print out the first n vowels in a given string

EdabitChallenges3.java

 private static int countMatches(Matcher matcher) {
    int counter = 0;
    while (matcher.find())
      counter++;
    return counter;
  }

  @Override
  public String firstNVowels(String str, int n) {
    Pattern pattern = Pattern.compile("(?i)[aeiou]");
    Matcher matcher = pattern.matcher(str);
    System.out.println(String.format("Number of matchers: %d", countMatches(matcher)));
    if (countMatches(matcher) < n) {
      return "invalid";
    }
    StringBuilder result = new StringBuilder();
    while (matcher.find()) {
      for (int i = 1; i <= n; i++) {
        result.append(matcher.group(i));
      }
    }
    return result.toString();
  }