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Cisco IOS and IOS-XR support filtering output using regular expressions. Cisco’s documentation on their regular expression format is available here, but we’ll go through some of the fundamentals and some useful tricks. Regex can be used to find just the specific information you’re looking for from usually long output.
Correct Use of .s
You’ve probably been using regular expressions in Cisco IOS without realizing it – ever notice that once in a long while you’ll see lines through a ”| include” filter that you didn’t expect to see? For example, say we’re looking for an interface on this router that has the assigned IP of 10.1.4.1 and try to filter ”show ip int brief” to obtain this info:
Router1#sh ip int bri | i 10.1.4.1 Loopback0 10.1.4.1 YES NVRAM up up Loopback4 10.104.120.40 YES NVRAM up up Port-channel1 10.104.120.24 YES manual up up Port-channel2 10.104.120.32 YES manual up up Vlan100 10.104.120.9 YES NVRAM up up
This showed a lot of interfaces that we weren’t looking for! This is because the . in the include statement is not interpreted literally – we inputted a regular expression, and IOS interpreted it as such. In regular expressions, . means “any character”, so they match any characters, including . and 4 in the above extra interfaces. For example, in the Loopback4 line, the regular expression 10.1.4.1 matched against 10.104.1 (the characters matched against the . in the regex are bold and red). If we wanted to match only the exact output 10.1.4.1, we need to escape the .s by placing a \ before them:
Router1#sh ip int bri | i 10\.1\.4\.1 Loopback0 10.1.4.1 YES NVRAM up up
Now, the .s are interpreted literally and we only see only the interface we wanted.
Filter on Multiple Components of a Line
As mentioned above, the . character means “any character”. The * character means “the previous character can exist 0 or more times”. Therefore, if we use .* within a regular expression, it reads like “any character can exist 0 or more times”. If we wanted to search for all TenGigabit interfaces that are up, this is easy using .*:
Router1#show int desc | i Te.*up Te2/0/0 up up Router2 Te3/0/1 up up Router3 Te4/0/0 up up Router4
Filter from Multiple Choices
We can allow multiple patterns to match using the | character, and we can group these using parenthesis. Want to list all routes in 10.0.0.0/23? We could use this regex:
This filters literally for anything that begins with
10.0.1.. This is not the most efficient way of searching for this, as we can reuse “Te” by grouping the OR statement using
Here’s an example of filtering a search for any line including either Te2/0/0 or Te3/0/1 – note that in the regex “Te” only occurs once but we use parenthesis to group the “or” statement:
Router1#show int desc | i Te(2/0/0|3/0/1) Te2/0/0 up up Router2 Te3/0/1 up up Router3
Note that for IOS-XR and IOS-XE, because they can accept multiple output filters (ie.
show int desc | include Te | exclude down), you must encapsulate any regular expressions that use
| with “”s.
That’s it for this tutorial on the basics. Come back for information on some advanced Cisco regular expression output filtering techniques.