title: How to use "regsub" or "regex" in Tcl to remove leading and trailing characters from a "start" period
body: <p>Extracting text parts from a sentence/line using the <code>regex</code> and <code>regsub</code> functions - left and right. For example the line:</p>
  
  <ul>
  <li>learning lesson number@12.4.1@_01.html</li>
  </ul>
  
  <p>I need <code>regsub</code> and/or<code>regex</code> to leave only the snippet between "@".</p>
  
  <blockquote>
    <p>Staying like this: <strong>12.1.4</strong> <em>the rest can erase all the left and right surplus.</em></p>
  </blockquote>
  
  <p>In another scenario, in <em>Bash</em> and not <em>Tcl</em>. I can do it like this:</p>
  
  <p><a href="https://i.stack.imgur.com/52TnT.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/52TnT.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/gl6iz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/gl6iz.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/VjebS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/VjebS.png" alt="enter image description here"></a></p>
  
  <p>I am asking this question, more for the sake of teaching. That is, learning to use the native features of this Tcl language instead of relying on the Unix-Like system command itself.</p>
  
  <p>How can I craft a regular expression to execute this command? Thanks in advance ...</p>