Matching empty strings
The Inline Regular Expression Builder makes it easy to work with expressions that need to match both a specific pattern or an empty string. The constructor of the class takes a bool that indicates whether or not to allow empty strings (the default is false).

    var builder = new RegexBuilder(allowEmptyString: true);

And that's all there is to it!

Once the class has been initialized you can always retrieve (but not change) this value via:

    var allowsAnEmptyString = builder.AllowsEmptyString;