How do I write regex to validate EIN numbers? <p>I want to validate that a string follows this format (using regex):</p> <pre><code>valid: 123456789 //9 digits valid: 12-1234567 // 2 digits + dash + 7 digits </code></pre> <p><strong>Here's an example, how I would use it:</strong></p> <pre><code>var r = new Regex("^[1-9]\d?-\d{7}$"); Console.WriteLine(r.IsMatch("1-2-3")); </code></pre> <p>I have the regex for the format with dash, but can't figure how to include the non-dash format??? </p>