This post makes excellent points! Just to expand a little on regex testing - having many test cases helps:

#!/usr/bin/env perl
use warnings;
use strict;
use Test::More;
#use re 'debug'; # optional

my $regex = qr/(foo|bar)(quz|baz)?$/;

# "match / doesn't match" tests
like   "foo",          $regex;
like   "bar",          $regex;
like   "foobar",       $regex;
like   "fooquz",       $regex;
like   "foobaz",       $regex;
like   "barbaz",       $regex;
like   "foobarquz",    $regex;
like   "foobarfooquz", $regex;
unlike "foobarquzbaz", $regex;
unlike "quzbaz",       $regex;
unlike "hello",        $regex;
unlike "foo ",         $regex;
# etc.

# check capture groups
ok "foobar"    =~ $regex;
is $1, "bar";
is $2, undef;
ok "foobarquz" =~ $regex;
is $1, "bar";
is $2, "quz";
ok "foobaz"    =~ $regex;
is $1, "foo";
is $2, "baz";
# etc.

# and/or check return values in list context
is_deeply [ "foobar" =~ $regex ],
    [ "bar", undef ];
is_deeply [ "fooquz" =~ $regex ],
    [ "foo", "quz" ];
is_deeply [ "barbaz" =~ $regex ],
    [ "bar", "baz" ];
# etc.

# an example with /g return values
is_deeply [ "foobarquzbaz" =~ /[aeiou]/g ],
    [ "o", "o", "a", "u", "a" ];
# same thing, written differently:
my @matches = "foobarquzbaz" =~ /[aeiou]/g;
is_deeply \@matches, [ "o", "o", "a", "u", "a" ];

done_testing;
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