title: Java: Search in HashMap keys based on regex?
body: <p>I'm building a thesaurus using a HashMap to store the synonyms. </p>
  
  <p>I'm trying to search through the words based on a regular expression: the method will have to take a string as parameter and return an array of results. Here's my first stab at it:</p>
  
  <pre><code>public ArrayList&lt;String&gt; searchDefinition(String regex) {
      ArrayList&lt;String&gt; results = new ArrayList&lt;String&gt;();
  
      Pattern p = Pattern.compile(regex);
  
      Set&lt;String&gt; keys = thesaurus.keySet();
      Iterator&lt;String&gt; ite = keys.iterator();
  
      while (ite.hasNext()) {
          String candidate = ite.next();
          Matcher m = p.matcher(candidate);
          System.out.println("Attempting to match: " + candidate + " to "  + regex);
          if (m.matches()) {
              System.out.println("it matches");
              results.add(candidate);
          }
      }   
  
      if (results.isEmpty()) {
          return null;
      }
      else {
          return results;
      }
  }
  </code></pre>
  
  <p>Now, this does not work as I would expect (or maybe I'm using regular expressions incorrectly). If I have the following keys in the hashmap:</p>
  
  <pre><code>cat, car, chopper
  </code></pre>
  
  <p>then by calling <code>searchDefinition("c")</code> or <code>searchDefinition("c*")</code> I get <code>null</code>.</p>
  
  <ol>
  <li>How do I make this work as expected?</li>
  <li>Is there a better data structure than HashMap to keep a <code>graph</code> like needed by a thesaurus? (curiosity only, as for this assignment we're asked to use Java Collection Map).</li>
  <li>Anything else I'm doing innapropriately in the code above?</li>
  </ol>
  
  <p>Thanks,
  Dan</p>
  
  <p>EDIT: I've corrected the example. It doesn't work even if I use the correct case.</p>