Regular Expression Examples and Applications:
---------------------------------------------

URLs:
-----

http://etext.lib.virginia.edu/helpsheets/regex.html
http://www.ieng.com/warp/public/459/26.html
http://tinlib.ujep.cz/cisco/data/doc/software/10_3/ptcgcr/70929.htm
http://www.ianai.net/ispf/BGP-201/BGP-201.PPT
http://www.conservatory.com/info/gawk.info.Regexp.html

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Special Symbols Used:

.	- Matches any single character
*	- Matches 0 or more sequences of the pattern
+	- Matches 1 or more sequences of the pattern
?	- Matches 0 or 1 occurrences of the pattern
^	- Matches the beginning of the input string
$	- Matches the end of the input string
_	- Matches a comma, left brace, right brace, left parenthesis
	  right parenthesis, the beginning of the input string,
	  or a space.
\char	- Used to dis-associate "special" characters with their special
	  properties and make them normal characters.

If you need to use one of the following "special" symbols, simply insert
a backslash in front of the character:

	\.
	\*
	\+
	\?

If you want to specify a range of matches, such as: a, e, i, o, or u, 
it can be done using brackets, [].  When using brackets, only one character 
must match in order for the regex to be true.

	[aeiou]	- regex to match anything with a vowel

You can also include ranges by using dashes..

	[a-dA-D]     - regex to match any char a-d, upper or lowercase.
	[a-dA-D\-]   - regex to match any char a-d, upper or lowercase.
		       Also matches for a hyphen.
	[a-dA-D\-\]] - regex to match any char a-d, upper or lowercase.
		       Also matches for a hyphen, or close bracket.

You can reverse the use of brackets by inserting a ^ character. 

	[^a-dqsv] - This example matches any letter -except- 
		    the ones listed
	[^\]d]	  - This example matches any letter -except- ] and d.


Using Multipliers:

You can create more complex regular expressions that instruct the IOS
to match multiple occurrences of a specified regular expression.  There
are special characters that may be used to be used as Multipliers.

*	- Matches 0 or more single- or multiple-char patterns
+	- Matches 1 or more single- or multiple-char patterns
?	- Matches 0 or 1 occurrences of the single- or multiple-char
	  patterns

	a*   - Matches any number of occurrences of the letter,
	       a, including none.

	a+   - Requires there to be at least one letter a in the string
	       to be matched.

	ba?b - Matches the string "bb" or "bab"

	\**  - Matches any number of asterisks.


To use multipliers with multiple-character patterns, enclose the pattern
in parentheses.  In the following example, the pattern matches any
number of the multiple-character string "ab":

	(ab)*

As a more complex example, the following pattern matches one or more 
instances of alphanumeric pairs (but not none; that is, an empty
string is not a match):

	([A-Za-z][0-9])+


Alternation:

Alternation allows you to specify alternative patterns to match against a 
string.  You separate the alternative patterns with a veritcal bar (|).
Exact one of the alternatives can match the input string.

	codex|telebit - Matches either "codex" or "telebit"


Anchoring:

You can instruct the IOS software to match a regular expression pattern 
against the beginning or the end of the input string.  That is, you can
specify that the beginning or end of an input string contain a specific
pattern.  You "anchor" these regular expressions to a portion of the
input string using the special characters shown below:

^	- Matches the beginning of the input string
$	- matches the end of the input string

	^abcd	- Matches an input string only if the string starts
		  with "abcd"

	[^abcd] - A range that matches any single letter, as long as
		  it is not the letters, a, b, c, or d.

	\.12$	- Matches an input string that ends with ".12"


BGP AS-Path Examples:
---------------------

^123_

This regular expression allows any announcement as long as its AS-Path
begins with AS 123.  This is not very secure since an announcement can
originate from any AS that is behind AS 123.

Matches the following:
	
	123
	123 321
	123 123 321
	123 321 456 981
	...


_123$

This regular expression matches any AS-Path that ends in AS 123.

Matches the following:

	123
	100 123
	100 200 123
	100 200 123 123
	...


^(123_)+$

This regular expression allows for prepending an AS, but -only- allows 
announcements from the specified AS, in this case AS 123.

Matches the following:

	123
	123 123
	123 123 123
	123 123 123 123
	...


^.*(_123)+$

This regular expression matches anything with 1 or more occurrences of 
the AS 123.

Matches the following:

	123
	100 123
	100 200 123 123
	100 200 300 123 123 123 123
	...


^(123)(_123)*$

This regular expression matches any value that BEGINS with 123.  It will
also match any # of occurrences of " 123" following the initial value
of 123, as well as 0 occurrences.  This is also used for permitting
pre-pended AS #s.

Matches the following:

	123
	123 123
	123 123 123
	123 123 123 123
	...


AS-Path ACL examples:

!
ip as-path access-list 1 seq 5 permit ^123
!
ip as-path access-list 2 seq 5 permit ^(_123)+$
!
ip as-path access-list 3 seq 5 permit ^(123)(_123)*$
!


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This file was written by: Eric Cables
This file was last modified on: 10/21/2000