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

Answer 1

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)