Java file extension regex

public static void main(String[] args) {
  Pattern fileExtensionPattern = Pattern.compile("\.\w+\z");

  boolean one = fileExtensionPattern.matcher("foo.bar").matches();
  boolean two = fileExtensionPattern.matcher("foo.b").matches();
  boolean three = fileExtensionPattern.matcher("foo.").matches();
  boolean four = fileExtensionPattern.matcher("foo").matches();

  System.out.println(one + " " + two + " " + three + " " + four);
}

public boolean isFilename(String filename) {
    int i=filename.lastInstanceOf(".");
    return(i != -1 && i != filename.length - 1)
}