I changed my mind about sitting this one out because this code from choroba is so fascinating, and worth exploring, as noted below. 

#!/usr/bin/env -S perl
 
#use warnings;      # suppress uninitialized warnings
use strict;
use feature qw{ say };
use experimental qw( signatures );


$_ = 'abXXXXacVVVVVad';
#$_ = 'abacad';

my $lim = 7;
say "Examining \'$_\' $lim times";
say "/a(.)/";
if (/a(.)/g) {

#say "/q(.)/";          # switch to see no-match example
#if (/q(.)/g) {

    say "\$1: $1";
    say "\$&: $&";
} else {
    say 'No match';
}

for my $try (1 .. $lim) {
    say "//";
    if (//g) {
        say "\$1: $1";
        say "\$&: $&";
    } else {
        say 'No match';
    }
}

$_ = '37.5BBBBB98UUUUU4.075QQQQQ42TTTT0.357SSS';
$lim = 5;
say "\nExamining \'$_\' $lim times";
say '/[^\d.](\d+(?:\.?\d*)?)/g';
if (/[^\d.](\d+(?:\.?\d*)?)/g) {
    say "\$1: $1";
    say "\$&: $&";
} else {
    say 'No match';
}

for my $try (1..$lim) {
    say "//";
    if (//g) {
        say "\$1: $1";
        say "\$&: $&";
    } else {
        say 'No match';
    }
}

exit(0);
__END__

[download]
  O U T P U T

  Examining 'abXXXXacVVVVVad' 7 times
  /a(.)/
  $1: b 
  $&: ab
  //
  $1: c
  $&: ac
  //
  $1: d
  $&: ad
  //
  No match
  //
  $1: b
  $&: ab
  //
  $1: c
  $&: ac
  //
  $1: d
  $&: ad
  //
  No match

  Examining '37.5BBBBB98UUUUU4.075QQQQQ42TTTT0.357SSS' 5 times
  /^\d.(\d+(?:\.?\d*)?)/g
  $1: 98
  $&: B98
  //
  $1: 4.075
  $&: U4.075
  //
  $1: 42
  $&: Q42
  //
  $1: 0.357
  $&: T0.357
  //
  No match
  //
  $1: 98
  $&: B98


S O M E  O B S E R V A T I O N S
The only use case I can infer from these code tweaks is the notion of determining the number of 'things' present in a string, as long as at least one of those 'things' is present, since there must be at least one match before //g can work its magic. I deliberately omitted a leading non-decimal in the second $_ to force the first match deeper in the string, but that's cool because as long as at least one 'thing' sub-expression is present, //g will find the rest (to the right). Also, note how the entire process starts over from the beginning if the number of $trys exceeds the number of 'things' present, as in $try (1..$lim) {. There must be other use cases for //g;. Any thoughts on how to exploit this facility?

Will

In reply to Re^4: Empty pattern in regex by perlboy_emeritus
in thread Empty pattern in regex by choroba

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