RegexFieldRule.cs

    public class RegexFieldRule : IRemoteFieldValidationRule
    {
        private readonly Regex _regex;

        public RegexFieldRule(Regex regex)
        {
            _regex = regex;
        }

        public void Validate(Accessor accessor, ValidationContext context)
        {
            var value = (string) accessor.GetValue(context.Target);
            if(value.IsEmpty())
            {
                return;
            }
            if(!_regex.IsMatch(value))
            {
                context.Notification.RegisterMessage(accessor, Token);
            }
        }

        public StringToken Token { get; set; }
    }