filtering - Select a particular rules out of a list

rule = {beta -> 4, alpha -> 2, x -> 4, z -> 2, w -> 0.8}

Select[rule, beta]
Select[rule, beta &]
Select[rule, beta -> _ &]

How would I select the rule that applies to beta only?

Or more general, how to select the rules apply to alpha and beta (or more variable, but not all of them)?

Thanks.

Answer

You can also use FilterRules:

rule = {beta -> 4, alpha -> 2, x -> 4, z -> 2, w -> 0.8};


FilterRules[rule, beta]
(* {beta -> 4} *)

FilterRules[rule, {beta, alpha}]
(* {beta -> 4, alpha -> 2} *)

Update: additional alternatives if you have V10:

KeyTake[rule,{alpha, x}]
(* or *) KeyTake[{alpha,x}][rule]

(* <|alpha->2,x->4|> *)
Normal@KeyTake[rule,{alpha, x}]
(*  {alpha->2,x->4} *)

KeySelect[rule, MatchQ[#,alpha|x]&]
(* or *) KeySelect[MatchQ[#,alpha|x]&][rule] 
(* <|alpha->2,x->4|>  *)
Normal@KeySelect[rule, MatchQ[#,alpha|x]&]
(*  {alpha->2,x->4} *)