It is not that certain inputs happen to correspond to a configuration solvable by an NFA.  It is that if you give it certain inputs it internally generates a data structure that actually IS a description of an NFA.  And the same is true of most RE engines.<p>As for _Mastering Regular Expressions_, you&#x27;ve demonstrated both what it says and that it is popular.  And further that it affected the terminology of a widely used project which has been embedded in many other projects.  (Like PHP.)<p>And yes, I agree that it did a lot of people a disservice.  But the damage is now long-done.  See for example <a href="https:&#x2F;&#x2F;msdn.microsoft.com&#x2F;en-us&#x2F;library&#x2F;e347654k%28v=vs.110%29.aspx" rel="nofollow">https:&#x2F;&#x2F;msdn.microsoft.com&#x2F;en-us&#x2F;library&#x2F;e347654k%28v=vs.110...</a> for yet another non-NFA that is described as one somewhere that programmers are likely to believe it.