Regex validates fine with online tool, but not in java application

Question

I have this regex used in Java as shown below:

private static final String DATE_TIME_REGEX = "^(20[1-5]\\d)-(0?[1-9]|1[012])-(0?[1-9]|[12]\\d|3[01])\\s([0-1]\\d)|(2[0-3]):([0-5]\\d):([0-5]\\d)$";

public static boolean validateDate(String dateStr) {
    return dateStr.matches(DATE_TIME_REGEX);
}

...to validate a date in this format:

private static final DateTimeFormatter dateTimeFormatter = DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss");

    String formattedDate = dateTimeFormatter.format(LocalDateTime.now());

...using this online tool

https://regexr.com/

...where adding this regex:

^(20[1-5]\d)-(0?[1-9]|1[012])-(0?[1-9]|[12]\d|3[01])\s([0-1]\d)|(2[0-3]):([0-5]\d):([0-5]\d)$

...works fine to validate a date string like:

2022-11-02 00:00:00

... but fails to validate (with no indication of the specifics) using the aforementioned validation code in java

To rule out the only caveat I can think of - when adding the code from the online tool into the java code, I have to add an extra \ before the other backslashes in the regex pattern when moving it from the online tool, i.e. \d and \s -> \\d and \\s. However, am highly dubious as to whether that is the issue

Here it doesn't match if I select Java: https://regex101.com/r/Dsp7TD/1 — Jorge Campos, Nov 25 '22 at 22:19
I also Like this site because it shows a visual representation of the Regex, if you know regex it becomes easier to debug it: https://www.debuggex.com/r/HBrpD2e_t-L9VUqe — Jorge Campos, Nov 25 '22 at 22:27

bobble bubble · Accepted Answer · 2022-11-26T01:15:24.600

The pattern contains one group and alternation that look to be out of place:

^(20[1-5]\d)-(0?[1-9]|1[012])-(0?[1-9]|[12]\d|3[01])\s([0-1]\d)|(2[0-3]):([0-5]\d):...
                                                      ^ HERE > ^

This makes your current pattern match ^regex101

EITHER the left part of the alternation from start up to the [0-1]\d hours-part
OR the right part from the 2[0-3] hours-part until the end

Using this with matches() cannot work because it gets matched against the full string:

public boolean matches()
Attempts to match the entire region against the pattern.
If the match succeeds then more information can be obtained...

Refactored by fixing the structure it works fine (using matches, the ^ and $ are redundant).

20[1-5]\d-(?:0?[1-9]|1[012])-(?:0?[1-9]|[12]\d|3[01])\s(?:[0-1]\d|2[0-3]):[0-5]\d:[0-5]\d

See this Java demo at tio.run or a demo at regex101 (pattern explained and code generator)

Regex validates fine with online tool, but not in java application

1 Answers1