JSFiddle - React, Tailwind, and code Playground
JavaScript
var str = '<span style="font-weight:bold;">te<span style="text-decoration:line-through; font-style:italic;">x</span>to</span>';
str = str
.replace(/<span.*?">/gi, '<b>')
.replace(/<.span>/gi, '</b>');
console.log( str );
alert( str );