Sample BGP regular expressions that can be used on Cisco router.
_100_ Going through AS 100
^100$ Directly connected to AS 100
_100$ Originated in AS 100
^100. Networks behind AS 100
^[0-9]+$ AS paths one AS long
^([0-9]+)(_\1)*$ Networks originating in neighboring AS
^$ Networks originated in local AS
.* Matches everything

BGP route selection criteria (on Cisco router) :

1. Exclude route with inaccessible next-hop
2. Prefer highest weight (local to router)
3. Prefer highest local preference (globa within AS)
4. Prefer routes that router originated
5. Prefer shortest AS paths (only length is compared)
6. Prefer lowest origin code (IGP
7. Prefer lowest MED
8. Prefer external (EBGP) path over internal (IBGP)
9. For IBGP paths, prefer path through closest IGP neighbor
10. For EBGP paths, prefer oldest (most stable) path
11. Prefer paths from router with the lowest BGP router-id

Before configuring BGP, you need gather information about your network and peer. Basically, configuring BGP on Juniper router is the same with Cisco or another router. Basic configuration is just configure BGP session the advertise our network.

Configuring BGP on Cisco router is simple enough. Before start configuring BGP, you need two consideration :
1. BGP only advertise routing entry that exist in IGP routing table
2. BGP is classfull routing protocol, you must include netmask in network statement BGP configuration if the network is not classfull.