There may be times when conditionally matching on all the networks prefixes may be too complicated, and identifying all routes from a specific organization is preferred, BGP Regular expression (RegEx) is one of the best solution. In such a case, path selection can be made by using a BGP AS_Path.
Regular expressions (regex) are used to parse through a large number of available ASNs (4,294,967,295). Regular expression are based on query modifiers used to select only the match routes with the appropriate content. The BGP table can be parsed with regular expression by using the command show bgp afi safi regexp regex-pattern.
BGP Regular Expression
Learning regular expression can take time, but the most common ones used in BGP involve the input string special characters: ^, $, and _.
The table below displays some common BGP regex.
Regular Expression Query Modifiers
| Modifier | Description |
| _ (underscore) | Matches a space |
| ^ (caret) | Indicates the start of a string |
| $ (dollar sign) | Indicates the end of a string |
| – (hyphen) | Indicates a range of numbers in brackets |
| [] (brackets) | Matches a single character or nesting within a range |
| [^] (caret in brackets) | Excludes the characters listed in brackets |
| () (parentheses) | Used for nesting of search patterns |
| | (pipe) | Provides OR functionality to the query |
| . (period) | Matches a single character, including a space |
| * (asterisk) | Matches zero or more characters or patterns |
| + (plus sign) | Matches one or more instances of the character or pattern |
| ? (question mark) | Matches one or no instances of the character or pattern |
\\ is an escape character | You’ll need this for BGP confederations |
Common BGP RegEx
Learning regex can take time, but the most common ones used in BGP involve the ^, $, and _.
The table below displays some common BGP regex.
Common BGP Regular Expressions
| Regular Expression | Meaning |
| ^$ | Local originating routes |
| permit ^200_ | Only routes from neighbor AS 200 |
| permit _200$ | Only routes originating from AS 200 |
| permit _200_ | Only routes that pass through AS 200 |
| permit ^[0-9]+ [0-9]+ [0-9]+? | Routes with three or fewer AS_Path entries |
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