Split an array based on regex

You can collect a map from this array:

String[] arr = {"A123", "blue", "A456", "red", "green",
        "B111", "purple", "C444", "blue", "green", "yellow", "pink"};

Map<String, List<String>> map = new LinkedHashMap<>();

// assume that first is the 'primary' element
List<String> list = null;
for (String str : arr) {
    // if this is a 'primary' element
    // regex:
    // \\D{1} - first character is non-digit
    // \\d+   - non-empty sequence of digits
    if (str.matches("\\D{1}\\d+")) {
        // put new entry into the map and initialise new list
        list = map.computeIfAbsent(str, el -> new ArrayList<>());
    } else {
        // otherwise, add the 'secondary' element to the list
        list.add(str);
    }
}

// output
map.forEach((k, v) -> System.out.println(k + "=" + v));
//A123=[blue]
//A456=[red, green]
//B111=[purple]
//C444=[blue, green, yellow, pink]

// convert a map to a 2d array, if needed
String[][] arr2d = map.entrySet().stream()
        .map(entry -> Stream
                .concat(Stream.of(entry.getKey()), entry.getValue().stream())
                .toArray(String[]::new))
        .toArray(String[][]::new);

// output
Arrays.stream(arr2d).map(Arrays::toString).forEach(System.out::println);
//[A123, blue]
//[A456, red, green]
//[B111, purple]
//[C444, blue, green, yellow, pink]

Here is an example:

@Test
public void testArr() {
    String[] strings = new String[]{
            "A123", "blue", "A456", "red", "green",
            "B111", "purple", "C444", "blue", "green", "yellow", "pink"
    };

    List<List<String>> output = new ArrayList<>();
    List<String> currentList = null;
    for (String s : strings) {
        if (s.matches("[A-Z][0-9]{3}")) {
            // start new sub-list
            currentList = new ArrayList<>();
            output.add(currentList);
        }
        currentList.add(s);
    }

    System.out.println(output);
}

It produces:

[[A123, blue], [A456, red, green], [B111, purple], [C444, blue, green, yellow, pink]]