I’m trying to make the hours 1 or two digits and optional by modifying the ISO_LOCAL_TIME formatter. This is more for parsing durations where hours can be optional. This is what I tried.

new DateTimeFormatterBuilder()
        .optionalStart()
        .appendValue(HOUR_OF_DAY, 1, 2, SignStyle.NOT_NEGATIVE)
        .appendLiteral(':')
        .optionalEnd()
        .appendValue(MINUTE_OF_HOUR, 2)
        .optionalStart()
        .appendLiteral(':')
        .appendValue(SECOND_OF_MINUTE, 2)
        .optionalStart()
        .appendFraction(NANO_OF_SECOND, 0, 9, true)
        .toFormatter()
        .withResolverStyle(STRICT);

This is what I am testing with.

12:34:56.789
2:34:56.789
34:56.789
34:56 (mm:ss not hh:mm)

But it fails on the last two.

How can I create a formatter which passes all 4 tests?