This is an addendum to my reply above, and fixes an error therein ((?R) vice (?-1)). It also avoids an  s///e executable substitution.

Try this for better results (still needs 5.10+):

use 5.010;

use warnings;
use strict;

use Test::More
  # tests => ?? + 1  # Test::NoWarnings adds 1 test
    'no_plan'
    ;
use Test::NoWarnings;

use constant TEST_SET => (
    [ q{()},   q{()} ],
    [ q{()'},  q{!()} ],
    [ q{(()')'},  q{!(!())} ],
    [ q{((()')')'},  q{!(!(!()))} ],
    [ q{(()' (()')')'},  q{!(!() !(!()))} ],
    [ q{(A & B)},   q{(A & B)} ],
    [ q{(A & B)'},  q{!(A & B)} ],
    [ q{(A & B | (C & D)')'},  q{!(A & B | !(C & D))} ],
    [ q{((A & B)' | (A & C & (A & B & D)'))},
      q{(!(A & B) | (A & C & !(A & B & D)))}
      ],
    [ q{((A | B)' & (C | (D & (E & F)))')'},
      q{!(!(A | B) & !(C | (D & (E & F))))}
      ],
    [ q{((A & B)' | (A & C & (A & Boff' & D | (A & (B' & D)')'))},
      q{(!(A & B) | (A & C & (A & !Boff & D | !(A & !(!B & D))))}
      ],
    );

FUNT:
for my $func_name (qw(xform_4)) {

    note "\n----- testing $func_name() ----- \n\n";
    *xform = do { no strict 'refs';  *$func_name; };

    VECTOR:
    for my $ar_vector (TEST_SET) {

        if (not ref $ar_vector) {
            note $ar_vector;
            next VECTOR;
            }

        my ($string, $expected) = @$ar_vector;

        is xform($string), $expected, qq{$string}

        }  # end for VECTOR

    }  # end for FUNT

done_testing;

# functions under test #############################################

sub xform_4 {  # needs 5.10+ for (?PARNO) and (?|...)

    my ($str,
        ) = @_;

    my $parenthetic = qr{ ( [(] (?: [^()]*+ | (?-1))* [)] ) }xmso;
    my $term        = qr{ ( [[:alpha:]] \w*               ) }xmso;

    1 while $str =~ s{ (?| $parenthetic | $term) ' }
                     {!$1}xmsgo;

    return $str;

    }
[download]

Output:

c:\@Work\Perl\monks\Learning_Perl_2017>perl xform_nested_terms_1.pl
#
# ----- testing xform_4() -----
#
ok 1 - ()
ok 2 - ()'
ok 3 - (()')'
ok 4 - ((()')')'
ok 5 - (()' (()')')'
ok 6 - (A & B)
ok 7 - (A & B)'
ok 8 - (A & B | (C & D)')'
ok 9 - ((A & B)' | (A & C & (A & B & D)'))
ok 10 - ((A | B)' & (C | (D & (E & F)))')'
ok 11 - ((A & B)' | (A & C & (A & Boff' & D | (A & (B' & D)')'))
1..11
ok 12 - no warnings
1..12
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.