What does the
_scale_id_regex = @ConfigurationSettings.AppSettings["ScaleIdRegEx"];
It's called a verbatim identifier. It allows you to name variables after reserved words. e.g.
string @string = string.Empty; object @object = new object(); int @int = 1; ...
The code you have is valid, but I don't believe
@ services any real purpose there. Since this got upvoted faster than I could refresh my page, here's what the ECMA C# Language Specification, section 9.4.2 says.
The prefix "@" enables the use of keywords as identifiers, which is useful when interfacing with other programming languages. The character @ is not actually part of the identifier, so the identifier might be seen in other languages as a normal identifier, without the prefix. An identifier with an @ prefix is called a verbatim identifier.
The code you posted is valid because this is allowed by the language specification, albeit discouraged.
[Note: Use of the @ prefix for identifiers that are not keywords is permitted, but strongly discouraged as a matter of style. end note]