Let \w+=\w+ = X for readability
(\w+=\w+;)*(\w+=\w+;?)?
=> (X;)*(X;?)?
=> (X;)*(X|X;)?               // ab? => a|ab
=> (X;)*|(X;)*(X|X;)          // ab? => a|ab
=> (X;)*|((X;)*X|(X;)*X;)     // a(b|c) => ab|ac
=> (X;)*|((X;)*X|(X;)+)       // a*a => a+
=> (X;)*|(X;)*X|(X;)+         // a|(b|c) => a|b|c
=> (X;)*|(X;)*X               // a*|a+ => a*
=> (X;)*(X)?
=> (\w+=\w+;)?(\w+=\w+)?