Hello,
Here is an article to remain or learn each metacharacter feature in ip as-path command.
. Any single character, including white space.
[] Any character listed between the brackets.
[^] Any character except those listed between the brackets. (The caret is placed before the sequence of literals.)
- (Hyphen) Any character in the range between the two literals separated by the hyphen.
? Zero or one instances of the character or pattern.
* Zero or more instances of the character or pattern.
+ One or more instances of the character or pattern.
^ Start of a line.
$ End of a line.
| Either of the literals separated by the metacharacter.
_ (Underscore) A comma, the beginning of the line, the end of the line, or a space.
HTH 
