Regular expression modelstate is false

I've created the following regex:

(^[%a-zA-Z][\w\-.]+)( [\w\-.]+|[\w\-.]+)([%A-Za-z.]$)

strings that should be allowed e.g.

test (working)
test test (working)
%test (working)
test% (working)
%test% (working)
% (not working)

I tried the following to make % only work:

(%|(^[%a-zA-Z][\w\-.]+)( [\w\-.]+|[\w\-.]+)([%A-Za-z.]$))

the defined pattern works fine with https://www.debuggex.com, but fails in my application, where the model looks like:

[Display(Name = "Publisher")]
[RegularExpression(@"(%|(^[%a-zA-Z][\w\-.]+)( [\w\-.]+|[\w\-.]+)([%A-Za-z.]$))", ErrorMessage = "Invalid Expression!")]    
public string SoftwarePublisher { get; set; }

any advice would be helpful. thanks in advance