Parsing an ISO 8601 date in Java8 when all fields (including separators, but not including years) are optional

Question

I have a requirement for parsing ISO8601 formatted strings in Java with various levels of accuracy. Some examples of the string I need to parse are:

• 2018

Answer 1

For the first cases with separators (-, :) one can use:

    DateTimeFormatter dtf = DateTimeFormatter
        .ofPattern("uuuu[-MM[-dd[['T']HH[:]mm[[:]ss[[.]SSS]]]]]");
    ParsePosition pos = new ParsePosition(0);
    TemporalAccessor result = dtf.parse(text, pos);

However neither uuuuMMdd nor [-] or ['-'] worked for me in Java 8.