RegExp for attribute values - expanding the allowedAttribute interface

[felixapitzsch]

I am going back to an issue raised seven years ago (#203). In reply to the need to restrict allowed values for allowedAttributes, @boutell proposed a new way to specify an allowedAttibute as
{ name: string; values: string[] } | { name: string; allowed: RegExp | Function }
... what I believe was later implemented as
{ name: string; multiple?: boolean | undefined; values: string[] }

However, the part with allowed: RegExp | Function seems to have never been implemented. Is this correct? I am wondering about the reasons not to implement this, because it is exactly want I am missing right now. 🙂

If there are no known downsides to this, I would like to advocate for expanding the allowedAttribute interface to include @boutell 's 'allowed' as an alternative into the 'values' property:
AllowedAttribute = string | { name: string; multiple?: boolean | undefined; values: ( string | RegExp | Function )[] };
... with the meaning that an evaluated attribute value would need to either satisfy the existing values:string/multiple logic or match at least one provided RegExp or make at least one provided function return true.

What are the opinions on this proposal? Any downsides? This interface could even be useful to specify other allowed values, for example for allowedStyles.