A map that uses a regular expression as a key.
Author: Manik Surtani (manik at surtani dot org)
Purpose: To create a Java Map that allowed me to pass in any arbitrary String as a key to it's get() method, and for the Map implementation to attempt to match this key against various regexp-keys it would store, and return any matching value (or null if nothing matches)
Dependencies: This class relies on v1.3 of Jakarta's Regexp package
Examples: Ok, that explanation above was rubbish. Nothing works quite as well as an example. Here we go:
RegexpKeyedMap map = new RegexpKeyedMap(); map.put( "^Green.*$", "Your sentence starts with Green" ); map.put( "^Blue.*$", "Your sentence starts with Blue" ); map.put( "Orange", "Your sentence contains the word Orange" ); // contains some stuff typed in my the user String sentence = "Green apples happen to be my favourite"; // responseText will be "Your sentence starts with Green" String responseText = map.get( sentence ); sentence = "I like green apples a lot"; // responseText will be null since nothing matched responseText = map.get( sentence ); sentence = "Green apples and Oranges are both quite good"; // responseText will be indeterminate - it could be either of "Your sentence starts with Green" // or "Your sentence contains the word Orange" - this is because the RegexpKeyedMap uses a HashMap // to store its contents and ordering of elements is not guaranteed. The first match is returned. responseText = map.get( sentence );
Source code:
package org.apache.regexp.collections; import java.util.HashMap; import java.util.Iterator; import org.apache.regexp.RE; import org.apache.regexp.RESyntaxException; /** * This map implementation uses a hashmap as the underlying storage. * Note that the keySet() method will return a set of regular expressions rather than actual keys. * The put() method uses a regexp as a key. * The get() method gets any value that matches one of the regexps. If there is more than one matching regexp, the first one * encountered is returned - and hence could be indeterminate! * * @author Manik Surtani * */ public class RegexpKeyedMap extends HashMap { public Object put(Object key, Object value) { if (key instanceof String) return super.put(key, value); else throw new RuntimeException("RegexpKeyedMap - only accepts Strings as keys."); } /** * The key passed in should always be a String. The map will return the first element whose key, treated as a regular expression, matches the key passed in * NOTE: It is possible for this map to have more than one return value, for example, if a key is passed into get() which matches more than one regexp. * * E.g., consider the following keys in the map - '[A-Za-z]*' and 'Hello'. Passing in 'Hello' as a key to the get() method would match either of the regexps, * and whichever apears first in the map (which is indeterminate) will be returned. * */ public Object get(Object key) { Iterator regexps = keySet().iterator(); String keyString; Object result = null; String stringToMatch = cleanKey( key ); while (regexps.hasNext()) { keyString = regexps.next().toString(); try { RE regexp = new RE(keyString); if (regexp.match(stringToMatch)) { result = super.get(keyString); break; } } catch (RESyntaxException e) { // invalid regexp. ignore? } } return result; } /** * Strip any 'dirty' chars from the key we are searching for, * otherwise we end up with funny results from the RE * * @param obj * @return */ private String cleanKey( Object obj ) { String retVal = obj.toString(); // remove any '^' from start of key - prevents the RE from matching !?!? return ( retVal.charAt(0) == '^' ) ? retVal.substring(1) : retVal; } }
Feedback: is very much appreciated! Let me know how this works for you, how it can be improved, etc.