I posted this also on the marshmallow github page, and was able to get a good response there.

https://github.com/marshmallow-code/marshmallow/issues/2829

from marshmallow import Schema, fields, validate

class RequestNumber(fields.String):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, metadata={'description': 'Request Number', 'example':'REQUEST12345'}, validate=validate.Regexp(regex=r"^REQUEST\d{3,9}$", error="Input string didn't match required format - REQUEST12345")), **kwargs)


class Api1():

    class Input1(Schema):
        request_number = RequestNumber()