There can be instances where identifying all the networks from a certain provider may be too complicated or numerous, and a regular expression may be more suitable to filter all networks from a certain AS. Regular expression can be used for this.
Quick guide to regular expression
| Modifier | Purpose |
| _ | Matches a space |
| ^ | Indicates the start of a string |
| $ | Indicates the end of a string |
| [] | Matches a single character with a range of characters |
| – | Indicates a range of numbers in brackets |
| [^] | Excludes the characters listed in the brackets |
| () | Nesting of search patterns uses parentheses |
| | | Acts as an OR logic to a query |
| . | Matches a single character including space |
| * | Matches zero or more characters, or pattern |
| + | Matches one or more instances of the character, or pattern |
| ? | Matches one or no instances of the character, or pattern |
Examples of Common Regular Expressions in BGP
^$ – Local originating routes
permit ^205_ – Only routes from neighbour AS 205
permit _205$ – Only routes originating from AS205
permit _205_ – Only routes that pass through AS205
permit ^[0-9]+ [0-9]+ [0-9]+? – Routes with three or fewer AS_Path entries
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