fixed validation of cron expression

[thejus-r]

Fixes Or Enhances

• fixes [Bug]: Cron validator #1529 and Cron validator accepting non conform cron string #1130
• no validates range values & steps
• added new test cases

// regexes.go
cronRegexString = `^(@(annually|yearly|monthly|weekly|daily|hourly|reboot)|@every (\d+(ns|us|µs|ms|s|m|h))+|(((\d+(-\d+)?|\*|\?|\d*L(?:W)?|\d*W|\d+#\d+|[A-Z]{3}(-[A-Z]{3})?)(\/\d+)?)(,(\d+(-\d+)?|\*|\?|\d*L(?:W)?|\d*W|\d+#\d+|[A-Z]{3}(-[A-Z]{3})?)(\/\d+)?)* ?){5,7})$`

Make sure that you've checked the boxes below before you submit PR:

• Tests exist or have been written that cover this particular change.

@go-playground/validator-maintainers

[coveralls]

coverage: 73.832%. remained the same
when pulling 4c289a8 on thejus-r:fix-cron-validation
into dede341 on go-playground:master.

[zemzale]

IMO this is better than what we have right now since it fixes the issues.

But this is still not perfect since as it would allow:

* 9-25 * * * 

Maybe update the test with something like this to have it documented?

	t.Skip("FIXME: validation on minute/hour/day/month/weekday range is not working")

	failingTests := []struct {
		value    string `validate:"cron"`
		tag      string
		expected bool
	}{
		{"60 * * * *", "cron", false},
		{"* 25 * * *", "cron", false},
		{"* * 32 * *", "cron", false},
		{"* * * 13 *", "cron", false},
		{"* * * * 8", "cron", false},
	}

	for i, test := range failingTests {
		errs := validate.Var(test.value, test.tag)
		if test.expected {
			if !IsEqual(errs, nil) {
				t.Fatalf(`Index: %d cron "%s" failed Error: %s`, i, test.value, errs)
			}
		} else {
			if IsEqual(errs, nil) {
				t.Fatalf(`Index: %d cron "%s" should have errs`, i, test.value)
			}
		}
	}