RegEx for ScheduleExpression that can be used in CloudFormation Template

Question

I was writing a CF template today that creates a periodic EventBridge rule. One of the parameters my template therefore accepts is the cron expression for setting the ScheduleExpression on this rule. I struggled for a while getting the expression right for my own use case, and wanted to add an AllowedPattern rule to my parameter. I went looking for a RegEx that I could use, and had no luck. There is a Gist, not updated in a few years, out there that presents this monster:

```
"^(rate$((1 (hour|minute|day))|(\d+ (hours|minutes|days)))$)|(cron\(\s*($|#|\w+\s*=|(\?|\*|(?:[0-5]?\d)(?:(?:-|/|\,)(?:[0-5]?\d))?(?:,(?:[0-5]?\d)(?:(?:-|/|\,)(?:[0-5]?\d))?)*)\s+(\?|\*|(?:[0-5]?\d)(?:(?:-|/|\,)(?:[0-5]?\d))?(?:,(?:[0-5]?\d)(?:(?:-|/|\,)(?:[0-5]?\d))?)*)\s+(\?|\*|(?:[01]?\d|2[0-3])(?:(?:-|/|\,)(?:[01]?\d|2[0-3]))?(?:,(?:[01]?\d|2[0-3])(?:(?:-|/|\,)(?:[01]?\d|2[0-3]))?)*)\s+(\?|\*|(?:0?[1-9]|[12]\d|3[01])(?:(?:-|/|\,)(?:0?[1-9]|[12]\d|3[01]))?(?:,(?:0?[1-9]|[12]\d|3[01])(?:(?:-|/|\,)(?:0?[1-9]|[12]\d|3[01]))?)*)\s+(\?|\*|(?:[1-9]|1[012])(?:(?:-|/|\,)(?:[1-9]|1[012]))?(?:L|W)?(?:,(?:[1-9]|1[012])(?:(?:-|/|\,)(?:[1-9]|1[012]))?(?:L|W)?)*|\?|\*|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)(?:(?:-)(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC))?(?:,(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)(?:(?:-)(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC))?)*)\s+(\?|\*|(?:[0-6])(?:(?:-|/|\,|#)(?:[0-6]))?(?:L)?(?:,(?:[0-6])(?:(?:-|/|\,|#)(?:[0-6]))?(?:L)?)*|\?|\*|(?:MON|TUE|WED|THU|FRI|SAT|SUN)(?:(?:-)(?:MON|TUE|WED|THU|FRI|SAT|SUN))?(?:,(?:MON|TUE|WED|THU|FRI|SAT|SUN)(?:(?:-)(?:MON|TUE|WED|THU|FRI|SAT|SUN))?)*)(|\s)+(\?|\*|(?:|\d{4})(?:(?:-|/|\,)(?:|\d{4}))?(?:,(?:|\d{4})(?:(?:-|/|\,)(?:|\d{4}))?)*))\))$"
```

However, when I tried to use this in a CF template...

```
Parameters:
  Schedule: {
    Type: String,
    Description: "AWS Schedule Expression representing how often the Lambda will run. Default is once per week, Sunday at noon.",
    AllowedPattern: "^(rate$((1 (hour|minute|day))|(\d+ (hours|minutes|days)))$)|(cron$\s*($|#|\w+\s*=|(\...  ...?)*))$)$"
    }
```
This failed and said my yaml was invalid. The column specified pointed to the cause being the escaped forward slashes, i.e. "/".  I have read some places that "/" is not a valid escape sequence in the version of yaml that CF uses. Is this true, and does anyone have a regex for ScheduleExpressions that works in a yaml CF template?

The Gist I can the above RegEx from is here: [https://gist.github.com/andrew-templeton/aca7fc6c166e9b8a46aa]()

Answer

Can you try this one?
```
AllowedPattern: >-
      ^(rate$((1 (hour|minute|day))|(\d+
      (hours|minutes|days)))$)|(cron\(\s*($|#|\w+\s*=|(\?|\*|(?:[0-5]?\d)(?:(?:-|/|\,)(?:[0-5]?\d))?(?:,(?:[0-5]?\d)(?:(?:-|/|\,)(?:[0-5]?\d))?)*)\s+(\?|\*|(?:[0-5]?\d)(?:(?:-|/|\,)(?:[0-5]?\d))?(?:,(?:[0-5]?\d)(?:(?:-|/|\,)(?:[0-5]?\d))?)*)\s+(\?|\*|(?:[01]?\d|2[0-3])(?:(?:-|/|\,)(?:[01]?\d|2[0-3]))?(?:,(?:[01]?\d|2[0-3])(?:(?:-|/|\,)(?:[01]?\d|2[0-3]))?)*)\s+(\?|\*|(?:0?[1-9]|[12]\d|3[01])(?:(?:-|/|\,)(?:0?[1-9]|[12]\d|3[01]))?(?:,(?:0?[1-9]|[12]\d|3[01])(?:(?:-|/|\,)(?:0?[1-9]|[12]\d|3[01]))?)*)\s+(\?|\*|(?:[1-9]|1[012])(?:(?:-|/|\,)(?:[1-9]|1[012]))?(?:L|W)?(?:,(?:[1-9]|1[012])(?:(?:-|/|\,)(?:[1-9]|1[012]))?(?:L|W)?)*|\?|\*|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)(?:(?:-)(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC))?(?:,(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)(?:(?:-)(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC))?)*)\s+(\?|\*|(?:[0-6])(?:(?:-|/|\,|#)(?:[0-6]))?(?:L)?(?:,(?:[0-6])(?:(?:-|/|\,|#)(?:[0-6]))?(?:L)?)*|\?|\*|(?:MON|TUE|WED|THU|FRI|SAT|SUN)(?:(?:-)(?:MON|TUE|WED|THU|FRI|SAT|SUN))?(?:,(?:MON|TUE|WED|THU|FRI|SAT|SUN)(?:(?:-)(?:MON|TUE|WED|THU|FRI|SAT|SUN))?)*)(|\s)+(\?|\*|(?:|\d{4})(?:(?:-|/|\,)(?:|\d{4}))?(?:,(?:|\d{4})(?:(?:-|/|\,)(?:|\d{4}))?)*))\))$
```    
This is similar to the one that you posted, but in a yaml-friendly way. It's passing CFN's validation.

