title: Lex: identifier vs integer
body: <p>I'm trying to create my own simple programming language. For this I need to insert some regex into Lex.
  I'm using the following regex to match identifiers and integers.</p>
  
  <pre><code>[a-zA-Z][a-zA-Z0-9]* /* identifier */ return IDENTIFIER;
  ("+"|"-")?[0-9]+ /* integer */ return INTEGER;
  </code></pre>
  
  <p>Now when I check for example an illegal identifier like:</p>
  
  <pre><code>0a = 1;
  </code></pre>
  
  <p>The leading zero is recognized as an integer followed by the 'a' recognized as an identifier. Instead of this I want this token '0a' to be recognized as an illegal character.
  How do I include this functionality? What regex do I have to adjust?</p>