'&branId=656121&'.match(/(?<subExp1>^|&)branId=(?<subExp2>[^&]*)(?<subExp3>&|$)/);
/*返回结果*/
["&branId=656121&", "&", "656121", "&", index: 0, input: "&branId=656121&",
groups: {subExp1: "&", subExp2: "656121", subExp3: "&"}]
?
'&branId=656121&'.match(/(?<subExp1>^|&)branId=(?<subExp2>[^&]*)(?<subExp3>&|$)/);
/*返回结果*/
["&branId=656121&", "&", "656121", "&", index: 0, input: "&branId=656121&",
groups: {subExp1: "&", subExp2: "656121", subExp3: "&"}]
?