Context

• joi version: 13.1.0

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

const schema = Joi.object().rename(/^someprefix-(\w+)$/g, '$1', { ignoreUndefined: true, multiple: true });
schema.validate({ 'someprefix-A': 1, 'someprefix-B': 2 }, { convert: true });

Which result you had ?

{ '$1': 1, 'someprefix-B': 2 }

renaming by capture group is not work.

What did you expect ?

from supports regexp, so why not add support capturing group to to parameter?