from re import sub

# var
# re_match = r":[a-z]+"
re_match = r"[a-z]+\n[a-z]+"
re_replace = "WOWOW"

# charge les données dans une variable
fp = Path(".").parent.joinpath("test.dat").resolve()
with fp.open() as f:
  data = f.read()

# match multiligne
print(sub(re_match, re_replace, data))

# mon fichier test.dat contient
# images/:id/huge
# images/:id/small
# documents/:id/main
# documents/:id/supplementary

# ce qui est lu dans la var "data"
# images/:id/huge\nimages/:id/small\ndocuments/:id/main\ndocuments/:id/supplementary\n

# ce qui sort après sub
# images/:id/WOWOW/:id/WOWOW/:id/WOWOW/:id/supplementary