title: use regex validator to ensure a string contains particular substring
body: <p>I want to use a regex validator to ensure that a certain string variable contains the substring <code>"www.youtube.com/watch?v="</code>, how would I do this?</p>
  
  <pre><code>[RegularExpression()]
  [Required(ErrorMessage = "Youtube link is Required")]
  [StringLength(100, ErrorMessage="Youtube link cannot exceed 50 characters")]
  public string YoutubeLink { get; set; }
  </code></pre>