您需要设置一个捕获组并在替换模式中使用反向引用.

You need to set a capturing group and use a back reference in the replacement pattern.

regEx.Pattern = "</span>([^>]*)</h3>"
hr2 = regEx.Replace(hr2,"$1</h3>")

或

regEx.Pattern = "</span>([^>]*</h3>)"
hr2 = regEx.Replace(hr2,"$1")