A Note on Regular Expressions Used in Cisco IOS

Regular expressions are commonly used to filter out show commands like show ip bgp and to identify a route in AS-Path Lists.

Character	Meaning
^	start of a string
$	end of a string
[]	range of characters
-	used to specify range ( i.e. [0-9] )
( )	logical grouping
.	any single character
*	repeat previous character zero or more times
+	repeat previous character one or more times
?	repeat previous character zero or one time
_	matches the space between AS numbers
\\	an escape character used for BGP confederations

Some commonly used examples:-

String	Meaning
.*	anything, all prefixes, will match everything
^$	match all prefixes learned from the local AS
^67_	match all prefixes learned from AS 67
_67$	match all prefixes originated in AS 67
_67_	match all prefixes any instance of AS 67
^[0-9]+$	match all prefixesprefixes originated from directly connected ASes

Now let's see the above expressions applied in the following real BGP table..

Expressions are used in the order of the table..