Context

• joi version: <=13.1.2

What are you trying to achieve or the steps to reproduce ?

Have available all possible regex modifier - e.g. global, multiline, ... currently all are ignored except 'i'.
https://github.com/hapijs/joi/blob/master/lib/types/string/index.js#L119

Example

The schema below (if I got it right) should match base64 encoded string with fixed line-length also - multiline modifier is used.

const schema = Joi.string().regex(/^(?:[A-Za-z0-9+\/]{4})*(?:[A-Za-z0-9+\/]{2}(==)?|[A-Za-z0-9+\/]{3}=?)?$/m;)