Python regex with negative matching

Question

I have a following text file as sample.txt :

xe-4/3/1.1596
xe-4/3/1.1528
ae2.670
xe-4/3/1.1503
ae2
xe-4/3/1.1478
xe-4/3/1.1475
xe-4/3/1.1469
xe-4/3/1
xe-4/3/1.3465
xe-4/0/0.670
xe-4/0/0
xe-4/3/1.1446
xe-4/0/0.544
xe-4/3/1.1437
gr-5/0/0
gr-5/0/0.10
lo0.16384
lo0.16385
em1
em1.0
cbp0
demux0
irb
pip0
pp0
ae0

This is the list of interfaces of a router. I need to print out the lines (interfaces) which contain: xe,ae,gr but the ones which does not containg dot, eg xe-4/3/1 , gr-5/0/0 , ae2 etc.

Trying the following code but it does not work:

import re

file = open('sample.txt','r')
string = file.read()

for f in string:
    matchObj = re.findall("(xe|ae|gr)[^.]*$", f)
    if matchObj:
        print f

Checked my regex (xe|ae|gr)[^.]*$ at http://regexr.com/ and it matches the lines I want. Could you tell me what am I doing wrong.

Answer 1

for f in string: will iterate over characters in file; you want to iterate over lines. I suggest the following code instead:

# use the with statement to open the file
with open('sample.txt') as file:
    for line in file:
        # use re.search to see if there is match on the line;
         # but we do not care about the actual matching strings
        if re.search("(xe|ae|gr)[^.]*$", line):
            print line