--- str <AB>  re (?^u:..)  
matched,  captured ()
matched,  captured ()


--- str <AB>  re (?^u:(.)(.))  
matched,  captured ("A", "B")
matched,  captured ("A", "B")


--- str <AB>  re (?^u:(X)?(.))  
matched,  captured (undef, "A")
matched,  captured (undef, "A")


--- str <AB>  re (?^u:XY)  
NO match


--- str <AB>  re (?^u:(X)Y)  
NO match


--- str <AB>  re (?^u:)  
matched,  captured ()
matched,  captured ()


--- str <>  re (?^u:..)  
NO match


--- str <>  re (?^u:(.)(.))  
NO match


--- str <>  re (?^u:(X)?(.))  
NO match


--- str <>  re (?^u:XY)  
NO match


--- str <>  re (?^u:(X)Y)  
NO match


--- str <>  re (?^u:)  
matched,  captured ()
matched,  captured ()