could anyone tell me what ip as-path access-list 100 deny .+_.+_.+_.+_.+_.+_.+_.+_.+_.+_.+_.+ stands for?!

I searched over the internet for the meaning of this regular expression, but I was unable to find any info.

2 Answers 2


Every time you see .+_, that regular expression represents a single BGP autonomous system. This as-path list denies BGP as paths equal to or longer than 12 paths long.

The regular expression works because . represents any character, + is a wild card which optionally repeats the previous character an unlimited number of times, and _ represents the space between the autonomous system paths. For more information, refer to the Cisco IOS Regular Expression guide, or this INE BGP Regular Expressions blog.

  • Hello Mike, thank you for your help and clarification. Very useful. Best regards.
    – ipermo
    Sep 16, 2013 at 12:31
  • you are most welcome good luck with your project Sep 16, 2013 at 12:37
  • 1
    As additional info, IOS has long supported 'bgp maxas-limit n' to make it easier to restrict AS-path size, however it is obviously global, while as-path ACL you can call more surgically.
    – ytti
    Sep 16, 2013 at 13:26

It'll be matching 12 AS-path list. But, I think a more appropriate way of matching it would be ^[0-9]+_[0-9]+_[0-9]+_[0-9]+_[0-9]+_[0-9]+_[0-9]+_[0-9]+_[0-9]+_[0-9]+_[0-9]+_[0-9]+$, because the expression you mentioned might just match any character ie. special characters and alphabets included.

  • 1
    AS-path regular expression are not 'normal' regular expressions, it's already a given that only numbers can be used.
    – Teun Vink
    May 9, 2015 at 8:47
  • 1
    I'm not completely sure this would work the way the OP needs it to. [0-9]+[0-9]+ doesn't account for the space between autonomous system paths. In plain terms, it says to match 0-9 ([0-9]) an infinite amount of times (+) followed directly by another set of infinite 0-9 sets ([0-9]+).
    – Ryan Foley
    May 9, 2015 at 15:07

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