How to implement letter-only for Wordpress contact form 7 plugin

Please give me example of letter-only validation for wordpress- contact form 7 plugin.

I already read about it in the Wordpress forum, but I didn't see how to implement it.

This is regex form letter only validation

/^[a-zA-Z]+$/

Or

Add following code in function.php. Change fields name according to your form

function my_wpcf7_validate_text( $result, $tag ) {

    $type = $tag['type'];
    $name = $tag['name'];
    $value = $_POST[$name] ;

    if ( strpos( $name , 'name' ) !== false ){
    $regex = '/^[a-zA-Z]+$/';
    $Valid = preg_match($regex,  $value, $matches );
            if ( $Valid > 0 ) {
            }
            else {
                $result->invalidate( $tag, wpcf7_get_message( 'invalid_name' ) );
            }
        }
        return $result;
}
add_filter( 'wpcf7_validate_text*', 'my_wpcf7_validate_text' , 10, 2 );

add_filter( 'wpcf7_messages', 'mywpcf7_text_messages' );
function mywpcf7_text_messages( $messages ) {
    return array_merge( $messages, array(
                                         'invalid_name' => array(
                                                                 'description' => __( "Name is invalid", 'contact-form-7' ),
                                                                 'default' => __( 'Name seems invalid.', 'contact-form-7' )
                                                                 )));
}