(?(?<=\w)(?!\w)|(?=\w)) # \b equivalent

##</code><code>##

(?(?<=\w)(?=\w)|(?!\w)) # \B equivalent

##</code><code>##

# \b equivalent:
(?(?<=      \w)     # if there is a word character left
      (?!   \w)     #    then there must be no word character right
  |   (?=   \w)     #    else there must be a  word character right
)

# \B equivalent:
(?(?<=      \w)     # if there is a word character left
      (?=   \w)     #    then there must be a  word character right
  |   (?!   \w)     #    else there must be no word character right
)

##</code><code>##

(?(DEFINE)
    (?<greeklish>            [\p{Greek}\p{Inherited}]   )
    (?<ungreeklish>          [^\p{Greek}\p{Inherited}]  )
    (?<greek_boundary>
        (?(?<=      (?&greeklish))
              (?!   (?&greeklish))
          |   (?=   (?&greeklish))
        )
    )
    (?<greek_nonboundary>
        (?(?<=      (?&greeklish))
              (?=   (?&greeklish))
          |   (?!   (?&greeklish))
        )
    )
)

##</code><code>##

     sub IsGreeklish {
         return <<'END';
     +utf8::IsGreek
     +utf8::IsInherited
     END
     }

##</code><code>##

# space boundary
(?(?<=      \S)     # if there is a nonspace character left
      (?!   \S)     #    then there must be no space character right
  |   (?=   \S)     #    else there must be a  space character right
)

##</code><code>##

#!/usr/bin/env perl

use v5.10;
use strict;
use warnings;

my $space_edge = qr{
    (?(?<=      \S)     # if there is a nonspace character left
          (?!   \S)     #    then there must be no space character right
      |   (?=   \S)     #    else there must be a  space character right
    )
}x;

while (<DATA>) {
    my $edges = 0;
    for my $str ( "foo", "()" ) {
        my $qstr = quotemeta($str);

        unless (/$qstr/) {
           #print "MISSING $str in: $_";
            next;
        }

        if (/${space_edge}${qstr}/) {
            $edges++;
            print "$str EDGE LEFT:  $_"
        }

        if (/${qstr}${space_edge}/) {
            $edges++;
            print "$str EDGE RIGHT: $_"
        }

        unless ($edges) {
            print "$str UNEDGED:    $_";
        }
    }
}
exit;

__END__
Put your foo in your pocket.
Put your foo() in your pocket.
Good food was had by all.
What fools these mortals be!
food is good to have.
That's a major snafoo there.
That's a major snafoo.
That's a major snafoo
That's a major snafoo()zle
That's a major snafoo() there.
That's a major snafoo()

##</code><code>##

foo EDGE LEFT:  Put your foo in your pocket.
foo EDGE RIGHT: Put your foo in your pocket.
foo EDGE LEFT:  Put your foo() in your pocket.
() EDGE RIGHT: Put your foo() in your pocket.
foo EDGE LEFT:  Good food was had by all.
foo EDGE LEFT:  What fools these mortals be!
foo EDGE LEFT:  food is good to have.
foo EDGE RIGHT: That's a major snafoo there.
foo UNEDGED:    That's a major snafoo.
foo EDGE RIGHT: That's a major snafoo
foo UNEDGED:    That's a major snafoo()zle
() UNEDGED:    That's a major snafoo()zle
foo UNEDGED:    That's a major snafoo() there.
() EDGE RIGHT: That's a major snafoo() there.
foo UNEDGED:    That's a major snafoo()
() EDGE RIGHT: That's a major snafoo()