Why is my regex to match words in PHP not working as expected? Ranged quantifiers on negated lookaheads are disqualifying unintended input strings

The {3,} in your non-capturing group mandates a minimum string length of 3 characters. If you want to allow Li, reduce it to {2,}.

The negated characters class inside your negated lookahead ((?![^aeiou]{4,}) has a minimum qualification of 4 consonants, so ntch satisfies that and disqualifies the input string. If you want to allow Drantch, increase it to (?![^aeiou]{5,}.

Code: (Demo)

$array = [
    'Nartinez',
    'Drantch',
    'Dratch',
    'Xtmnprwq',
    'Yelendez',
    'Boldberg',
    'Yelenovich',
    'Allash',
    'Mohamed',
    'Li',
];

$regex = <<<REGEX
/
^
(?![aeiou]{3,})
(?:
   D(?![^aeiou]{5,}[aeiou]*)
   (?![aeiou]{4,})
){2,}
$
/ix
REGEX;

var_export(preg_grep($regex, $array));

Output:

array (
  0 => 'Nartinez',
  1 => 'Drantch',
  2 => 'Dratch',
  4 => 'Yelendez',
  5 => 'Boldberg',
  6 => 'Yelenovich',
  7 => 'Allash',
  8 => 'Mohamed',
  9 => 'Li',
)

As for improving your pattern’s readability, it would be better to express your exact intention, then generate a set of negated lookaheads before the "core" requirement that all characters must be letters and have a minimum character length.

$regex = <<<REGEX
/
^
(?![aeiou]{3})    #doesn't start with 3 consecutive vowels
(?!.*[aeiou]{4})  #doesn't contain 4 consecutive vowels
(?!.*[^aeiou]{5}) #doesn't contain 5 consecutive consonants
[a-z]{2,}         #contains only letters, minimum of 2 characters
$
/ix
REGEX;