Paths that originate from Border Gateway Protocol (BGP) autonomous system (AS) 222 and are learned from AS 222 or any AS directly attached to AS 222 are allowed by the AS path filter. Regular expressions are used to locate character strings that match a particular pattern.
The caret (^) character indicates that the subsequent characters should match the start of the string. Each router in the path prepends its AS number to the beginning of the AS path? therefore, the first AS number in the AS path is the AS from which the path is learned. The expression [09] indicates a single digit from 0 through 9. The asterisk (*) character indicates zero or more sequences of the previous expression. When combined, the expression [09]* indicates any number of digits, including a set of no digits. Therefore, the ip as-path access-list 1 permit ^[09]*_222$ command allows paths that are learned from any AS number, including AS 222.
The dollar sign ($) character indicates that the preceding characters should match the end of the string. The originating router will insert its AS number into the AS path, and subsequent routers will prepend their AS numbers to the beginning of the AS path string. The last AS number in the AS path is the originating AS; therefore, the ip as-path access-list 1 permit ^[09]*_222$ command allows paths that originate from AS 222.
The underscore (_) character is used to indicate a comma, a brace, the start or end of an input string, or a space. When used between two AS path numbers, the _ character indicates that the ASes are directly connected. Therefore, the ip as-path access-list 1 permit ^[09]*_222$ command indicates that if the path is not learned from AS 222 directly, the AS from which the path is learned must be directly connected to AS 222.
The ip as-path access-list 1 permit ^[09]*_222$ command does not permit paths that are learned from AS 222 and originate from any directly attached AS. To permit paths that are learned from AS 222 and originate from AS 222 or any directly attached AS, you should issue the ip as-path access-list 1 permit ^222_ [09]*$ command.
The ip as-path access-list 1 permit ^[09]*_222$ command does not permit paths that are learned from AS 222 and originate from a directly attached AS numbered from 0 through 9. To permit paths that are learned from AS 222 and originate from a directly attached AS numbered from 0 through 9, you should issue the ip as-path access-list 1 permit ^222_[09]$ command.
The ip as-path access-list 1 permit ^[09]*_222$ command does not permit paths that originate from AS 222 and are learned from a directly attached AS numbered from 0 through 9. To permit paths that originate from AS 222 and are learned from a directly attached AS numbered from 0 through 9, you should issue the ip as-path access-list 1 permit ^[09]_222$ command.