title: RegExp match balise [] with A-Z 0-9 inside
body: <p>Say I have something like:</p>
  
  <pre><code>$content = " some text [TAG_123] and other text";
  </code></pre>
  
  <p>I want to match <code>[TAG_123]</code>.</p>
  
  <p>To be more precise : <code>[</code> followed by one uppercase <code>A-Z</code> , followed by zero or several <code>0-9A-Z_</code>, followed by <code>]</code>.</p>
  
  <p>I tried : </p>
  
  <pre><code>$reg = "/\[[A-Z]+[0-9A-Z_]*/"; // =&gt; this match [TAG_123
  
  $reg = "/\[[A-Z]+[0-9A-Z_]*\]/"; // =&gt; this doesn't work ???
  </code></pre>