Regular Expressions

Prioritize regex readability. For guidance, refer to Writing better Regular Expressions in PHP.

Use DEFINE for recurring patterns and sprintf for reusable definitions:

final class RegexHelper
{
    /** @return array<string, string> */
    public function images(string $htmlContent): array
    {
        $pattern = '
            (?'image' # Capture named group
                (?P>img) # Recurse img subpattern from definitions
            )
        ';

        preg_match_all($this->createRegex($pattern), $htmlContent, $matches);

        return $matches['image'];
    }

    private function createRegex(string $pattern): string
    {
        return sprintf($this->getDefinitions(), preg_quote($pattern, '~'));
    }

    private function getDefinitions(): string
    {
        return "~
            (?(DEFINE) # Allows defining reusable patterns
                (?'attr'(?:\s[^>]++)?) # Capture HTML attributes
                (?'img'<img(?P>params)>) # Capture HTML img tag with its attributes
            )
            %s #Allows adding dynamic regex using sprintf
            ~ix";
    }
}

Use Regex101 for testing patterns.

TIP

There is a less popular, hidden PHP germ sscanf function that can be used for parsing strings and simplify your code in some cases.