how to extract the string using regular expressions

I have the list :

a1 = {"struct name1 {","struct name2{","struct name3{","struct name4{","struct name5{","struct lastStruct{"}

I tried to extract the struct names "name1",..."lastStruct" with this but I failed hard :(

StringCases[a1, RegularExpression["[^struct^{^\\s]"]]

Which returned:

{{"n", "a", "m", "e", "1"}, {"n", "a", "m", "e", "2"}, {"n", "a", "m", "e", "3"}, {"n", "a", "m", "e", "4"}, {"n", "a", "m", "e", "5"}, {"l", "a", "S"}}

Any ideas?