private static final String LIST_REGEX = "^(\d+)(,\d+)*$";
private static final Pattern LIST_PATTERN = Pattern.compile(LIST_REGEX);

public static void main(String[] args) {
    final String list = "1,2,3,4,5";
    final Matcher matcher = LIST_PATTERN.matcher(list);
    System.out.println(matcher.matches());
    for (int i = 0, n = matcher.groupCount(); i < n; i++) {
        System.out.println(i + "t" + matcher.group(i));
    }
}

true
0   1,2,3,4,5
1   1

private static final String LIST_REGEX = "^\[?(\d+(?:,\d+)*)\]?$";
private static final Pattern LIST_PATTERN = Pattern.compile(LIST_REGEX);

public static void main(String[] args) {
    final String list = "[1,2,3,4,5]";
    final Matcher matcher = LIST_PATTERN.matcher(list);

    matcher.find();
    int i = 0;

    String[] vals = matcher.group(1).split(",");

    System.out.println(matcher.matches());
    System.out.println(i + "t" + matcher.group(1));

    for (String x : vals) {
       i++;
       System.out.println(i + "t" + x);
    }
}

true
0   1,2,3,4,5
1   1
2   2
3   3
4   4
5   5

for (String segment : "1,2,3,4,5".split(","))
    System.out.println(segment);

Pattern pattern = Pattern.compile("(\d),?");
for (Matcher m = pattern.matcher("1,2,3,4,5");; m.find())
     m.group(1);

for (String segment : "!!!!![1,2,3,4,5] //"
                          .replaceFirst("^\D*(\d(?:,\d+)*)\D*$", "$1")
                          .split(","))
    System.out.println(segment);