It's a library to parse cron strings. It has no external dependencies in production code.

Despite being free of dependencies in production code, it's a gradle project to manage test dependencies.

Simply import it as a gradle project in your favorite IDE.

You can use it as a standalone jar to parse cron. For your convenience a compiled jar resides in lib catalog. You can simply call:

java -jar cron-parser.jar "<cron>

For example:

java -jar cron-parser.jar "*/15 0 1,15 * 1-5 /usr/bin/find"

You should expect the following output:

minute        0 15 30 45
hour          0
day of month  1 15
month         1 2 3 4 5 6 7 8 9 10 11 12
day of week   1 2 3 4 5
command       /usr/bin/find

You can use it as a library in your project:

CronPattern pattern = CronPattern.compile("*/15 0 1,15 * 1-5 /usr/bin/find");
if (pattern.isValid()) System.out.println(pattern.getCommand);

Which should print /usr/bin/find.

1. Make the standalone jar more extensible by interpreting additional input arguments as parameters for the formatter.
2. Add more validation, make the exception messages more usable.
3. Add "fail-slow" validation to list all the exceptions (all mistakes in cron string). The effect should be similar to Validated in scala-cats / vavr.
4. The CRON_PATTERN in CronPattern is written to be as much readable as possible. The extended regex could provide additional validation now included in classes implementing CronElement;
5. A nice idea would be to build a schema in a fluent manner. Something like:

var pattern = CronPattern.builder()
    .timeSegment("minute", 0, 59)
    .timeSegment("hour", 0, 23)
    ...
    .stringSegment("command")
    .stringSegment("command arguments")
    .build()
    .compile("*/15 0 1,15 * 1-5 /usr/bin/find --help");
if (pattern.isValid()) System.out.println(pattern.getSegment("command arguments")); // should print "--help"