Per that perlop discussion I wrapped // in quotes with 'm' and tried:

perl -le 'print for a .. z' | perl  -nle 'if (/d/ .. /h/) { next unles
+s "m//"; print }'

[download]

and got:

  % perl -le 'print for a .. z' | perl  -nle 'if (/d/ .. /h/) { next unless "m//"; print }'
  d
  e
  f
  g
  h

And then I pedantically did:

for my $c ( 'a'..'z') {
    next unless ($c =~ /[d-h]/); say $& if $&;
}

[download]

and got:

  d
  e
  f
  g
  h

I guess I don't understand. Isn't 'd e f g h' what is expected? I've never really trusted one-liners. Brian Foy wrote an interesting piece on SO, to wit:

https://stackoverflow.com/questions/22652393/regex-1-variable-reset

except his example using //; did not work for me. He expected all vars to be cleared but when I ran his code:

# The regex capture variables are only reset on the next successful ma
+tch.
# This way, Perl saves a lot of time by not affecting variables when m
+atches
# fail. As such, only use those variables with a guard, to wit:
#   if ( /abc/ ) {    # this tests for /abc/ success and now it's OK t
+o use $&
#       ...
#   }

# Here's an extended demonstration, with a special surprise at the end
+:

say "First long example...\n";

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

# successful match
/(P)(erl)/;
print "First match\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# unsuccessful match
/(P)(ython)/;
print "Failed capture\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# successful match again
/(Pe)(r)(l)/;
print "Three captures\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# successful match, fewer captures
/(Perl)/;
print "One capture\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# successful match, no captures
/Perl/;
print "No captures\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

# successful match, no pattern, special case
//;
print "No nothing\n";
print "\$1: $1\n\$2: $2\n\$3: $3\n\$&: $&\n\n";

[download]

I got:

  $1: 
  $2: 
  $3: 
  $&: 

  First match
  $1: P
  $2: erl
  $3: 
  $&: Perl

  Failed capture
  $1: P
  $2: erl
  $3: 
  $&: Perl

  Three captures
  $1: Pe
  $2: r
  $3: l
  $&: Perl

  One capture
  $1: Perl
  $2: 
  $3: 
  $&: Perl

  No captures
  $1: 
  $2: 
  $3: 
  $&: Perl

  No nothing
  $1: 
  $2: 
  $3: 
  $&: Perl

As you can see in 'No nothing' $& was not cleared for me as it was for him, as he reported in that piece. I don't trust using $n, $`, $& or $' unless I explicitly test for TRUE after the regex executes. Am I being overly paranoid?

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

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