Juniper JUNOSE 11.1.X - COMMAND REFERENCE A TO M 4-9-2010 Command Reference Manual page 788

JUNOSe 11.1.x Command Reference Guide A to M
Global Configuration
Mode
746
ip as-path access-list
^ matches any AS number except the ones specified when used as a first
item within brackets
_ underscore; used in implementations on routers from other vendors on
either side of a path to specify a literal and disallow substring matching.
Allowed but not required in our CLI.