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 7 describes the metacharacters supported for regular expression pattern-matching.
 Table 7:  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 E-series router, you are likely to use the backslash 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 8 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 8:  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 about using AS-path access lists, see Access Lists. For information about using community lists, see Community Lists.

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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