There is no single one correct solution to the exercises.

Solutions might differ regarding complexity, how general they are and how easy they are to understand and change. These solutions are just here to give you an idea about how to solve the exercises if you get stuck.

Meta

m = re.match(r"........\....", filename)

Quantifier

Valid filename

m = re.match(r".+\..{3,}", filename)

Valid email

m = re.match(r".+@.{3,}\..{2,}", email)

Character classes

Better IP validator

m = re.match(r"192\.168\.1\.[0-9]{1,3}", ip_address)

Hexadecimal validator

m = re.match(r"0x[0-9A-F]{1,4}", hex_string)

Email revisited

m = re.match(r"[a-zA-Z]+@.{3,}\.[a-zA-Z]{2,}", email)

Anchors

Valid filenames Windows

m = re.search(r'^[^\/:*?"]+\.[^\/:*?"]{3}$', filename)

Is integer

m = re.search(r"^-?\d+$", string)

Groups

submatch

pattern = r"^invoice_[\w]+\.pdf$"

Exercise Phone number

return bool(re.match(r"(\+?\d{5}|\d{4})/\d{7,}", number))

Exercise Valid hour

return re.match(r"([01]\d|[2][0-3]):[0-5]\d", string) is not None

Valid ip address

m = re.match(r"^192\.168\.1\.([0-9]|\d\d|1\d\d|2[0-5][0-5])$", ip_address

Sub

Exercise

fix = re.sub(r'"(\w+\.\w{3,4})"', r'"images/\1"', line)

Extended Exercise

fix = re.sub(r'src=\"(?!images)(?:[a-zA-Z0-9/_])*?/?(\w+\.\w+)\"', r'src="images/\1"', line)

or alternative solution (credits go to Benny):
fix = re.sub(r'(src=.*)(?<!images)(\"|/)(\w+\.\w{,4}) ', r'src="images/\3"', line)