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Someone at my company created a regex string that is working as intended but I want to do similar things in the future and don't understand the mechanics of it. 

import re

config = """
interface Ethernet1/2
 description Management
 ip address 192.168.1.190 255.255.255.0
 ip pim sparse-dense-mode
!
interface Ethernet1/3
 description to-IntDMVPN
 ip address 10.0.0.93 255.255.255.252
 ip pim sparse-dense-mode
!
router eigrp 1
 network 172.16.0.0 0.0.0.255
 redistribute ospf 1 metric 1 1 1 1 1
!
"""

intf_obj_list = re.compile(r'^interface\s*(\S+)(.+?)(?:^\S+|\Z)', re.S|re.M).findall(config)

print(str(intf_obj_list))

I understand up to ^interface\s*(\S+) which matches the router interfaces but adding the (.+?) also matches the /n on the first line of each interface config even though the dot is supposed to match any character but the newline. Now (?:^\S+|\Z) adds matching of the body of the configuration under the interface and I don't understand that section at all. From what I could find ?: is another way to configure backreferencing which after a fair amount of googling, I'm still not clear on and I'm not sure what the ^\S+|\Z within it are accomplishing. Finally, the re.S I think allows it to continue to run through the body while encountering newlines and re.M allows it to run through multiple lines. If anyone can help me to break down what's happening here I would greatly appreciate it. Thanks

lmsherman
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    I would recommend going over some tutorials to better grasp regex. A great source of learning is also [https://regex101.com](https://regex101.com/r/HTml93/3). Just copy-paste the regex there and you will see a detailed explanation to the right – Tomerikoo Mar 22 '20 at 14:39
  • Yeah, the question might appear lazy but I've done quite a bit of research trying to figure this out including regex101. From what i can tell regex101 doesn't allow you to put in the `re.S|re.M` and the output I get from what I can plug in there doesn't match what I get from running the program within Python. `(?:^\S+|\Z)` shows up as a non-capturing group as well. – lmsherman Mar 22 '20 at 15:01
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    Yes you can add flags. In the edge of the regex bar you can select them (there is even a little flag to indicate that). Use the actual link I put in my comment, I took the bother to put your regex there and match the flags – Tomerikoo Mar 22 '20 at 15:04

0 Answers0