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copied!<p>As shown by @Maglob, the basic approach is to test the conversion from string to date using <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/text/SimpleDateFormat.html" rel="noreferrer">SimpleDateFormat.parse</a>. That will catch invalid day/month combinations like 2008-02-31.</p> <p>However, in practice that is rarely enough since SimpleDateFormat.parse is exceedingly liberal. There are two behaviours you might be concerned with:</p> <p><strong>Invalid characters in the date string</strong> Surprisingly, 2008-02-2x will "pass" as a valid date with locale format = "yyyy-MM-dd" for example. Even when isLenient==false.</p> <p><strong>Years: 2, 3 or 4 digits?</strong> You may also want to enforce 4-digit years rather than allowing the default SimpleDateFormat behaviour (which will interpret "12-02-31" differently depending on whether your format was "yyyy-MM-dd" or "yy-MM-dd")</p> <h2>A Strict Solution with the Standard Library</h2> <p>So a complete string to date test could look like this: a combination of regex match, and then a forced date conversion. The trick with the regex is to make it locale-friendly.</p> <pre><code> Date parseDate(String maybeDate, String format, boolean lenient) { Date date = null; // test date string matches format structure using regex // - weed out illegal characters and enforce 4-digit year // - create the regex based on the local format string String reFormat = Pattern.compile("d+|M+").matcher(Matcher.quoteReplacement(format)).replaceAll("\\\\d{1,2}"); reFormat = Pattern.compile("y+").matcher(reFormat).replaceAll("\\\\d{4}"); if ( Pattern.compile(reFormat).matcher(maybeDate).matches() ) { // date string matches format structure, // - now test it can be converted to a valid date SimpleDateFormat sdf = (SimpleDateFormat)DateFormat.getDateInstance(); sdf.applyPattern(format); sdf.setLenient(lenient); try { date = sdf.parse(maybeDate); } catch (ParseException e) { } } return date; } // used like this: Date date = parseDate( "21/5/2009", "d/M/yyyy", false); </code></pre> <p>Note that the regex assumes the format string contains only day, month, year, and separator characters. Aside from that, format can be in any locale format: "d/MM/yy", "yyyy-MM-dd", and so on. The format string for the current locale could be obtained like this:</p> <pre><code>Locale locale = Locale.getDefault(); SimpleDateFormat sdf = (SimpleDateFormat)DateFormat.getDateInstance(DateFormat.SHORT, locale ); String format = sdf.toPattern(); </code></pre> <h2>Joda Time - Better Alternative?</h2> <p>I've been hearing about <a href="http://joda-time.sourceforge.net/" rel="noreferrer">joda time</a> recently and thought I'd compare. Two points:</p> <ol> <li>Seems better at being strict about invalid characters in the date string, unlike SimpleDateFormat</li> <li>Can't see a way to enforce 4-digit years with it yet (but I guess you could create your own <a href="http://joda-time.sourceforge.net/api-release/org/joda/time/format/DateTimeFormatter.html" rel="noreferrer">DateTimeFormatter</a> for this purpose)</li> </ol> <p>It's quite simple to use:</p> <pre><code>import org.joda.time.format.*; import org.joda.time.DateTime; org.joda.time.DateTime parseDate(String maybeDate, String format) { org.joda.time.DateTime date = null; try { DateTimeFormatter fmt = DateTimeFormat.forPattern(format); date = fmt.parseDateTime(maybeDate); } catch (Exception e) { } return date; } </code></pre>

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