Using Regular Expressions

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Using Regular Expressions
You can use regular expressions when defining AS-path access lists and community lists to more easily filter routes. A regular expression uses special characters—often referred to as metacharacters—to define a pattern that is compared with an input string.

AS-path Lists

For an AS-path access list, the input string is the AS path of the routes to which the list is applied via the route-map or neighbor filter-list commands. If the AS path matches the regular expression in the access list, then the route matches the access list.

Example

The following commands apply access list 1 to routes inbound from BGP peer 10.5.5.2. Access list 1 uses a regular expression to deny routes originating in AS 32.
host1(config-router)#neighbor 10.5.5.2 remote-as 32
host1(config-router)#neighbor 10.5.5.2 filter-list 1 in
host1(config-router)#exit
host1(config)#ip as-path access-list 1 deny 32$
Community Lists

For a community list, the input string is the community attribute of the routes to which the list is applied via a route-map command. If the community attribute matches the regular expression in the community list, then the route matches the community list.

Example

The following commands apply route map 5 to routes forwarded to BGP peer 10.5.5.4. Route map 5 uses a regular expression to match community numbers ending with 305, setting the weight of matching routes to 150.
host1(config-router)#neighbor 10.5.5.4 remote-as 425
host1(config-router)#neighbor 10.5.5.4 route-map 5 out
host1(config-router)#exit
host1(config)#route-map 5 permit 10
host1(config-route-map)#match community 305$
host1(config-route-map)#set weight 150
Community Numbers

When you use a regular expression to match a community number, use the appropriate format for the community number in the community list. If you issue the ip bgp-community new-format command, the community number has the format AA:NN where AA is a number that identifies the autonomous system, and NN is a number that identifies the community within the autonomous system. Otherwise, the community number is an integer in the range 1-4294967295.

Metacharacters

Each regular expression consists of one or more metacharacters and zero or more complete or partial AS or community numbers. Table 1-3 describes the metacharacters supported for regular expression pattern-matching.

Table 1-3 Supported regular expression metacharacters

Metacharacter

Description

^

Matches the beginning of the input string.
Alternatively, when used as the first character within brackets—[^ ]—matches any number except the ones specified within the brackets.

$

Matches the end of the input string.

.

Matches any single character, including white space.

*

Matches 0 or more sequences of the immediately previous character or pattern.

+

Matches 1 or more sequences of the immediately previous character or pattern.

?

Matches 0 or 1 sequence of the immediately previous character or pattern.

()

Specifies patterns for multiple use when followed by one of the multiplier metacharacters: asterisk *, plus sign +, or question mark ?

[ ]

Matches any enclosed character; 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, a {, or a }. Placed on either side of a string to specify a literal and disallow substring matching. Numerals enclosed by underscores can be preceded or followed by any of the characters listed above.

|

Matches characters on either side of the metacharacter; logical OR.

Using Metacharacters as Literal Tokens

You can remove the special meaning of a metacharacter by preceding it with a backslash (\). Such a construction denotes that the metacharacter is not treated as a metacharacter for that regular expression. It is simply a character or token with no special meaning, just as a numeral has no special meaning. The backslash applies only to the character immediately following it in the regular expression.

On the ERX system, you are likely to do this only for the parentheses characters, ( or ). BGP indicates a segment of an AS path that is of type AS-confed-set or AS-confed-seq by enclosing that segment within parentheses.

Example

The following AS-path access list uses a regular expression to match routes having an AS-path attribute that begins with any AS-confed-set or AS-confed-seq:
host1(config)#ip as-path access-list 1 permit ^$
The following AS-path access list uses a regular expression to match routes having an AS-path attribute that ends with any AS-confed-set or AS-confed-seq:
host1(config)#ip as-path access-list 1 permit $$
The following AS-path access list uses a regular expression to match routes having an AS-path attribute that includes the specific AS-confed-set or AS-confed-seq, (100 200):
host1(config)#ip as-path access-list 1 permit $100 200$
Regular Expression Examples

Table 1-4 shows some representative regular expressions that might be used in an AS-path access list or community list along with sample attribute values that match or do not match the regular expression.

