【全网最短】用正则表达式→一行代码动态匹配→获取原始字符串中指定开始、结尾之间的字符串（截取两个字符串之间及字符串前后内容的方法）

let getBetweenStr = (originStr, beginStr, endStr) => (originStr.match(new RegExp(`${beginStr}(\S*)${endStr}`)) || [, ""])[1];



console.log(getBetweenStr('aaabccc','a','c'));    //aabcc

console.log(getBetweenStr('aaabccc','aa','cc'));    //abc

console.log(getBetweenStr('aaabccc','aaa','ccc'));    //b