Users Guide

Regular
Expression
Denition
^ (caret) Matches the beginning of the input string. Alternatively, when used as the rst character within brackets [^ ], this
matches any number except the ones specied within the brackets.
$ (dollar) Matches the end of the input string.
. (period) Matches any single character, including white space.
* (asterisk) Matches 0 or more sequences of the immediately previous character or pattern.
+ (plus) Matches 1 or more sequences of the immediately previous character or pattern.
? (question) Matches 0 or 1 sequence of the immediately previous character or pattern.
( ) (parenthesis) Species patterns for multiple use when one of the multiplier metacharacters follows: asterisk *, plus sign +, or
question mark ?
[ ] (brackets) Matches any enclosed character and species a range of single characters.
- (hyphen) Used within brackets to specify a range of AS or community numbers.
_ (underscore) Matches a ^, a $, a comma, a space, or a {, or a }. Placed on either side of a string to specify a literal and disallow
substring matching. You can precede or follow numerals enclosed by underscores by any of the characters listed.
| (pipe) Matches characters on either side of the metacharacter; logical OR.
As seen in the following example, the expressions are displayed when using the show commands. To view the AS-PATH ACL conguration,
use the show config command in CONFIGURATION AS-PATH ACL mode and the show ip as-path-access-list command in
EXEC Privilege mode.
For more information about this command and route ltering, refer to Filtering BGP Routes.
The following example applies access list Eagle to routes inbound from BGP peer 10.5.5.2. Access list Eagle uses a regular expression to
deny routes originating in AS 32. The rst lines shown in bold create the access list and lter. The second lines shown in bold are the regular
expression shown as part of the access list lter.
Example of Using Regular Expression to Filter AS Paths
Dell(config)#router bgp 99
Dell(conf-router_bgp)#neigh AAA peer-group
Dell(conf-router_bgp)#neigh AAA no shut
Dell(conf-router_bgp)#show conf
!
router bgp 99
neighbor AAA peer-group
neighbor AAA no shutdown
neighbor 10.155.15.2 remote-as 32
neighbor 10.155.15.2 shutdown
Dell(conf-router_bgp)#neigh 10.155.15.2 filter-list 1 in
Dell(conf-router_bgp)#ex
Dell(conf)#ip as-path access-list Eagle
Dell(config-as-path)#deny 32$
Dell(config-as-path)#ex
Dell(conf)#router bgp 99
Dell(conf-router_bgp)#neighbor AAA filter-list Eagle in
Dell(conf-router_bgp)#show conf
!
router bgp 99
neighbor AAA peer-group
neighbor AAA filter-list Eaglein
neighbor AAA no shutdown
neighbor 10.155.15.2 remote-as 32
neighbor 10.155.15.2 filter-list 1 in
neighbor 10.155.15.2 shutdown
Dell(conf-router_bgp)#ex
Dell(conf)#ex
186
Border Gateway Protocol IPv4 (BGPv4)