The Prime That Wasn’t
That's not a true Regular Expression: it uses back-tracking, and to the author's credit, he notes that in the article.<p>True regular expressions can't recognize primes.  It's important to keep these kind of distinctions in mind for two reasons.<p>First, without clear, precisely defined categories of formal languages, you'll fail to recognize unsolvable (or undecidable) problems and waste a lot of time.  Personally, I put "malware dectection" in this category, as determining whether a given file constitutes a virus seems equivalent to solving the halting problem.<p>Second, sloppy usage like using "Regular Expression" for something else is one of those common usage issues that allow people to argue based on the name ("regular expression") to lure the unsuspecting in, but then fall back on extreme technicalities to trick people into agreeing with the argument when they might not otherwise have agreed.  A lot like I've done in this comment.