let reg = /^\s+/;
let reg1 = /\s+$/
let str = ' hello world ';
console.log(str.length);
let a = str.replace(reg, '');
console.log(a, a.length);
let b = str.replace(reg1, '');
console.log(b, b.length);
let str = ' hello world ';
console.log(str.length);
let reg2 =/\S(?!=\S)\s+\s(?!=\s)/ ;
let c = str.replace(reg2, '');
console.log(c, c.length);
为什么reg2中增加了^$就会变成null(用reg.test的时候)