Table 1-4 Sample regular expressions

Regular Expression

Matches Any AS-path or Community Attribute That...

^12

Begins with 12, such as the following:
12 23 42    12 629
12    1245 19
but not
58 12 7

[^12]

Includes any numeral except 1 or 2, such as the following:
44 73 46    5 69
8
but not
1145 19    12
2 49

305$

Ends with 305, such as the following:
89 611 305    305
42 305    19 1305
6666:305

.5

Includes any one character followed by numeral 5, such as the following:
89 611 35    33 252 12 998
600:500

1.9

Includes a sequence of three characters where the first character is numeral 1 and the third character is numeral 9, such as the following:
179 35 24    33 252 129 48
2129 14    600:2129
321:94

.*

Includes any character; matches all AS paths and community lists

42*

Includes a number having a numeral 4 followed by zero or more instances of numeral 2, such as the following:
67 42 51    314
33 252 422 48    3142
4 339 78    31422

1(37)*

Includes a sequence having a numeral 1 followed by zero or more instances of the pattern 37, such as the following:
137 42 21    1373737 29 4
1
but not
4 3737 78

42+

Includes a number having a numeral 4 immediately followed by one or more instances of numeral 2, such as the following:
67 42 21    33 252 422 48
but not
4 329 78

1(37)+

Includes a sequence having a numeral 1 immediately followed by one or more instances of the pattern 37, such as the following:
1373737 29 4    4 37137 78
137 42 21
but not
4 372 21    21 37 5
1 456 88    1

42?

Includes a number having a numeral 4 followed by zero or only one instance of numeral 2, such as the following:
67 42 71    4 359 78
but not
33 252 422 48

1(37)?

Includes a sequence having a numeral 1 followed by zero or only one instance of the pattern 37, such as the following:
137 42 21    53 612 49
1
but not
4 13737 78

7..

Includes a sequence of three characters where the first character is numeral 7, such as the following:
600 700 100    25 7771
In the following examples, the three characters are 7, space, 8:
307 800     6127 888 999

^7..

Includes a number ranging from 700 to 799, such as the following:
6127 723 999     700 100 600
but not
25 7771    307 800

^7..$

Consists only of a number ranging from 700 to 799, such as the following:
723     700
but not
25 7771    307 800
6127 723 999    700 100 600

[621]

Includes any of the numerals 6, 2, or 1, such as the following:
60 43    34 545 92
200    710
86 53
The regular expression [162] has the same results.

[0-9]

Includes any number in the range 0-9

^$22 431$

(AS-path attribute only) Begins with the AS-confed-set or AS-confed-seq (22 431), such as the following:
(22 431) 102    (22 431) 55 76
but not
43 (22 431) 5    22 431 59

{41 19}

(AS-path attribute only) Includes the AS-set or AS-seq {41 19}, such as the following:
{41 19} 53     76 {41 19} 17
255 {41 19}
but not
3 41 19     41 19 532

101 102 | 103 105

Includes either sequence 101 102 or sequence 103 105, such as the following:
43 101 102 5    103 105 22
but not
19 102 101    102 103

_200_

Includes the number 200 (as opposed to the pattern consisting of numeral 2, numeral 0, numeral 0), such as the following:
33 200 422 48    ^200$
^200 500$
but not
33 20 422 48     51 2005
Our implementation of regular expressions is not literal. Substring matching is enabled by default. Specifying 200 (no underscores) results in a match on 200 and on 2005. The underscore metacharacter disables substring matching.

For information on using AS-path access lists, see Access Lists on page 1-16. For information on using community lists, see Community Lists on page 1-30.

