Flags#
When using re functions or creating a compiled regex you can specify additional flags that affect the behavior of regex.
The re module supports flags (in parentheses - a short version of flag):
re.ASCII(re.A)re.IGNORECASE(re.I)re.MULTILINE(re.M)re.DOTALL(re.S)re.VERBOSE(re.X)re.LOCALE(re.L)re.DEBUG
In this subsection the re.DOTALL flag is covered. Information about other flags
is available in documentation.
re.DOTALL#
Regex can also be used for multiline string.
For example, from sh_cdp string you need to get a device name, platform and IOS:
In [2]: sh_cdp = '''
...: Device ID: SW2
...: Entry address(es):
...: IP address: 10.1.1.2
...: Platform: cisco WS-C2960-8TC-L, Capabilities: Switch IGMP
...: Interface: GigabitEthernet1/0/16, Port ID (outgoing port): GigabitEthernet0/1
...: Holdtime : 164 sec
...:
...: Version :
...: Cisco IOS Software, C2960 Software (C2960-LANBASEK9-M), Version 12.2(55)SE9, RELEASE SOFTWARE (fc1)
...: Technical Support: http://www.cisco.com/techsupport
...: Copyright (c) 1986-2014 by Cisco Systems, Inc.
...: Compiled Mon 03-Mar-14 22:53 by prod_rel_team
...:
...: advertisement version: 2
...: VTP Management Domain: ''
...: Native VLAN: 1
...: Duplex: full
...: Management address(es):
...: IP address: 10.1.1.2
...: '''
Of course, in this case it is possible to divide a string into parts and work with each string separately, but you can get necessary data without splitting.
In this expression, strings with required data are described:
In [3]: regex = r'Device ID: (\S+).+Platform: \w+ (\S+),.+Cisco IOS Software.+ Version (\S+),'
In this case, there will be no match because by default a dot means any character other than a new line character:
In [4]: print(re.search(regex, sh_cdp))
None
You can change default behavior by using re.DOTALL flag:
In [5]: match = re.search(regex, sh_cdp, re.DOTALL)
In [6]: match.groups()
Out[6]: ('SW2', 'WS-C2960-8TC-L', '12.2(55)SE9')
Since new line character is now included, combination .+ captures everything between data.
Now try to use this regex to get information about all neighbors from sh_cdp_neighbors_sw1.txt file.
SW1#show cdp neighbors detail
-------------------------
Device ID: SW2
Entry address(es):
IP address: 10.1.1.2
Platform: cisco WS-C2960-8TC-L, Capabilities: Switch IGMP
Interface: GigabitEthernet1/0/16, Port ID (outgoing port): GigabitEthernet0/1
Holdtime : 164 sec
Version :
Cisco IOS Software, C2960 Software (C2960-LANBASEK9-M), Version 12.2(55)SE9, RELEASE SOFTWARE (fc1)
Technical Support: http://www.cisco.com/techsupport
-------------------------
Device ID: R1
Entry address(es):
IP address: 10.1.1.1
Platform: Cisco 3825, Capabilities: Router Switch IGMP
Interface: GigabitEthernet1/0/22, Port ID (outgoing port): GigabitEthernet0/0
Holdtime : 156 sec
Version :
Cisco IOS Software, 3800 Software (C3825-ADVENTERPRISEK9-M), Version 12.4(24)T1, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
-------------------------
Device ID: R2
Entry address(es):
IP address: 10.2.2.2
Platform: Cisco 2911, Capabilities: Router Switch IGMP
Interface: GigabitEthernet1/0/21, Port ID (outgoing port): GigabitEthernet0/0
Holdtime : 156 sec
Version :
Cisco IOS Software, 2900 Software (C3825-ADVENTERPRISEK9-M), Version 15.2(2)T1, RELEASE SOFTWARE (fc3)
Technical Support: http://www.cisco.com/techsupport
Search for all regex matches:
In [7]: with open('sh_cdp_neighbors_sw1.txt') as f:
...: sh_cdp = f.read()
...:
In [8]: regex = r'Device ID: (\S+).+Platform: \w+ (\S+),.+Cisco IOS Software.+ Version (\S+),'
In [9]: match = re.finditer(regex, sh_cdp, re.DOTALL)
In [10]: for m in match:
...: print(m.groups())
...:
('SW2', '2911', '15.2(2)T1')
At first glance, it seems that instead of three devices there was only one device in output. However, if you look at the results the tuple has Device ID from the first neighbor and platform and IOS from the last neighbor.
A short output to ease understanding of result:
Device ID Local Intrfce Holdtme Capability Platform Port ID
SW2 Gi 1/0/16 171 R S C2960 Gi 0/1
R1 Gi 1/0/22 158 R C3825 Gi 0/0
R2 Gi 1/0/21 177 R C2911 Gi 0/0
This is because there is a .+ combination between desired parts of the output.
Without re.DOTALL flag, such an expression would capture everything before
new line character, but with a flag it captures the longest possible piece of text
because + is greedy.
As a result, regex describes a string from the first Device ID to the
last place where Cisco IOS Software.+ Version match occurs.
This situation occurs very often when using re.DOTALL and in order to
correct it remember to disable greedy behavior:
In [10]: regex = r'Device ID: (\S+).+?Platform: \w+ (\S+),.+?Cisco IOS Software.+? Version (\S+),'
In [11]: match = re.finditer(regex, sh_cdp, re.DOTALL)
In [12]: for m in match:
...: print(m.groups())
...:
('SW2', 'WS-C2960-8TC-L', '12.2(55)SE9')
('R1', '3825', '12.4(24)T1')
('R2', '2911', '15.2(2)T1')