3 ms·

Only Rob could pull this feat. Not for the lesser mortals :-)

• tgv 4y ago
  It is for lesser mortals, too. I think everyone with half a CS degree should be able to pull it off. That said: it's an implementation which doesn't behave like normal regexps (it's a left-shortest match first instead of the common left-longest) and it's a back-tracker. Because this implementation doesn't allow much choice, expensive examples will look contrived, but extending this implementation with character sets, groups and choices (i.e. a|b) will make its exponential nature felt very quickly.
  • eesmith 4y ago
    The book, two pages later, gives a left-longest implementation of matchstar. See https://archive.org/details/practiceofprogra0000kern/page/226/mode/2up?q=matchstar https://archive.org/details/practiceofprogra0000kern/page/22... . > extending this implementation The exercises at the end of the chapter ask the student to extension the implementation along these lines. Including support for utf8. > exponential nature Both the essay and the book highlight this issue. The latter comments that some commercial greps (at the time) also had exponential behavior. https://archive.org/details/practiceofprogra0000kern/page/226/mode/2up?q=exponential https://archive.org/details/practiceofprogra0000kern/page/22...
• abecedarius 4y ago
  It's a natural approach to anyone familiar with the pattern matchers explained in old Lisp books. This is not just hindsight because when I first read this chapter in Beautiful Code I stopped after the problem statement and wrote something similar before reading the rest. (I didn't choose exactly the same grep sublanguage to implement -- it was something like supporting the most basic escaping instead of ^ and $.)