编写一个函数，化简表达式（仅限加减法）

Question

我考虑用正则去做，正则写半天没写出来，求高人解救
测试规则如下：

simplify("3x+y-x+2y+5xy-y-xy") => 2x+2y+4xy

simplify("2xy-yx") =>xy

simplify("xzy+zby") =>byz+xyz

simplify("xzy+zby") =>byz+xyz

simplify("y-x")＝>-x+y