Answer

~~I'm still testing it to be sure, but this *should* work according to the [current EventBridge documentation](https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-cron-expressions.html)~~:

```yml
AllowedPattern: # [REMOVED, see below]
```

**Update:** I have refactored the pattern and, while it adds significant complexity, it *does* seem to handle all the edge cases I can think to test and all backslashes were removed (to prevent backslash-escape pandemonium).

It's also up on [github](https://github.com/EvanK/aws-eventbridge-schedule-expressions-regex), if you find any issues with it:

```yml
AllowedPattern: "^(?:rate[(](?:(?:1[ ]+(hour|minute|day))|(?:[0-9]+[ ]+(hours|minutes|days)))[)])|(?:cron[(](?:(?:(?:[0-5]?[0-9])|[*])(?:(?:[-](?:(?:[0-5]?[0-9])|[*]))|(?:[/][0-9]+))?(?:[,](?:(?:[0-5]?[0-9])|[*])(?:(?:[-](?:(?:[0-5]?[0-9])|[*]))|(?:[/][0-9]+))?)*)[ ]+(?:(?:(?:[0-2]?[0-9])|[*])(?:(?:[-](?:(?:[0-2]?[0-9])|[*]))|(?:[/][0-9]+))?(?:[,](?:(?:[0-2]?[0-9])|[*])(?:(?:[-](?:(?:[0-2]?[0-9])|[*]))|(?:[/][0-9]+))?)*)[ ]+(?:(?:[?][ ]+(?:(?:(?:[1]?[0-9])|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|[*])(?:(?:[-](?:(?:[1]?[0-9])|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|[*]))|(?:[/][0-9]+))?(?:[,](?:(?:[1]?[0-9])|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|[*])(?:(?:[-](?:(?:[1]?[0-9])|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|[*]))|(?:[/][0-9]+))?)*)[ ]+(?:(?:(?:[1-7]|(?:SUN|MON|TUE|WED|THU|FRI|SAT))[#][0-5])|(?:(?:(?:(?:[1-7]|(?:SUN|MON|TUE|WED|THU|FRI|SAT))L?)|[L*])(?:(?:[-](?:(?:(?:[1-7]|(?:SUN|MON|TUE|WED|THU|FRI|SAT))L?)|[L*]))|(?:[/][0-9]+))?(?:[,](?:(?:(?:[1-7]|(?:SUN|MON|TUE|WED|THU|FRI|SAT))L?)|[L*])(?:(?:[-](?:(?:(?:[1-7]|(?:SUN|MON|TUE|WED|THU|FRI|SAT))L?)|[L*]))|(?:[/][0-9]+))?)*)))|(?:(?:(?:(?:(?:[1-3]?[0-9])W?)|LW|[L*])(?:(?:[-](?:(?:(?:[1-3]?[0-9])W?)|LW|[L*]))|(?:[/][0-9]+))?(?:[,](?:(?:(?:[1-3]?[0-9])W?)|LW|[L*])(?:(?:[-](?:(?:(?:[1-3]?[0-9])W?)|LW|[L*]))|(?:[/][0-9]+))?)*)[ ]+(?:(?:(?:[1]?[0-9])|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|[*])(?:(?:[-](?:(?:[1]?[0-9])|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|[*]))|(?:[/][0-9]+))?(?:[,](?:(?:[1]?[0-9])|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|[*])(?:(?:[-](?:(?:[1]?[0-9])|(?:JAN|FEB|MAR|APR|MAY|JUN|JUL|AUG|SEP|OCT|NOV|DEC)|[*]))|(?:[/][0-9]+))?)*)[ ]+[?]))[ ]+(?:(?:(?:[12][0-9]{3})|[*])(?:(?:[-](?:(?:[12][0-9]{3})|[*]))|(?:[/][0-9]+))?(?:[,](?:(?:[12][0-9]{3})|[*])(?:(?:[-](?:(?:[12][0-9]{3})|[*]))|(?:[/][0-9]+))?)*)[)])$"
```

RegEx for ScheduleExpression that can be used in CloudFormation Template

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