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

# use Regexp::Common;       
# ^^^ Not used. I'm so lazy, I just peeked at $RE{quoted}
# to construct the "$quoted" expression below, by slightly
# modifying it (see "$") to satisfy the third clause.
# And actually 2nd test case below is to test how it works,
# it seems there's not a similar one among your 18.

my $quoted = qr/
(?:(?|
    (?:(?<!\\)\")(?:[^\\\"]*(?:\\.[^\\\"]*)*)(?:\"|$)|
    (?:(?<!\\)\')(?:[^\\\']*(?:\\.[^\\\']*)*)(?:\'|$)
))
/x;

my $re = qr/(?:$quoted|[^ ])+\K(?: |$)/;

my @tests = ( 
    q(This 'isn\'t nice.'),
    q(This 'isn\'t nice.),
    q(This \"isnt unnice.\"),
);

for my $t ( @tests ) {
    say "[$_]" for split $re, $t;
}

__END__

[This]
['isn\'t nice.']
[This]
['isn\'t nice.]
[This]
[\"isnt]
[unnice.\"]