ValidationExtensions.cs

using System;
using System.Linq.Expressions;
using System.Text.RegularExpressions;
using System.Web.Mvc;
using System.Web.Mvc.Html;

public static class ValidationExtensions
{
   public static MvcHtmlString ValidationMessageLabelFor<TModel, TProperty>(this HtmlHelper<TModel> html, Expression<Func<TModel, TProperty>> expression, string errorClass = "error")
   {
       string elementName = ExpressionHelper.GetExpressionText(expression);
       MvcHtmlString normal = html.ValidationMessageFor(expression);
       if (normal != null)
       {
           string newValidator = Regex.Replace(normal.ToHtmlString(), @"<span([^>]*)>([^<]*)</span>", string.Format("<label for=\\"{0}\\" $1>$2</label>", elementName), RegexOptions.IgnoreCase);
           if (!string.IsNullOrWhiteSpace(errorClass))
               newValidator = newValidator.Replace("field-validation-error", errorClass);
           return MvcHtmlString.Create(newValidator);
       }
       return null;
   }
}