Parse any date in Java

Question

I know this question is asked quite a bit, and obviously you can't parse any arbitrary date. However, I find that the python-dateutil library is able to parse every date I throw at it, all while requiring absolutely zero effort in figuring out a date format string. Joda time is always sold as being a great Java date parser, but it still requires you to decide what format your date is in before you pick a Format (or create your own). You can't just call DateFormatter.parse(mydate) and magically get a Date object back.

For example, the date "Wed Mar 04 05:09:06 GMT-06:00 2009" is properly parsed with python-dateutil:

import dateutil.parser
print dateutil.parser.parse('Wed Mar 04 05:09:06 GMT-06:00 2009')

but the following Joda time call doesn't work:

    String date = "Wed Mar 04 05:09:06 GMT-06:00 2009";
    DateTimeFormatter fmt = ISODateTimeFormat.dateTime();
    DateTime dt = fmt.parseDateTime(date);
    System.out.println(date);

And creating your own DateTimeFormatter defeats the purpose, since that seems to be the same as using SimpleDateFormatter with the correct format string.

Is there a comparable way to parse a date in Java, like python-dateutil? I don't care about errors, I just want it to mostly perfect.

Answer 1

Have you considered porting the Python utility to Java?

Answer 2

What I have seen done is a Date util class that contains several typical date formats. So, when DateUtil.parse(date) is called, it tries to parse the date with each date format internally and only throws exceptions if none of the internal formats can parse it.

It is basically a brute force approach to your problem.

Answer 3

Your best bet is really asking help to regex to match the date format pattern and/or to do brute forcing.

Several years ago I wrote a little silly DateUtil class which did the job. Here's an extract of relevance:

private static final Map<String, String> DATE_FORMAT_REGEXPS = new HashMap<String, String>() {{
    put("^\\d{8}$", "yyyyMMdd");
    put("^\\d{1,2}-\\d{1,2}-\\d{4}$", "dd-MM-yyyy");
    put("^\\d{4}-\\d{1,2}-\\d{1,2}$", "yyyy-MM-dd");
    put("^\\d{1,2}/\\d{1,2}/\\d{4}$", "MM/dd/yyyy");
    put("^\\d{4}/\\d{1,2}/\\d{1,2}$", "yyyy/MM/dd");
    put("^\\d{1,2}\\s[a-z]{3}\\s\\d{4}$", "dd MMM yyyy");
    put("^\\d{1,2}\\s[a-z]{4,}\\s\\d{4}$", "dd MMMM yyyy");
    put("^\\d{12}$", "yyyyMMddHHmm");
    put("^\\d{8}\\s\\d{4}$", "yyyyMMdd HHmm");
    put("^\\d{1,2}-\\d{1,2}-\\d{4}\\s\\d{1,2}:\\d{2}$", "dd-MM-yyyy HH:mm");
    put("^\\d{4}-\\d{1,2}-\\d{1,2}\\s\\d{1,2}:\\d{2}$", "yyyy-MM-dd HH:mm");
    put("^\\d{1,2}/\\d{1,2}/\\d{4}\\s\\d{1,2}:\\d{2}$", "MM/dd/yyyy HH:mm");
    put("^\\d{4}/\\d{1,2}/\\d{1,2}\\s\\d{1,2}:\\d{2}$", "yyyy/MM/dd HH:mm");
    put("^\\d{1,2}\\s[a-z]{3}\\s\\d{4}\\s\\d{1,2}:\\d{2}$", "dd MMM yyyy HH:mm");
    put("^\\d{1,2}\\s[a-z]{4,}\\s\\d{4}\\s\\d{1,2}:\\d{2}$", "dd MMMM yyyy HH:mm");
    put("^\\d{14}$", "yyyyMMddHHmmss");
    put("^\\d{8}\\s\\d{6}$", "yyyyMMdd HHmmss");
    put("^\\d{1,2}-\\d{1,2}-\\d{4}\\s\\d{1,2}:\\d{2}:\\d{2}$", "dd-MM-yyyy HH:mm:ss");
    put("^\\d{4}-\\d{1,2}-\\d{1,2}\\s\\d{1,2}:\\d{2}:\\d{2}$", "yyyy-MM-dd HH:mm:ss");
    put("^\\d{1,2}/\\d{1,2}/\\d{4}\\s\\d{1,2}:\\d{2}:\\d{2}$", "MM/dd/yyyy HH:mm:ss");
    put("^\\d{4}/\\d{1,2}/\\d{1,2}\\s\\d{1,2}:\\d{2}:\\d{2}$", "yyyy/MM/dd HH:mm:ss");
    put("^\\d{1,2}\\s[a-z]{3}\\s\\d{4}\\s\\d{1,2}:\\d{2}:\\d{2}$", "dd MMM yyyy HH:mm:ss");
    put("^\\d{1,2}\\s[a-z]{4,}\\s\\d{4}\\s\\d{1,2}:\\d{2}:\\d{2}$", "dd MMMM yyyy HH:mm:ss");
}};

/**
 * Determine SimpleDateFormat pattern matching with the given date string. Returns null if
 * format is unknown. You can simply extend DateUtil with more formats if needed.
 * @param dateString The date string to determine the SimpleDateFormat pattern for.
 * @return The matching SimpleDateFormat pattern, or null if format is unknown.
 * @see SimpleDateFormat
 */
public static String determineDateFormat(String dateString) {
    for (String regexp : DATE_FORMAT_REGEXPS.keySet()) {
        if (dateString.toLowerCase().matches(regexp)) {
            return DATE_FORMAT_REGEXPS.get(regexp);
        }
    }
    return null; // Unknown format.
}

(cough, double brace initialization, cough, it was just to get it all to fit in 100 char max length ;) )

You can easily expand it yourself with new regex and dateformat patterns.