Search for a regexp in a java arraylist

Question

ArrayList <String> list = new ArrayList(); 
list.add("behold");
list.add("bend");
list.add("bet");
list.add("bear");
list.add("beat");
list.add("become");
list.add("begin"); 

There is a way to search for the regexp bea.* and get the indexes like in ArrayList.indexOf ?

EDIT: returning the items is fine but I need something with more performance than a Linear search

Answer 1

Herms got the basics right. If you want the Strings and not the indexes then you can improve by using the Java 5 foreach loop:

import java.util.regex.Pattern;
import java.util.ListIterator;
import java.util.ArrayList;

/**
 * Finds the index of all entries in the list that matches the regex
 * @param list The list of strings to check
 * @param regex The regular expression to use
 * @return list containing the indexes of all matching entries
 */
List<String> getMatchingStrings(List<String> list, String regex) {

  ArrayList<String> matches = new ArrayList<String>();

  Pattern p = Pattern.compile(regex);

  for (String s:list) {
    if (p.matcher(s).matches()) {
      matches.add(s);
    }
  }

  return matches
}

Answer 2

Is there a built-in method? Not that I know of. However, it should be rather easy to do it yourself. Here's some completely untested code that should give you the basic idea:

import java.util.regex.Pattern;
import java.util.ListIterator;
import java.util.ArrayList;

/**
 * Finds the index of all entries in the list that matches the regex
 * @param list The list of strings to check
 * @param regex The regular expression to use
 * @return list containing the indexes of all matching entries
 */
List<Integer> getMatchingIndexes(List<String> list, String regex) {
  ListIterator<String> li = list.listIterator();

  List<Integer> indexes = new ArrayList<Integer>();

  while(li.hasNext()) {
    int i = li.nextIndex();
    String next = li.next();
    if(Pattern.matches(regex, next)) {
      indexes.add(i);
    }
  }

  return indexes;
}

I might have the usage of Pattern and ListIterator parts a bit wrong (I've never used either), but that should give the basic idea. You could also do a simple for loop instead of the while loop over the iterator.

Answer 3

One option is to use Apache Commons CollectionUtils "select" method. You would need to create a Predicate object (an object with a single "evaluate" method that uses the regular expression to check for a match and return true or false) and then you can search for items in the list that match. However, it won't return the indexes, it will return a collection containing the items themselves.