Laravel collection roman numeral conversion

roman_numerals.php

<?php

$roman = ['i' => 1, 'v' => 5, 'x' => 10, 'l' => 50, 'c' => 100, 'd' => 500, 'm' => 1000];

$value = 'IX';
$result = 0;
$prevValue = 0;

if (preg_match('/[^ivxlcdm]|i{4,}|x{4,}|c{4,}|m{4,}|v{2,}|l{2,}|d{2,}/i', $value)) {
    return "invalid format";
}

return str($value)->ucsplit('')
  ->reverse()
  ->values()
  ->map(fn ($item) => $roman[strtolower($item)])
  ->reduce(function ($carry, $item) use (&$prevValue) {
      $result = ($item >= $prevValue) ? $carry + $item : $carry - $item;
      $prevValue = $item;

      return $result;
  }, 0);