title: Regular expressions C# (xxxx-xxxx mask)
body: <p>I can not understand how to make regular expression
  For example I have mask:
  xxxx-xxxx-xxxx where "x" can be a-z or A-Z or 0-9</p>
  
  <p>And have code:</p>
  
  <pre><code>            string[] numbers = 
                   {
                       "1a3F-5t5C-FIG0-InFo", 
                       "444-234-2245", 
                       "444.-2344-2245", 
                   };
          string sPattern = "^[a-z0-9A-z]{4}-[a-z0-9A-z]{4}-[a-z0-9A-z]{4}$-[a-z0-9A-z]{4}$";
          var validList = new List&lt;string&gt;();
  
          foreach (string s in numbers)
          {
              if (Regex.IsMatch(s, sPattern))
              {
                  validList.Add(s);
              }
          }
          Assert.IsTrue(validList.Count==1);
  </code></pre>
  
  <p>But it doesn't work... I have some mistakes in expression. Could you explain why is it wrong?</p>