Starting with version 2.6.0, when this attribute is applied on a data field AutoFixture will try to generate a value that matches the specified regular expression.
Let's take as an example the following type:
Prior to version 2.6.0 if we request an anonymous instance from AutoFixture, by default we would get back an instance of the above type with it's properties containing values similar to those below.
However, from version 2.6.0 AutoFixture can handle requests with regular expressions, through the RegularExpressionAttribute class, by issuing a new request for the specified regular expression pattern.
The idea behind this new feature is that members of a Regular Expression (Regex) can be generated by first transforming the Regex into a Finite-state machine, particularly a Deterministic finite automaton.