Create a regular expression for Cron statement

Question

I need to write an regular expression that matches Cron time expressions, like these ones:

• 28 2 7 1 1
• 28 2 7 1 *
• 28 2 7 * *
• 28 2 * * *
• 28 * * * *
• * * * * *

Answer 1

To validate cron expressions in general, it'll depend greatly on the specific implementation you're using

Rules

General Format

In general, most adhere to the the following format:

Field name	Mandatory?	Allowed values	Special characters
Seconds	No*	0-59	* / , -
Minutes	Yes	0-59	* / , -
Hours	Yes	0-23	* / , -
Day of month	Yes	1-31	* / , - L W
Month	Yes	1-12 or JAN-DEC	* / , -
Day of week	Yes	0-6 or SUN-SAT	* / , - L #
Year	No*	1970–2099	* / , -

^{*where seconds and years are non-standard and sometimes not included}

Predefined Scheduling Macros:

Some flavors allow predefined time periods like this:

Entry	Equivalent to
@annually	0 0 0 1 1 * *
@yearly	0 0 0 1 1 * *
@monthly	0 0 0 1 * * *
@weekly	0 0 0 * * 0 *
@daily	0 0 0 * * * *
@hourly	0 0 * * * * *
@reboot

Intervals

Some flavors allow using the @every <duration> syntax with the following timespan units:

Unit	Definition
ns	nanosecond
us, µs	microsecond
ms	millisecond
s	second
m	minute
h	hour

Validating

Predefined Macros Regex

To validate predefined macros, you can use the following regex:

/@(annually|yearly|monthly|weekly|daily|hourly|reboot)/

Which will pass the following test cases:

@daily
@hourly

@every Regex

To validate @every durations, you can use the following regex:

/@every (\d+(ns|us|µs|ms|s|m|h))+/

Which will pass the following test cases:

@every 5s
@every 20h30m

Individual Cron Terms

Validating cron terms in the regular expression is a little trickier since there are so many variations.

Just focusing in on a single term, the following are all valid:

• Two or more numbers separated by a ,
• Two numbers separated by a / or -
• A 1-2 digit number
• A single *

To validate a single term, we can use the following regex:

/(\d+,)+\d+|(\d+(\/|-)\d+)|\d+|\*/

^{where \d+ just guarantees you have 1 or more numeric digits}

Which will pass the following test cases:

1,2,3
1,2
1/2
1-2
1
*

Combining Cron Terms

To check the full expression, we can just make sure we have {5,7} terms with the following regex:

/(((\d+,)+\d+|(\d+(\/|-)\d+)|\d+|\*) ?){5,7}/

If we wanted to distinguish between each term, we'd need to validate numbers within a certain range:

Allowed values	Regex
0-59	[1-5]?[0-9]
0-23	2[0-3]|1[0-9]|[0-9]
1-31	3[01]|[12][0-9]|[1-9]
1-12	1[0-2]|[1-9]
0-6	[0-6]
1970–2099	19[7-9][0-9]|20[0-9][0-9]

If however, we just want to make sure something looks like a regex expression without worrying about which term is days vs. hours, the expression stays a lot cleaner, so that's what I'll go with for simplicity

Putting it all together

By combining the above statements, we can have a relatively simple sanity check that the term looks regex-y with the following expression:

/(@(annually|yearly|monthly|weekly|daily|hourly|reboot))|(@every (\d+(ns|us|µs|ms|s|m|h))+)|((((\d+,)+\d+|(\d+(\/|-)\d+)|\d+|\*) ?){5,7})/

Additional Resources

• crontab.guru
• regexr.com