正则表达式\B

\B表示非单词边界

const a='example:a+b=3'
const b=/\B/g
console.log(a.replace(b,'*'))//e*x*a*m*p*l*e:a+b=3
const re=/\Bend\w*\b/g
str='send end ck lender'
result=str.replace(re,'-')
console.log(result)//s- end ck l-

var dateStr = '2020/05/21';
var reg = /(\d{4})\/(\d{2})\/(\d{2})/;
dateStr = dateStr.replace(reg, '$1-$2-$3') 
console.log(dateStr)//2020-05-21