A typical AS_PATH filter might look like this:
ip as-path access-list 83 permit ^1_701 _( _5646_ _1240_ ) .*
The string of characters following the permit keyword is a regular expression. The regex is composed of literals and metacharacters. Literals are just text characters that describe what the regex will try to match. In this example, 1, 701, 5646, and 1240 are literals describing autonomous system numbers .
Metacharacters are special regular-expression characters that act as operators, telling the regex how to perform matches. Table B-1 shows the metacharacters available for use with Cisco IOS; the remainder of this appendix describes how each of the metacharacters are used.
Table B-1. Regular-Expression Metacharacters Relevant to AS_PATH Access Lists