st = ['(800) 555-1212',
'1-800-555-1212',
'800-555-1212x1234',
'800-555-1212 ext. 1234',
'work 1-(800) 555.1212 #1234']​

My objective is to segregate the phone numbers so that I can isolate each individual group viz. '800', '555', '1212' and the optional '1234'.

I have tried out the following code.

p1 = re.compile(r'(\d{3}).*(\d{3}).*(\d{4}).*(\d{4})?')
step1 = [re.sub(r'\D','',p1.search(t).group()) for t in st]
p2 = re.compile(r'(\d{3})(\d{3})(\d{4})(\d{4})?')
step2 = [p2.search(t).groups() for t in step1]

p1 and p2 being the two patterns to fetch the desired output.

for i in range(len(step2)):
print step2

('800', '555', '1212', None)
('800', '555', '1212', None)
('800', '555', '1212', '1234')
('800', '555', '1212', '1234')
('800', '555', '1212', '1234')

One more (modification of yours):

import re
pattern = re.compile('.*(\d{3})[^\d]*(\d{3})[^\d]*(\d{4})[^\d]*(\d{4})?$')
print [[pattern.match(s).group(i) for i in range(1,5)] for s in st]

#[['800', '555', '1212', None], ['800', '555', '1212', None], ['800', '555', '1212', '1234'], ['800', '555', '1212', '1234'], ['800', '555', '1212', '1234']]

Instead of trying to match the entire string and capturing the desired substrings, you can just match digits with lenghts 3 or 4.

Demo on Regex101: https://regex101.com/r/XNbb79/1

import re

st = ['(800) 555-1212',
'1-800-555-1212',
'800-555-1212x1234',
'800-555-1212 ext. 1234',
'work 1-(800) 555.1212 #1234']

for b in [re.findall('\d{3,4}', a) for a in st]:
    if len(b) == 3:
        print "number does not have extension"
        print b
    if len(b) == 4:
        print "number has extension"
        print b

Output:

number does not have extension
['800', '555', '1212']
number does not have extension
['800', '555', '1212']
number has extension
['800', '555', '1212', '1234']
number has extension
['800', '555', '1212', '1234']
number has extension
['800', '555', '1212', '1234']

I think re.findall and the similarity of the groups allow you a simpler approach:

>>> import re
>>> from pprint import pprint
>>> res = [re.findall(r'\d{3,4}', s) for s in st]
>>> pprint res
[['800', '555', '1212'],
 ['800', '555', '1212'],
 ['800', '555', '1212', '1234'],
 ['800', '555', '1212', '1234'],
 ['800', '555', '1212', '1234']]