title: Check if input matches STEAM ID Format
body: <p>I understand how to match it but I don't know the criteria for example, Email matcher would use: <code>/^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/</code> but I need to know what a steam id would use.</p>
  
  <p>Demo Steam ID: STEAM_0:1:20206720</p>
  
  <p>Could someone give me a criteria for STEAM IDs?</p>
  
  <p>EDIT:
  I'm using the regex expression provided by pimvdb, but it still replys that a steam id(STEAM_0:1:20206720) is incorrect.</p>
  
  <p>My code is below:</p>
  
  <pre><code>function verifySteamID(){
      var elem = document.getElementById('item_name');
      var emailExp = /^STEAM_[0-5]:[01]:\d+$/;
      if(elem.value.match(emailExp)){
          document.getElementById("error").setAttribute("class", "hidden");
          return true;
      }else{
          document.getElementById("error").setAttribute("class", "unhidden");
          elem.focus();
          return false;
      }
      }
  </code></pre>
  
  <p>Which is called by:</p>
  
  <pre><code>&lt;input type="submit" value="Donate" id="donatebtn" onclick="return verifySteamID()" /&gt;
  </code></pre>