title: Using ?=. in regular expression
body: <p>I saw the phrase</p>
  
  <pre><code>^(?=.*[A-Z])(?=.*[a-z])(?=.*[0-9])[A-Za-z0-9_#@%\*\-]{8,24}$
  </code></pre>
  
  <p>in regex, which was password checking mechanism. I read few courses about regular expressions, but I never saw combination <code>?=.</code> explained.</p>
  
  <p>I want know how it works. In the example it is searching for at least one capital letter, one small letter and one number. I guess it's something like "if".</p>