Groups

(...)

Capturing

(?P<obj>...)

Capturing group named "obj"

(?P=obj)

Match the named group "obj"

\g{Z}

Match the named or numbered group Z

\Y

Match the Y'th captured group

(?>...)

Atomic group: This group does not allow any backtracking to occur

(?|...)

Duplicate group numbers

(?#...)

Comment between parenthesis

(?:...)

Matches but does not capture the group

(?&obj)

Recurse into the named group "obj"

(?Y)

Recurse into numbered group Y

(?R)

Recurse into entire pattern

\g<Z>

Recurse into the named or numbered group Z