A regular expression that matches 1-3:6-8:13-15 etc

I have a string pattern that can be

1. a number (ex 1)
2. a number range (ex 1-2)
3. a list of numbers (ex 1:4:7)
4. a list of ranges (ex 1-2:5-7:12-15)

So, string like "1", "1-4:4", "6:9-12:20" are all valid

I have created a pattern on RegExr.com so far

/(\d(-\d)?)(:(\d(-\d)?))?/g

which is almost working, but not quite, as it can't go past the first pair. For example:

1-2:7-9:12-23
       ^

the character denoted with the arrow is not matched for some reason.

Can someone please advise?

You could try this pattern: /^(\d+(-\d+)?)(:(\d+(-\d+)?))*$/gm

This is modified version of your pattern, which added start/end string anchors, replaced ? quantifier of your last group to * (zero or more) quantifier, and + quantifier for matching group of digit numbers.

See regex demo here.

Each group could start from the beginning of the string or with ":": /^|:/. This way regex could be simplified into a single group definition:

public static void main(String[] args) {
    String pattern = "(?:(?:^|:)(\\d+)(?:-(\\d+))?)+";
    Stream.of(
        "1",
        "1-2.",
        "1:4:7",
        "1-2:5-7:12-15",
        "1-4:4",
        "6:9-12:20",
        "123",
        "123-234",
        "123:456:789",
        "123-234:567-789:1234-1567",
        "123-456:456",
        "678:999-1234:2012"
    ).forEach(it -> {
        System.out.println(it + " => " + it.matches(pattern));
    });
}

Stdout:

1 => true
1-2. => false
1:4:7 => true
1-2:5-7:12-15 => true
1-4:4 => true
6:9-12:20 => true
123 => true
123-234 => true
123:456:789 => true
123-234:567-789:1234-1567 => true
123-456:456 => true
678:999-1234:2012 => true