Discussion

Table 13-1. AS path regular expression operators
Operator Description Match example
{m,n} Match at least m and at most n repetitions of term. 65500{2,3}—Match only "65500 65500"
and "65500 65500 65500".
? Match zero or one repetition of term; equivalent to {0,1}. 65500?—Match only "65500" and "65500
65500".
{m } Match exactly m repetitions of term. 65500{2}—Match only "65500 65500".
{m,} Match m or more repetitions of term. 65500{2,}—Match "65500 65500",
"65500 65500 65500", "65500 65500
65500 65500", and so on.
* Match zero or more repetitions of term; equivalent to {0,}. 65500*—Match "65500", "65500 65500", and
so on; also match a path that does not contain
"65500".
+ Match one or more repetitions of term; equivalent to {1,}. 65500 + —Match "65500", "65500
65500", and so on.
. (dot) Match any single instance of any term. 65500.—Match 65500 if it appears anywhere
in the AS path. A more exact way to create this
match is ".* 65500 .*".
| Match one of the terms on either side of the pipe; the
terms can include other operators.
( ) Match a group of terms enclosed in the parentheses.
You must include the term when defining the pattern to match, but the operator is
optional. When you leave out an operator, the AS path exactly matches what you
type for the term.
The JUNOS regular expressions for AS paths are, for the most part, identical to Unix
regular expressions. There are, however, a few differences. The main difference is
that the basic unit of matching is an entire AS number, not an individual character.
This means that the JUNOS regular expression treats 65500 as a single entity when it
performs any matching operations, not as the five individual digits 6, 5, 5, 0, and 0. In
other words, the AS number is effectively a single integer. A second difference is that
with Unix, you need to type the operators ^ (which matches the beginning of a
string) and $ (which matches the end of a string), but in the JUNOS regular expressions,
these operators are always assumed to be present. So, with Unix you would
type ^65500$ to match the string 65500, but in the AS path regular expression you just
need to type 65500.
Looking back at our recipe, the AS path match term and operator are:
65500{4}
(65500 | 65505)—Match either "65500" or
"65505".
– Match a range; the terms can include other operators. 65500-65505—Match 65500, 65501,
65502, 65503, 65504, or 65505.
"( )" Match a null AS path.
^
Indicates the character at the beginning of an AS path.
$ Indicates the character at the end of an AS path.
446 | Chapter 13: BGP
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