Can I replace the binding operator with the smart match operator in Perl?

Question

How can I write this with the smart match operator (~~)?

use 5.010;

my $string = '12 23 34 45 5464 46';

while ( $string =~ /(\d\d)\s/g ) {
    say $1;
}

Answer 1

Interesting. perlsyn states:

Any ~~ Regex pattern match $a =~ /$b/

so, at first glance, it seems reasonable to expect

use strict; use warnings;
use 5.010;

my $string = '12 23 34 45 5464 46';

while ( $string ~~ /(\d\d)\s/g ) {
    say $1;
}

to print 12, 23, etc but it gets stuck in a loop, matching 12 repeatedly. Using:

$ perl -MO=Deparse y.pl

yields

while ($string ~~ qr/(\d\d)\s/g) {
    say $1;
}

looking at perlop, we notice

qr/STRING/msixpo

Note that 'g' is not listed as a modifier (logically, to me).

Interestingly, if you write:

my $re = qr/(\d\d)\s/g;

perl barfs:

Bareword found where operator expected at C:\Temp\y.pl line 5, 
near "qr/(\d\d)\s/g"
syntax error at C:\Temp\y.pl line 5, near "qr/(\d\d)\s/g"

and presumably it should also say something if an invalid expression is used in the code above

Answer 2

Is the expected behaviour to output to first match endlessly? Because that's what this code must do in its current form. The problem isn't the smart-match operator. The while loop is endless, because no modification ever occurs to $string. The /g global switch doesn't change the loop itself.

What are you trying to achieve? I'm assuming you want to output the two-digit values, one per line. In which case you might want to consider:

say join("\n", grep { /^\d{2}$/ } split(" ",$string));

Answer 3

If we go and look at what these two variants get transformed into, we can see the reason for this.

---

• First lets look at the original version.

  perl -MO=Deparse -e'while("abc" =~ /(.)/g){print "hi\n"}'
  

  while ('abc' =~ /(.)/g) {
      print "hi\n";
  }
  

  As you can see there wasn't any changing of the opcodes.

• Now if you go and change it to use the smart-match operator, you can see it does actually change.

  perl -MO=Deparse -e'while("abc" ~~ /(.)/g){print "hi\n"}'
  

  while ('abc' ~~ qr/(.)/g) {
      print "hi\n";
  }
  

  It changes it to qr, which doesn't recognize the /g option.

  This should probably give you an error, but it doesn't get transformed until after it gets parsed.

  ---

  The warning you should have gotten, and would get if you used qr instead is:

  syntax error at -e line 1, near "qr/(.)/g"

The smart-match feature was never intended to replace the =~ operator. It came out of the process of making given/when work like it does.

Most of the time, when(EXPR) is treated as an implicit smart match of $_.
...

Answer 4

To be honest, I'm not sure you can use the smart match operator for this. In my limited testing, it looks like the smart match is returning a boolean instead of a list of matches. The code you posted (using =~) can work without it, however.
What you posted doesn't work because of the while loop. The conditional statement on a while loop is executed before the start of each iteration. In this case, your regex is returning the first value in $string because it is reset at each iteration. A foreach would work however:

my $string = '12 23 34 45 5464 46';
foreach my $number ($string =~ /(\d\d)\s/g) {
    print $number."\n";
}