<?php
error_reporting(E_ALL);
ini_set('display_errors', '1');
$patterns = [
    '/\b(?:orally|oral|intravenous|\biv\b)\b/i',
    '/\b(?:intravenous|\biv\b|intramuscular|\bim\b)\b/i',
];
foreach ($patterns as $p) {
    $r = preg_match($p, 'iv');
    echo $p, ' => ', var_export($r, true), ' last=', preg_last_error(), PHP_EOL;
}