Metacharacter	Description
^	Matches the beginning of the input string. Alternatively, when used as the first character within brackets—[^ ]—matches any number except the ones specified within the brackets.
$	Matches the end of the input string.
.	Matches any single character, including white space.
*	Matches 0 or more sequences of the immediately previous character or pattern.
+	Matches 1 or more sequences of the immediately previous character or pattern.
?	Matches 0 or 1 sequence of the immediately previous character or pattern.
()	Specifies patterns for multiple use when followed by one of the multiplier metacharacters: asterisk *, plus sign +, or question mark ?
[ ]	Matches any enclosed character; 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, a {, or a }. Placed on either side of a string to specify a literal and disallow substring matching. Numerals enclosed by underscores can be preceded or followed by any of the characters listed above.
\|	Matches characters on either side of the metacharacter; logical OR.

Regular Expression	Matches Any AS-path or Community Attribute That...
^12	Begins with 12, such as the following: 12 23 42 12 629 12 1245 19 but not 58 12 7
[^12]	Includes any numeral except 1 or 2, such as the following: 44 73 46 5 69 8 but not 1145 19 12 2 49
305$	Ends with 305, such as the following: 89 611 305 305 42 305 19 1305 6666:305
.5	Includes any one character followed by numeral 5, such as the following: 89 611 35 33 252 12 998 600:500
1.9	Includes a sequence of three characters where the first character is numeral 1 and the third character is numeral 9, such as the following: 179 35 24 33 252 129 48 2129 14 600:2129 321:94
.*	Includes any character; matches all AS paths and community lists
42*	Includes a number having a numeral 4 followed by zero or more instances of numeral 2, such as the following: 67 42 51 314 33 252 422 48 3142 4 339 78 31422
1(37)*	Includes a sequence having a numeral 1 followed by zero or more instances of the pattern 37, such as the following: 137 42 21 1373737 29 4 1 but not 4 3737 78
42+	Includes a number having a numeral 4 immediately followed by one or more instances of numeral 2, such as the following: 67 42 21 33 252 422 48 but not 4 329 78
1(37)+	Includes a sequence having a numeral 1 immediately followed by one or more instances of the pattern 37, such as the following: 1373737 29 4 4 37137 78 137 42 21 but not 4 372 21 21 37 5 1 456 88 1
42?	Includes a number having a numeral 4 followed by zero or only one instance of numeral 2, such as the following: 67 42 71 4 359 78 but not 33 252 422 48
1(37)?	Includes a sequence having a numeral 1 followed by zero or only one instance of the pattern 37, such as the following: 137 42 21 53 612 49 1 but not 4 13737 78
7..	Includes a sequence of three characters where the first character is numeral 7, such as the following: 600 700 100 25 7771 In the following examples, the three characters are 7, space, 8: 307 800 6127 888 999
^7..	Includes a number ranging from 700 to 799, such as the following: 6127 723 999 700 100 600 but not 25 7771 307 800
^7..$	Consists only of a number ranging from 700 to 799, such as the following: 723 700 but not 25 7771 307 800 6127 723 999 700 100 600
[621]	Includes any of the numerals 6, 2, or 1, such as the following: 60 43 34 545 92 200 710 86 53 The regular expression [162] has the same results.
[0-9]	Includes any number in the range 0-9
^\(22 431\)	(AS-path attribute only) Begins with the AS-confed-set or AS-confed-seq (22 431), such as the following: (22 431) 102 (22 431) 55 76 but not 43 (22 431) 5 22 431 59
{41 19}	(AS-path attribute only) Includes the AS-set or AS-seq {41 19}, such as the following: {41 19} 53 76 {41 19} 17 255 {41 19} but not 3 41 19 41 19 532
101 102 \| 103 105	Includes either sequence 101 102 or sequence 103 105, such as the following: 43 101 102 5 103 105 22 but not 19 102 101 102 103
_200_	Includes the number 200 (as opposed to the pattern consisting of numeral 2, numeral 0, numeral 0), such as the following: 33 200 422 48 ^200$ ^200 500$ but not 33 20 422 48 51 2005 Our implementation of regular expressions is not literal. Substring matching is enabled by default. Specifying 200 (no underscores) results in a match on 200 and on 2005. The underscore metacharacter disables substring matching.

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