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I'm building a thesaurus using a HashMap to store the synonyms. 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: public ArrayList<String> searchDefinition(String regex) { ArrayList<String> results = new ArrayList<String>(); Pattern p = Pattern.compile(regex); Set<String> keys = thesaurus.keySet(); Iterator<String> 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; } } 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: cat, car, chopper then by calling searchDefinition("c") or searchDefinition("c*") I get null . How do I make this work as expected? Is there a better data structure than HashMap to keep a graph like needed by a thesaurus? (curiosity only, as for this assignment we're asked to use Java Collection Map). Anything else I'm doing innapropriately in the code above? Thanks, Dan EDIT: I've corrected the example. It doesn't work even if I use the correct case.

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