Java DateTimeFormatter parsing with special characters

I'm having some troubles getting my format right for using DataTimeFormatter to parse a date time String in the format of:

20200915095318.883[-4:EDT]

I have a DateTimeFormatter pattern that works to produce this format, but it doesn't work to parse the same String. That pattern is:

yyyyMMddHHmmss.SSS'['x':'z']'

If there are other libraries for DateTime parsing that are more appropriate for this type of need I'll happily consider those as well. Right now my solution is a one off regex matcher for when I encounter this DateTime String and to manually build its parts. Which is gross.

java.time, the modern Java date and time API that I think you are already using, is indeed the best library for the job. This works since Java 9:

	DateTimeFormatter formatter = new DateTimeFormatterBuilder()
			.appendPattern("uuuuMMddHHmmss")
			.appendFraction(ChronoField.NANO_OF_SECOND, 0, 9, true)
			.appendLiteral('[')
			.appendOffset("+Hmm", "+0")
			.appendLiteral(':')
			.appendZoneText(TextStyle.SHORT)
			.appendLiteral(']')
			.toFormatter(Locale.ENGLISH);
	
	String s = "20200915095318.883[-4:EDT]";
	
	OffsetDateTime odt = OffsetDateTime.parse(s, formatter);
	ZonedDateTime zdt = ZonedDateTime.parse(s, formatter);
	
	if (odt.getOffset().equals(zdt.getOffset())) {
		System.out.println(odt);
		System.out.println(zdt);
	} else {
		System.out.println("Something’s wrong here");
	}

Output is:

> 2020-09-15T09:53:18.883-04:00
> 2020-09-15T09:53:18.883-04:00[America/New_York]

Parsing into an OffsetDateTime uses the offset, -4, while parsing into a ZonedDateTime uses the time zone abbreviation, EDT. I check that the two agree about the offset because else I would not know which of them to believe. In your example string they do agree.

I am passing +Hmm as pattern to appendOffset(). Other choices include +H, +H:mm, +Hmmss and +H:mm:ss. Lower case mm and ss mean that minutes and seconds of the offset are optional. You should choose based on how an offset would look like if it is not a whole number of hours.

What do I mean by since Java 9? java.time, DateTimeFormatterBuilder and its appendOffset method are all in Java 8, but only since Java 9 does that method accept a pattern with just one H for one-digit hour in it.

Your only problem is that the -4 in your string needs a leading zero, as @deHaar indicated in his comment, i.e.

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyyMMddHHmmss.SSS'['x':'z']'");
LocalDateTime ldt = LocalDateTime.parse("20200915095318.883[-04:EDT]", formatter);