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Tried to create a (simplified) regex for cron patterns for myself and came up with the following one:

^(((\d+(,\d+)*)|(\d+-\d+)|(\*(\/\d+)?))(\s+(\d+(,\d+)*)|(\d+-\d+)|(\*(\/\d+)?)){4})$

This is generated by 

var variant1 = "\\d+(,\\d+)*"; var variant2 = "\\d+-\\d+"; var variant3 = "\\*(\\/\\d+)?"; var variants = "(" + variant1 + ")|(" + variant2 + ")|(" + variant3 + ")"; var regex_cron = "^((" + variants + ")(\\s+" + variants + "){4})$";

For a lot of cron patterns this is working just fine:

* 4 45 6 7 5 4 45 6 7 */5 3,4 45,6,8 6 5

But if I use an asterisk in "group" 2 or higher, it fails:

4 * * * *

I know that there are a couple of regular expressions for recognizing cron patterns available on the internet, but I really would like to know where I went wrong during creating the regex.

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The probleme in your regex lies in the grouping of the second part:

var regex_cron = "^((" + variants + ")(\\s+" + variants + "){4})$"; ---

should be :

var regex_cron = "^((" + variants + ")(\\s+(" + variants + ")){4})$"; ! !

Because as you wrote it the \s+ (---) belongs only to the first part of the OR and the 2 next one don't match because there is a space.

May be it would be better practice to group the variants var together:

var variants = "((" + variant1 + ")|(" + variant2 + ")|(" + variant3 + "))";

There is a good place to test your regexp: http://www.regexr.com/

And this regexp could be simplified, and in the current state it doesn't match the string that will be the cron command, that should come before the $ matching the end of the line.

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  • You're right and it's kind of obvious once someone spells it out for me. Thx. Actually I don't check entries of the crontab file but patterns for cron4j (which can be entered by the user and therefore have to be checked). Commented Dec 25, 2014 at 11:59

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