REGO compare functions: Add Regex

[lebogg]

In #185 , @d-pizhuk proposed regular expression for target values. As you can see there, this is possible in Rego. Would be a nice feature, I can imagne.

What do you think? @oxisto @anatheka @immqu

[anatheka]

In principle, regular expressions are not bad, but for the metrics, I currently can't think of an example where they would be useful. Do you have a specific example in mind?

[lebogg]

Yes, just look at the referenced PR :)
But as en example, check if AES with GCM is used, with all key size variants:

• ^AES-(128|192|256)-GCM$
• ^AES-\d{3}-GCM$ (because all variants are considered secure currently)

[anatheka]

In that case, I would use the array comparison, because it’s much easier to read and understand. You can clearly list all allowed values, so it’s obvious to others what’s valid. Furthermore, it’s simpler to maintain—adding or removing values is quick and doesn’t involve tweaking a complicated regex pattern which can accidentally match things you didn’t intend.