Python network automation, BGP and IOS regular expressions (regex). Search output (Part 2).

Key Takeaways

[Music] follow every trap every road additional strain analyze and try to feel you laugh those lines up on your face [Music] Oh Oh [Music] this is one of multiple videos discussing regular expressions another regular expression is a star this matches zero or more sequences of the character preceding the star or asterisk it also acts as a wild-card for matching any number of characters so five star matches any occurrence of the number five including none this matches the character eighteen dot because we escaping the special meaning of this value and any characters are that to follow 18 so on a switch in my journey through topology if I type shall run type include in user dot star that includes all lines that have user followed by zero or more characters so this is looking for any character repeated zero more times if we do this we get that output that's looking for use followed by zero or more occurrences of our so we don't want zero or more occurrences of our we want the user followed by zero or more occurrences of something so we get output like that now in Python we can do something similar so let me show you some Python theory on the Python documentation page they talk about regular expression subtext it's very similar to what you'll find on Cisco dodge in default mode matches any character except a newline star or asterisk causes the resulting regular expression to match zero or more repetitions of the preceding regular expression as many repetitions as are possible so this matches a a B or a followed by any number of bees so again use a dot star gives us that but the user star gives us that notice we have zero or more occurrences of the letter R ok so cat reg X 3 dot py here's a Python script that I've created and we are searching for user dot star just like we did on the Cisco IOS here so again we should if our Python script works properly see this information returned so I'll run the script Python reg X 3 dot py and notice that's what's returned now the great thing about regular expressions is you can use show commands to test things before you add them to scripts or to BGP statements so as an example looking at the regular expressions in BGP in this example notice they've got a permit dot star that permits anything so they've got an IP access path access list one two nine an underscore means space for is the digit fall and end of line and then this matches anything else so this is like a permit any any so they are denying BGP advertisements from this neighbor so inbound from that neighbor based on this IP autonomous system private access list so here they are denying all routes that originate in autonomous system for and then they are permitting all other routes so this is denying all BGP advertisements that originated from autonomous system for but then they are permitting everything else now you can test that kind of thing in the CLI again by typing so run pipe include and let's include e that will show me all a lines that have a in them and as you can see there are many lines if I say e dollar that shows me all lines that end in E in this case only control plane if I touch to run we can see a whole bunch of lines let's look for lines that end in zero so firstly without the dollar that's all lines that include zero such as this line here and this line here that if we say zero dollar that's all lines that end in zero so notice all of these lines end in a zero if we say space $0 that's all lines that have a space to then a zero and then end of line so notice space zero end of line that's different to what we saw over here here we weren't looking for a space here we were just looking for lines of that end in zero but now we're looking for lines for that end in space zero and that's what they doing here they're looking for all bgp advertisements that originated from autonomous system call if you look at BGP routing tables you'll notice that the autonomous system that rad originates from exactly the end so notice they don't say for dollar they say space a for dollar because otherwise you would match autonomous system numbers like sixty five thousand and four or one thousand and four and so forth and so on now in subsequent videos I'll continue talking about various regular expressions hope you enjoyed with this video if you did please like it and please subscribe to my youtube channel I wish you all the very best [Music]