Java RegEx - check if a string ends with an exact number of digits

Question

The Java matches method returns 'true' for all of the following

// check if a string ends with an exact number of digits (yyyyMMdd 8 digits)

    String s20180122_1 = "fileNamePrefix_20171219";
    String s20180122_2 = "fileNamePrefix_20171219131415111";

    System.out.println(s20180122_1.matches(".*\\d{8}$"));
    System.out.println(s20180122_2.matches(".*\\d{8}$"));
    System.out.println(s20180122_2.matches(".*\\d{8,8}$"));

Since the s20180122_2 has more digits I expect the 2nd and 3rd checks to return 'false', but they don't. How do I enforce the exact (8 digits only. No more, no less) match?

Thanks for your help.

Kirill Polishchuk · Accepted Answer · 2018-01-22 22:35:26Z

3

This should work:

.*[^\d]\d{8}$

or if Java regex engine supports \D group, you can use it instead of [^\d]

answered Jan 22, 2018 at 22:35

Kirill Polishchuk

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5 Comments

badbee Over a year ago

Very interesting. It works. Can you explain the logic please?

Kirill Polishchuk Over a year ago

@badbee, the main issue with you regex that it doesn't set the left boundary of the digit part, which in your example is _. From your example you can use this regex: .*_\d{8}$. To make it a bit more generic you can use [^\d] (any character that is not a digit) instead of _ character.

badbee Over a year ago

Thank you for the explanation.

slestak Over a year ago

This regex did not work as expected on oracle. returned true with 9 digits. Still working at it. I actually want to test for ending in 6 or 8 digits.

slestak Over a year ago

This got it. Couldn't edit my previous comment. .*[:digit:{6|8}]$

GalAbra · Accepted Answer · 2018-01-22 22:43:22Z

1

This regex would validate that the last 8 characters are digits in yyyyMMdd form (and the year is within the 20th-21st centuries:

.*[^\d](19|20)\d{2}(0[1-9]|1[0-2])(0[1-9]|[12]\d|3[01])$

answered Jan 22, 2018 at 22:43

GalAbra

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