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Regular Expressions as Filters
Regular expressions are used to filter AS paths or community lists. A regular expression is a special
character used to define a pattern that is then compared with an input string.
For an AS-path access list, as shown in the previous commands, if the AS path matches the regular
expression in the access list, the route matches the access list.
The following lists the regular expressions accepted in the Dell Networking OS.
Regular Expression Definition
^ (caret) Matches the beginning of the input string. Alternatively, when used as the first
character within brackets [^ ], this matches any number except the ones specified
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) Specifies patterns for multiple use when one of the multiplier metacharacters
follows: asterisk *, plus sign +, or question mark ?
[ ] (brackets) Matches any enclosed character and specifies 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 configuration, 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 filtering, 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 first lines shown in bold create
the access list and filter. The second lines shown in bold are the regular expression shown as part of the
access list filter.
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
Border Gateway Protocol IPv4 (BGPv4)
183