title: Regex: Combination of String Length and Starts With patterns
body: <p>I want to check if the string contains exactly 11 characters, not more or less, and also if it starts with the numbers '09', so my pattern is:</p>
  
  <pre><code>Regex rg = new Regex(@"^(?=09)(?={11})");
  Console.WriteLine(rg.IsMatch("09123456789"));
  </code></pre>
  
  <p><code>^(?=09)</code> is working correctly, but when I add second part, <code>(?={11})</code>, an exception will be thrown. What's the right pattern?</p>