Why does this regex fails to check accurately?

Question

I have the following regex method which does the matches in 3 stages for a given string. But for some reason the Regex fails to check some of the things. As per whatever knowledge I have gained by working they seem to be correct. Can someone please correct me what am I doing wrong here?

I have the following code:

public class App {
    public static void main(String[] args) {
        String identifier = "urn:abc:de:xyz:234567.1890123";

        if (identifier.matches("^urn:abc:de:xyz:.*")) {
            System.out.println("Match ONE");

            if (identifier.matches("^urn:abc:de:xyz:[0-9]{6,12}.[0-9]{1,7}.*")) {
                System.out.println("Match TWO");

                if (identifier.matches("^urn:abc:de:xyz:[0-9]{6,12}.[a-zA-Z0-9.-_]{1,20}$")) {
                    System.out.println("Match Three");
                }
            }
        }

    }
}

Ideally, this code should generate the output

Match ONE
Match TWO
Match Three

Only when the identifier = "urn:abc:de:xyz:234567.1890123.abd12" but it provides the same output event if the identifier does not match the regex such as for the following inputs:

"urn:abc:de:xyz:234567.1890123"
"urn:abc:de:xyz:234567.1890ANC"
"urn:abc:de:xyz:234567.1890123"
"urn:abc:de:xyz:234567.1890ACB.123"

I am not understanding why is it allowing the Alphanumeric characters after the . and also it does not care about the characters after the second ..

I would like my Regex to check that the string has the following format:

String starts with urn:abc:de:xyz:
Then it has the numbers [0-9] which range from 6 to 12 (234567).
Then it has the decimal point .
Then it has the numbers [0-9] which range from 1 to 7 (1890123)
Then it has the decimal point ..
Finally it has the alphanumeric character and spcial character which range from 1 to 20 (ABC123.-_12).

This is an valid string for my regex: urn:abc:de:xyz:234567.1890123.ABC123.-_12

This is an invalid string for my regex as it misses the elements from point 6: urn:abc:de:xyz:234567.1890123

This is also an invalid string for my regex as it misses the elements from point 4 (it has ABC instead of decimal numbers). urn:abc:de:xyz:234567.1890ABC.ABC123.-_12

See [the regex reference](https://stackoverflow.com/questions/22937618/reference-what-does-this-regex-mean). `.` not only matches the dot character. It matches almost everything. — Sweeper, Feb 06 '21 at 08:43
Because `.` has special meaning in regex. That’s why your `.*` at the end works - it means (almost) any character 0 or more times. — Boris the Spider, Feb 06 '21 at 08:43
@Sweeper How can I ensure there is a `.` and the required characters in my String? How can I modify my `regex` matching expression to check for these things? — BATMAN_2008, Feb 06 '21 at 08:57
@BoristheSpider Thanks for the response. How can I modify it to make it work and check for the things I need? — BATMAN_2008, Feb 06 '21 at 08:57
I'm not sure what you are checking for... What exactly do you want to match, and what do you not want to match? — Sweeper, Feb 06 '21 at 08:58
@Sweeper I have modified the question and added the constraint which I am looking to add to my regex. Please let me know if you are able to understand and help me with this. — BATMAN_2008, Feb 06 '21 at 09:05

alex-dl · Accepted Answer · 2021-02-06T09:13:19.967

2

This part of the regex:

[0-9]{6,12}.[0-9]{1,7} matches 6 to 12 digits followed by any character followed by 1 to 7 digits

To match a dot, it needs to be escaped. Try this:

^urn:abc:de:xyz:[0-9]{6,12}\.[0-9]{1,7}\.[a-zA-Z0-9\-_]{1,20}$

edited Feb 06 '21 at 09:13

answered Feb 06 '21 at 09:00

alex-dl

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But if I escape the `.` character then it would not check if there is a `.` character or not right? – BATMAN_2008 Feb 06 '21 at 09:07
@BATMAN_2008 No, it would only match exactly the dot, if you escape it. Otherwise it would match any non-newline character. – Sweeper Feb 06 '21 at 09:10
The backslash-dot sequence does match a dot. After matching the dot it can be removed between the last square brackets, if no more dots need be be matched. – alex-dl Feb 06 '21 at 09:11

score 0 · Answer 2 · answered Feb 06 '21 at 11:41

0

This will match with any number of dot alphanum at the end of the string as your examples:

^urn:abc:de:xyz:\d{6,12}\.\d{1,7}(?:\.[\w-]{1,20})+$

Demo & explanation

answered Feb 06 '21 at 11:41

Toto

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Why does this regex fails to check accurately?

2 Answers2