Why does an empty regex always return true?

Question

PHP

$uri = 'contacts';

var_dump(preg_match('//', $uri)); // true

Но ведь строка не пустая

Answer 1

Because this regular expression is treated as "the string contains an empty substring" and is true for any string. The expression "string is empty" looks like /^$/, but it's much easier to check with the usual equality ($url === '').