You are correct. Sometimes the information though meant for good, can be inaccurate or incomplete such as the information on this page which would have resolved your issue.
In addition to the book referenced, the juniper website does offer good explanation on the basics fo using regex and on the exact case you have mention here. For example, the explanations on this and the example cited works for you.
https://www.juniper.net/documentation/en_US/junos/topics/usage-guidelines/policy-configuring-as-path-regular-expressions-to-use-as-routing-policy-match-conditions.html
AS path the match: Path of any length that begins with AS numbers 4, 5, 6
Regular Expression: 4 5 6 .* <==== The problem is that I don't think that this would work.
It would have to "4 5 6".*
This should have been written as 456.* which would match your case.