How to trace the execution of a regex matching?

Question

Is there a convenient way to get a trace of the regex engine's states during the evaluation of a m// or s/// expression?

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BTW, I'm aware of that there is a Regexp::Debugger package available through CPAN (and it is amazingly cool), but I don't see a way to get anything like a trace from it; I don't want to step through a potentially huge number of steps.

Answer 1

Yes. Turn the regex engine into debug mode and it'll print what it's doing:

use re 'debug'; 

my $str = "abcdefg";
$str =~ m/[ef]+/;

Which gives an output of:

Compiling REx "[ef]+"
Final program:
   1: PLUS (13)
   2:   ANYOF[ef] (0)
  13: END (0)
stclass ANYOF[ef] plus minlen 1 
Matching REx "[ef]+" against "abcdefg"
Matching stclass ANYOF[ef] against "abcdefg" (7 bytes)
   4 <abcd> <efg>            |  1:PLUS(13)
                                  ANYOF[ef] can match 2 times out of 2147483647...
   6 <abcdef> <g>            | 13:  END(0)
Match successful!
Freeing REx: "[ef]+"