$re = '/^(repeating-)?(linear|radial|conic)-gradient\(([^()]|rgb[a]?\([^()]*\))*\)$/';
$str = 'radial-gradient(circle at 5px 5px,#0c0d0d70 2px,#ffffff00 0px,#ffffff00 0px) 0 0 / 8px 8px, linear-gradient(180deg, var(--wp--preset--color--base) 0%,#000000 200%)';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php