Or

'' =~ /|/;
[download]

Or

'' =~ /(?:)/;
[download]

Or

'g' =~ /g{0}/;
[download]

Nice idea, thanks! I'm using /(?:)/ for now. It's still a bit of a workaround though, since it affects the debug output - you see a lot of "Matching REx "(?:)"" instead of "Matching REx """:

use warnings;
use strict;

'g'=~/g/;
my $regex = 'm{}g';  # user input
my $text = 'x';      # user input
eval qq{
    use re 'debug';
    ''=~/(?:)/;
    print "<\$&>\\n" while \$text =~ $regex;
};

__END__

Compiling REx "(?:)"
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0 
Compiling REx ""
Final program:
   1: NOTHING (2)
   2: END (0)
minlen 0 
Matching REx "(?:)" against ""
   0 <> <>                   |   0| 1:NOTHING(2)
   0 <> <>                   |   0| 2:END(0)
Match successful!
Matching REx "(?:)" against "x"
   0 <> <x>                  |   0| 1:NOTHING(2)
   0 <> <x>                  |   0| 2:END(0)
Match successful!
<>
Matching REx "(?:)" against "x"
   0 <> <x>                  |   0| 1:NOTHING(2)
   0 <> <x>                  |   0| 2:END(0)
Match possible, but length=0 is smaller than requested=1, failing!
   1 <x> <>                  |   0| 1:NOTHING(2)
   1 <x> <>                  |   0| 2:END(0)
Match successful!
<>
Matching REx "(?:)" against ""
   1 <x> <>                  |   0| 1:NOTHING(2)
   1 <x> <>                  |   0| 2:END(0)
Match possible, but length=0 is smaller than requested=1, failing!
Match failed
Freeing REx: ""
Freeing REx: "(?:)"
[download]

Or /(?#)/

They are nice for short scripts but a source of errors in larger apps, because they break orthogonality.

Anyway I'm confused about the result you are expecting... the empty pattern would always match for every position

use strict;
use warnings;
use feature 'say';


my $run=0;

sub test {
    $run++;
    my $regex = 'm{}g';                 # user input
    my $text = 'xx';                    # user input
    print "--- run $run\n";
    pos($text)=0;
    eval <<"__CODE__";
        say pos(\$text),"<\$&>" while \$text =~ $regex;
__CODE__
}

test();

say "successful /g/" if 'g' =~ /g/;
test();

say "successful /x/" if 'x' =~ /x/;
test();

my $empty='';
say "successful //" if ' ' =~ /$empty/;
test();
[download]

--- run 1
0<>
1<>
2<>
successful /g/
--- run 2
successful /x/
--- run 3
1<x>
2<x>
--- run 4
1<x>
2<x>
[download]

so you are looking for a way to reproduce run 1?

I wasn't able to reset the regex engine here

update

tybalt's suggestions seem to work well:

...
for my $tybalt (qw/ () | (?:) g{0}/) {
    'g' =~ /g/;
    say "successful /$tybalt/" if ' ' =~ /$tybalt/;
    test();
}
[download]

successful /()/
--- run 5
0<>
1<>
2<>
successful /|/
--- run 6
0<>
1<>
2<>
successful /(?:)/
--- run 7
0<>
1<>
2<>
successful /g{0}/
--- run 8
0<>
1<>
2<>
[download]

Thanks for testing!

I'm confused about the result you are expecting... the empty pattern would always match for every position

I want to allow users to enter any regex, I run it via eval, and it should behave "normally". The /g/ is just an example of something that my code was doing, which the user knows nothing about. So it seemed to me to be the most "normal" to have the regex behave as if it's the first regex in the program.

I'm not sure if you should even allow the empty regex.

The result surprised me and might not be intuitive for your user.

Unless he has to learn Perl.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

Try

'' =~ /()/;
[download]

Can't test now, but probably this one has side effects by setting $1.

Cheers Rolf
_{(addicted to the Perl Programming Language :)
Wikisyntax for the Monastery FootballPerl is like chess, only without the dice}

That's why I posted the other ones.

On the other hand, does it really matter? If the user does not expect $1 to be set by //, whether it has a value or is undef is irrelevant.

You should accept a regex pattern as a parameter (as the variable name implies) instead of a fragment of Perl source code! This would avoid this and many other issues.