> 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?

In reply to Re^2: Empty pattern in regex by perlboy_emeritus
in thread Empty pattern in regex by choroba

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