regex to match as_path regex

check the roads

2019-06-08

While working on input validation for my soon-to-be-released looking glass app Hyperglass, I had to find a way to validate regex with regex. Google searches returned nothing of relevance, so I built it myself. I figured this might be useful for others in the future:

Supported lookup patterns

Expression	Match
`_65000$`	Originated by AS65000
`^65000\`	Received from AS65000
`_65000_`	Via AS65000
`_65000_65001_`	Via AS65000 and AS65001
`_65000(_.+_)65001$`	Anything from AS65001 that passed through AS65000

AS_PATH regex pattern for asplain

^(\^|^\_)(\d+\_|\d+\$|\d+\(\_\.\+\_\))+$

AS_PATH regex pattern for asdot

^(\^|^\_)((\d+\.\d+)\_|(\d+\.\d+)\$|(\d+\.\d+)\(\_\.\+\_\))+$

Notes

If using these patterns with Python as I am, you’ll want to make sure you use the raw string literal flag, e.g. r"pattern". For example:

re_bgp_aspath_default = r"^(\^|^\_)(\d+\_|\d+\$|\d+\(\_\.\+\_\))+$"

This will ensure that Python doesn’t interpret the regex escape characters as Python recape characters, and returns the raw string.