Do you mean something like this?
>>> import re
>>> a = "I am a programmer"
>>> reg = re.compile(r'I am (.*?)$')
>>> print('How long have you been {}'.format(*reg.findall(a)))
How long have you been a programmer
r'I am (.*?)$'
matches I am
and then everything else to the end of the string.
To match one word after, you can do:
>>> a = "I am an apple"
>>> reg = re.compile(r'I am (\w+).*?$')
>>> print('How long have you been {}'.format(*reg.findall(a)))
How long have you been an