Disclaimer

Parsing a string back to date/time value in an unknown format is inherently impossible (let’s face it, what does 3/3/3 actually mean?!), all we can do is “best effort”

Important

This solution doesn’t throw an Exception, it returns a boolean, this is by design. Any Exceptions are used purely as a guard mechanism.

2018

Since it’s now 2018 and Java 8+ has the date/time API (and the rest have the ThreeTen backport). The solution remains basically the same, but becomes slightly more complicated, as we need to perform checks for:

• date and time
• date only
• time only

This makes it look something like…

public static boolean isValidFormat(String format, String value, Locale locale) {
    LocalDateTime ldt = null;
    DateTimeFormatter fomatter = DateTimeFormatter.ofPattern(format, locale);

    try {
        ldt = LocalDateTime.parse(value, fomatter);
        String result = ldt.format(fomatter);
        return result.equals(value);
    } catch (DateTimeParseException e) {
        try {
            LocalDate ld = LocalDate.parse(value, fomatter);
            String result = ld.format(fomatter);
            return result.equals(value);
        } catch (DateTimeParseException exp) {
            try {
                LocalTime lt = LocalTime.parse(value, fomatter);
                String result = lt.format(fomatter);
                return result.equals(value);
            } catch (DateTimeParseException e2) {
                // Debugging purposes
                //e2.printStackTrace();
            }
        }
    }

    return false;
}

This makes the following…

System.out.println("isValid - dd/MM/yyyy with 20130925 = " + isValidFormat("dd/MM/yyyy", "20130925", Locale.ENGLISH));
System.out.println("isValid - dd/MM/yyyy with 25/09/2013 = " + isValidFormat("dd/MM/yyyy", "25/09/2013", Locale.ENGLISH));
System.out.println("isValid - dd/MM/yyyy with 25/09/2013 12:13:50 = " + isValidFormat("dd/MM/yyyy", "25/09/2013  12:13:50", Locale.ENGLISH));
System.out.println("isValid - yyyy-MM-dd with 2017-18--15 = " + isValidFormat("yyyy-MM-dd", "2017-18--15", Locale.ENGLISH));

output…

isValid - dd/MM/yyyy with 20130925 = false
isValid - dd/MM/yyyy with 25/09/2013 = true
isValid - dd/MM/yyyy with 25/09/2013 12:13:50 = false
isValid - yyyy-MM-dd with 2017-18--15 = false

Original Answer

Simple try and parse the String to the required Date using something like SimpleDateFormat

Date date = null;
try {
    SimpleDateFormat sdf = new SimpleDateFormat(format);
    date = sdf.parse(value);
    if (!value.equals(sdf.format(date))) {
        date = null;
    }
} catch (ParseException ex) {
    ex.printStackTrace();
}
if (date == null) {
    // Invalid date format
} else {
    // Valid date format
}

You could then simply write a simple method that performed this action and returned true when ever Date was not null…

As a suggestion…

Updated with running example

I’m not sure what you are doing, but, the following example…

import java.text.ParseException;
import java.text.SimpleDateFormat;
import java.util.Date;

public class TestDateParser {

    public static void main(String[] args) {
        System.out.println("isValid - dd/MM/yyyy with 20130925 = " + isValidFormat("dd/MM/yyyy", "20130925"));
        System.out.println("isValid - dd/MM/yyyy with 25/09/2013 = " + isValidFormat("dd/MM/yyyy", "25/09/2013"));
        System.out.println("isValid - dd/MM/yyyy with 25/09/2013 12:13:50 = " + isValidFormat("dd/MM/yyyy", "25/09/2013  12:13:50"));
    }

    public static boolean isValidFormat(String format, String value) {
        Date date = null;
        try {
            SimpleDateFormat sdf = new SimpleDateFormat(format);
            date = sdf.parse(value);
            if (!value.equals(sdf.format(date))) {
                date = null;
            }
        } catch (ParseException ex) {
            ex.printStackTrace();
        }
        return date != null;
    }

}

Outputs (something like)…

java.text.ParseException: Unparseable date: "20130925"
isValid - dd/MM/yyyy with 20130925 = false
isValid - dd/MM/yyyy with 25/09/2013 = true
isValid - dd/MM/yyyy with 25/09/2013 12:13:50 = false
    at java.text.DateFormat.parse(DateFormat.java:366)
    at javaapplication373.JavaApplication373.isValidFormat(JavaApplication373.java:28)
    at javaapplication373.JavaApplication373.main(JavaApplication373.java:19)

Not correct. For isValidFormat(“yyyy-MM-dd”, “2017-18–15”); not throw any Exception.

isValid - yyyy-MM-dd", "2017-18--15 = false

Seems to work as expected for me – the method doesn’t rely on (nor does it throw) the exception alone to perform it’s operation