为何正则匹配捕获不了“\”反斜杠？

Question

var str = "\abc";// 字符串是获取到的，不能更改为了\\abc

var re = /^\abc/;

console.log(re.test(str));//true

console.log(str.match(re));

//["abc", index: 0, input: "abc"] 没有'\'

console.log(re.exec(str));

//["abc", index: 0, input: "abc"] 没有'\'

var re2 = /^\\abc/;

console.log(re2.test(str));//false

console.log(str.match(re2));//null

console.log(re2.exec(str));//null

var re3 = /^\\\abc/;

console.log(re3.test(str));//false

console.log(str.match(re3));//null

console.log(re3.exec(str));//null

var re4 = /^\\\\abc/;

console.log(re4.test(str));//false

console.log(str.match(re4));//null

console.log(re4.exec(str));//null