title: The question mark (?) in flex
body: <p>I cannot understand what the question means in a regular expression in flex. For example, in my notes it says:</p>
  
  <blockquote>
    <p><strong>[ a-z ]?</strong>  corresponds to 0 or 1 repetition (used for optional parts).</p>
  </blockquote>
  
  <p>and another example says:</p>
  
  <blockquote>
    <p><strong>-?[1-9][ 0-9 ]∗</strong> corresponds to any nonzero integer.</p>
  </blockquote>
  
  <p>I do not understand why these regular expressions correspond to their explanations. Can somebody explain these two? </p>