(?=pattern) - zero-width positive look-ahead
(?!pattern) - zero-width negative look-ahead
(?<=pattern) - zero-width positive look-behind
(?<!pattern) - zero-width negative look-behind
(?{ code }) - zero-width assertion with embedded code
(??{ code }) - postponed subexpression
(?>pattern) - "independent" subexpression
(?(condition)yes-pattern|no-pattern) - conditional expression
(?(condition)yes-pattern)