PHP Regex match words in a string excluding one specific word

Question

I have a text ($txt), an array of words ($words) i want to add a link and a word ($wordToExclude) that must be not replaced.

$words = array ('adipiscing','molestie','fringilla');
$wordToExclude = 'consectetur adipiscing';


$txt = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Quisque
mattis tincidunt dolor sed consequat. Sed rutrum, mauris convallis bibendum 
dignissim, ligula sem molestie massa, vitae condimentum neque sem non tellus.
Aenean dolor enim, cursus vel sodales ac, condimentum ac erat. Quisque
lobortis libero nec arcu fringilla imperdiet. Pellentesque commodo, 
arcu et dictum tincidunt, ipsum elit molestie ipsum, ut ultricies nisl
neque in velit. Curabitur luctus dui id urna consequat vitae mattis
turpis pretium. Donec nec adipiscing velit.'

I want to obtain this result:

$txt = 'Lorem ipsum dolor sit amet, consectetur adipiscing elit. Quisque
mattis tincidunt dolor sed consequat. Sed rutrum, mauris convallis bibendum 
dignissim, ligula sem <a href="#">molestie</a> massa, vitae condimentum neque sem non tellus.
Aenean dolor enim, cursus vel sodales ac, condimentum ac erat. Quisque
lobortis libero nec arcu <a href="#">fringilla</a> imperdiet. Pellentesque commodo, 
arcu et dictum tincidunt, ipsum elit <a href="#">molestie</a> ipsum, ut ultricies nisl
neque in velit. Curabitur luctus dui id urna consequat vitae mattis
turpis pretium. Donec nec <a href="#">adipiscing</a> velit.'

Answer 1

$result = preg_replace(
    '/\b                 # Word boundary
    (                    # Match one of the following:
     (?<!consectetur\s)  #  (unless preceded by "consectetur "
     adipiscing          #  adipiscing
    |                    # or
     molestie            #  molestie
    |                    # etc.
     fringilla
    )                    # End of alternation
    \b                   # Word boundary
    /ix', 
    '<a href="#">\1</a>', $subject);