Re^5: Possible to have regexes act on file directly (decompose regex)

> For some regexes, it is very easy, for others, it is very difficult or even impossible.

it's often much simpler as you might think, cause you can decompose regexes into smaller and easier parts.

 perl -e'use re 'debug';qr/x{100}.*y{100}/'
Compiling REx "x{100}.*y{100}"
Final program:
   1: CURLY {100,100} (5)
   3:   EXACT <x> (0)
   5: STAR (7)
   6:   REG_ANY (0)
   7: CURLY {100,100} (11)
   9:   EXACT <y> (0)
  11: END (0)
anchored "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"... at 0 floating "yyyyyyyyyy
+yyyyyyyyyyyyyyyyyyyy"... at 100..2147483647 (checking floating) minle
+n 200 
Freeing REx: "x{100}.*y{100}"
[download]

In this case you start looking for 'x'x100 in a sliding window of size >200 from the beginning. Then you search backwards from the end in sliding windows for 'y'x100.

Like this even greedy matches can be handled (mostly) and the total match might even cover terrabytes.

Cheers Rolf

( addicted to the Perl Programming Language)

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Re^6: Possible to have regexes act on file directly (decompose regex) by Laurent_R (Canon) on May 02, 2014 at 21:07 UTC
Thank you for you very interesting point. If I have understood you correctly (I think I did), this is IMHO no longer a straight regex pattern match, but this is really becoming a very basic parser using several regexes. Or it can be viewed as building your own very basic custom pseudo-regex engine in pure Perl, using Perl's built-in regex engine for matching the various components of the original regex pattern. Yes, I believe that this can indeed work on a number of cases, but this can quickly become hairy for somewhat complicated patterns having more than one `*` or `+` quantifiers, alternations, captured matches, look-around assertions, etc. I have actually built a few times very primitive parsers using several regexes to parse progressively data that I could not figure out how to analyze with a single regex. This works, but this means that if the pattern changes, then you don't only need to change the pattern, but you also probably need to write different code around it. But all this is really speculation so long as the OP does not tell us which kind of regexes she or he wants to use.	[reply] [d/l] [select]
Re^7: Possible to have regexes act on file directly (decompose regex) by LanX (Saint) on May 02, 2014 at 21:55 UTC
It's recursive, I don't know if all cases can be coveredš but most cases are much easier then one might think: `> perl -e'use re 'debug'; qr/(x{100}.y{100})/' Compiling REx "(x{100}.y{100})" Final program: 1: CURLYX[0] {0,32767} (18) 3: OPEN1 (5) 5: CURLY {100,100} (9) 7: EXACT <x> (0) 9: STAR (11) 10: REG_ANY (0) 11: CURLY {100,100} (15) 13: EXACT <y> (0) 15: CLOSE1 (17) 17: WHILEM[1/1] (0) 18: NOTHING (19) 19: END (0) minlen 0 Freeing REx: "(x{100}.y{100})"` [download] > But all this is really speculation so long as the OP does not tell us which kind of regexes she or he wants to use. Yes the monastery has the tendency to answer kindergarten questions with a PHD thesis. I certainly won't start to implement it, just showing the theoretical approach. Cheers Rolf ( addicted to the Perl Programming Language) update š) with an opcode interpreter	[reply] [d/l]

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