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.\"]


##</code><code>##

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


##</code><code>##

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

##</code><code>##

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

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

# and later:
    my $got = [ split $re, $str ];