x:Key="_blink">  
                    <ColorAnimationUsingKeyFrames AutoReverse="True" BeginTime="00:00:00" Storyboard.TargetName="validationBorder" Storyboard.TargetProperty="(Border.BorderBrush).(SolidColorBrush.Color)" RepeatBehavior="00:00:02">  
                        <SplineColorKeyFrame KeyTime="00:00:00.5" Value="#00FF0000"/>  
                    </ColorAnimationUsingKeyFrames>  
                </Storyboard>  
            </Border.Resources>  
            <Border.Triggers>  
                <EventTrigger RoutedEvent="FrameworkElement.Loaded">  
                    <BeginStoryboard Storyboard="{StaticResource _blink}" />  
                </EventTrigger>  
            </Border.Triggers>  
            <AdornedElementPlaceholder/>  
        </Border>  
    </DockPanel>  
</ControlTemplate>