Ken, thanks for your writeup, including forcing me to learn a bit about Test -- I needed that! :-)

I'd rather stick with Perl's slice capability since it reads more cleanly (to me, anyway) and doesn't require copying and modifying the source array. Here's how I ended up implementing and testing pyrange():

#!/usr/bin/env perl

use v5.36;
use constant {
    AREF    => 0,
    PYTHON  => 1,
    EXP     => 2,
};

use Test::More;

my @test_array = 'a' .. 'g';

my @tests = (
    [\@test_array, '[:3]',    'abc'],
    [\@test_array, '[:-3]',   'abcd'],
    [\@test_array, '[3:]',    'defg'],
    [\@test_array, '[-3:]',   'efg'],
    [\@test_array, '[-3:-1]', 'ef'],   # testing double-ended ranges
    [\@test_array, '[-3:-3]', ''],
    [\@test_array, '[3:-1]',  'def'],
);

plan tests => 0+@tests;

for my $test (@tests) {
    is get_array_slice_by_python_expr($test->@[AREF, PYTHON]),
        $test->[EXP], "Testing: $test->[PYTHON]";
}

sub get_array_slice_by_python_expr ($aref, $python) {

    ### $re modified to return undef instead of '' when endpoint omitt
+ed
    state $re = qr{^ \[ ( -? \d+ )? : ( -? \d+ )? \] $}x;

    my ($start, $stop) = $python =~ $re;
    my $range_aref = pyrange($aref, $start, $stop);
    return join '', @$range_aref;
}

sub pyrange($aref, $start=undef, $stop=undef) {
    if (!defined $start) {
        $start = 0;
    }
    elsif ($start < 0) {
        $start = @$aref + $start;
    }
    if (!defined $stop) {
        $stop = $#$aref;
    }
    elsif ($stop >= 0) {
        $stop = $stop - 1;
    }
    else {
        $stop = @$aref + $stop - 1;
    }
    return [@$aref[$start .. $stop]];
}
[download]

Output:

$ ./test
1..7
ok 1 - Testing: [:3]
ok 2 - Testing: [:-3]
ok 3 - Testing: [3:]
ok 4 - Testing: [-3:]
ok 5 - Testing: [-3:-1]
ok 6 - Testing: [-3:-3]
ok 7 - Testing: [3:-1]
[download]

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