% ^ \s* ( # <1> < \s* ([a-zA-Z:]+) # <2/> (?: \s*[a-zA-Z:]* \s* = \s* (?:'[^']*'|"[^"]*") )* \s* (/\s*)? # <3/> > (?:[^<>]* | (?1))* # Update: added * to (?:) (?(3)| <\s*/\s*\2\s*> ) ) # \s* $ %x