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Here's the same code with one debug print added to show what's going on at the beginning of each time around the "while 1" loop.

#!/usr/bin/perl # http://perlmonks.org/?node_id=1189605 use strict; use warnings; $| = 1; sub err { die "ERROR: ", s/\G/<@_>/r, "\n" } sub crossmap { my ($left, $right) = @_; [ map { my $prefix = $_ ; map $prefix.$_, @$right } @$left ]; } sub expr { my $answer = [ '' ]; print("\ndebug ", s/\G/<<you are here with [@$answer]>>/r, " end " +), $answer = /\G\|/gc ? [ @$answer, @{ expr() } ] : /\G\d+/gc ? crossmap $answer, [ $& ] : /\G\[\d+\]/gc ? crossmap $answer, [ $& =~ /\d/g ] : /\G\(/gc ? ( crossmap($answer, expr()), /\G\)/gc || err "missing ')'")[0] : return $answer while 1; } while(<DATA>) { chomp; print "$_ => "; my $answer = expr(); /\G\z/gc or err "incomplete parse"; local $" = ','; print "@$answer\n"; } __DATA__ (65|70) (3[678]|4[1678]) 5[45] (6[4569]|7[01])

It prints the following ( showing both where the current position in the input is (the \G) and the contents of the variable $answer at the time).:

(65|70) => debug <<you are here with []>>(65|70) end debug (<<you are here with []>>65|70) end debug (65<<you are here with [65]>>|70) end debug (65|<<you are here with []>>70) end debug (65|70<<you are here with [70]>>) end debug (65|70<<you are here with [65 70]>>) end debug (65|70)<<you are here with [65 70]>> end 65,70 (3[678]|4[1678]) => debug <<you are here with []>>(3[678]|4[1678]) end debug (<<you are here with []>>3[678]|4[1678]) end debug (3<<you are here with [3]>>[678]|4[1678]) end debug (3[678]<<you are here with [36 37 38]>>|4[1678]) end debug (3[678]|<<you are here with []>>4[1678]) end debug (3[678]|4<<you are here with [4]>>[1678]) end debug (3[678]|4[1678]<<you are here with [41 46 47 48]>>) end debug (3[678]|4[1678]<<you are here with [36 37 38 41 46 47 48]>>) end + debug (3[678]|4[1678])<<you are here with [36 37 38 41 46 47 48]>> end + 36,37,38,41,46,47,48 5[45] => debug <<you are here with []>>5[45] end debug 5<<you are here with [5]>>[45] end debug 5[45]<<you are here with [54 55]>> end 54,55 (6[4569]|7[01]) => debug <<you are here with []>>(6[4569]|7[01]) end debug (<<you are here with []>>6[4569]|7[01]) end debug (6<<you are here with [6]>>[4569]|7[01]) end debug (6[4569]<<you are here with [64 65 66 69]>>|7[01]) end debug (6[4569]|<<you are here with []>>7[01]) end debug (6[4569]|7<<you are here with [7]>>[01]) end debug (6[4569]|7[01]<<you are here with [70 71]>>) end debug (6[4569]|7[01]<<you are here with [64 65 66 69 70 71]>>) end debug (6[4569]|7[01])<<you are here with [64 65 66 69 70 71]>> end 64 +,65,66,69,70,71

I hope this is helpful. At each debug output you can see what the /\G.../gc has stepped over and what the new value of $answer is.

My problem with explaining this type of parser is that I have been working with parsers like this for well over a year, and it all comes as second nature to me. If you have more specific questions I'll be willing to take a shot at answering them.


In reply to Re^3: Regular Expression Test by tybalt89
in thread Regular Expression Test by leoberbert

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