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

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.