Re^2: Empty pattern in regex

    > but it is intended to use the last successfully matched regexp

Whatever its intent, it is one confusing puppy. //; always matches, always returns TRUE, but it never changes $&. $& is always whatever the previous regex set it to, whether it matched or not, effectively a NOP. Consider this:

$_ = 'Hello Perl';
say '$_ = \'Hello Perl\';';
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# unsuccessful match
/Python/;
print "No match, \/Python\/\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# successful match
if (//) {
    print "No nothing, if (\/\/) {\n";
    print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";
} else {
    print "\/\/ unsuccessfull match";
}
 
# successful match, no captures
/Perl/;
print "Match \/Perl\/, No captures\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# successful match, empty pattern
if (//) {
    print "No nothing, if (\/\/) {\n";
    print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";
} else {
    print "\/\/ unsuccessfull match";
}
 
# successful match, no captures
/Perl/;
print "Match \/Perl\/, No captures\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# successful match, no pattern, empty parens
if (/()/) {
#//;
    print "No nothing, if \(\/\(\)\/\) {\n";
    print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";
} else {
    print "\/\/ unsuccessfull match";
}
[download]

Which results in:

  $_ = 'Hello Perl';
  $1: 
  $2: 
  $3: 
  $&: 

  No match, /Python/
  $1:  
  $2: 
  $3: 
  $&: 

  No nothing, if (//) {
  $1: 
  $2: 
  $3: 
  $&: 

  Match /Perl/, No captures
  $1: 
  $2: 
  $3: 
  $&: Perl

  No nothing, if (//) {
  $1: 
  $2: 
  $3: 
  $&: Perl

  Match /Perl/, No captures
  $1: 
  $2: 
  $3: 
  $&: Perl
  
  No nothing, if (/()/) {
  $1: 
  $2: 
  $3: 
  $&:

What is or was the purpose of this construction? How would one use it?

Comment on Re^2: Empty pattern in regex Download Code

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Re^3: Empty pattern in regex by choroba (Cardinal) on Oct 19, 2023 at 19:02 UTC
> What is or was the purpose of this construction? How would one use it? `#!/usr/bin/perl use warnings; use strict; use feature qw{ say }; $_ = 'abacad'; say "/a(.)/"; if (/a(.)/g) { say "\$1: $1"; say "\$&: $&"; } else { say 'No match'; } for my $try (1 .. 3) { say "//"; if (//g) { say "\$1: $1"; say "\$&: $&"; } else { say 'No match'; } }` [download] Output: `/a(.)/ $1: b $&: ab // $1: c $&: ac // $1: d $&: ad // No match` [download] Update: If I remember correctly, this was the original reason the feature was introduced: `#!/usr/bin/perl use warnings; use strict; use feature qw{ say }; my $x = 'found 11'; my $y = 'found 12'; if ($x =~ /found (\d+)/ && $y =~ //) { # No need to repeat the long r +egex! Yay! say "Found $1."; }` [download] `map{substr$_->[0],$_->[1]\|\|0,1}[\\|\|{},3],[[]],[ref qr-1,-,-1],[{}],[sub{}^ARGV,3]`	[reply] [d/l] [select]
Re^4: Empty pattern in regex by perlboy_emeritus (Scribe) on Oct 20, 2023 at 20:08 UTC
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	[reply] [d/l]
Re^3: Empty pattern in regex by hv (Prior) on Oct 19, 2023 at 19:56 UTC
`//` always matches, always returns TRUE, but it never changes $&. That is not correct, in either aspect: `% perl -wle '"a" =~ /a/; "b" =~ // or print "did not match, did not re +turn TRUE"' did not match, did not return TRUE % perl -wle '"a" =~ /.*/; q{$& changed} =~ // and print $&' $& changed %` [download]	[reply] [d/l] [select]
Re^4: Empty pattern in regex by perlboy_emeritus (Scribe) on Oct 19, 2023 at 20:44 UTC
I concede, I give up and I surrender. Too confusing, why I get the result I do and you get the results you do. I'll sit this one out and just study the results y'all get. :-)	[reply]