title: How to check if a string contains ALL of required characters in php
body: <p>I am trying to check if a string contains letters A, B, and C and nothing else. Is there a shorthand way of doing this using <code>preg_match</code>? </p>
  
  <p>To give an example of what I want, the check should return</p>
  
  <pre><code>ABC //TRUE
  AABC //TRUE
  ABBCC //TRUE
  ABCD //FALSE
  </code></pre>
  
  <p>Thus, a string <strong>MUST</strong> contain <em>A</em>, <em>B</em>, and <em>C</em> at least once, and no other characters. </p>