Exercise 1 solution

    import re

    EMAIL = re.compile(r"[\w.+-]+@[\w-]+(?:\.[\w-]+)+")

    def redact_emails(text):
        return EMAIL.subn("[email]", text)

    print(redact_emails("Write to ann@example.com or bob.smith+x@mail.co.uk today."))
    print(redact_emails("Nothing to hide here, only a@ and @b."))
    # ('Write to [email] or [email] today.', 2)
    # ('Nothing to hide here, only a@ and @b.', 0)

(All lines were run and printed exactly what is shown.)

Explanation: subn does the replacement and returns the new text together with the number of replacements, so one call gives both answers.