SOURCE

//////  工具函数：HTML实体与字符互转
var entityMap = {
    escape: {
        '&': '&',
        '<': '&lt;',
        '>': '&gt;',
        '"': '&quot;',
        "'": '&apos;',
    },
    unescape: {
        '&amp;': "&",
        '&apos;': "'",
        '&gt;': ">",
        '&lt;': "<",
        '&quot;': '"',
    }
};
var entityReg = {
    escape: RegExp('[' + Object.keys(entityMap.escape).join('') + ']', 'g'),
    unescape: RegExp('(' + Object.keys(entityMap.unescape).join('|') + ')', 'g')
}

// 将HTML转义为实体
function escape(html) {
    if (typeof html !== 'string') return ''
    return html.replace(entityReg.escape, function (match) {
        return entityMap.escape[match]
    })
}

// 将实体转回为HTML
function unescape(str) {
    if (typeof str !== 'string') return ''
    return str.replace(entityReg.unescape, function (match) {
        return entityMap.unescape[match]
    })
}
////// 工具函数：HTML实体与字符互转 结束


const record = '<div class="news-item">(.*?)</div></div>'
const field = '<h3 class="title">(.*)</h3>'
let sourceData = `<h3 class="title">标题-不需要的1</h3>
<div class="news-item"><h3 class="title">标题1标题1</h3><div>新闻概览1新闻概览1</div><div>2019-05-12</div></div>
<div class="news-item"><h3 class="title">标题2标题2</h3><div>新闻概览2新闻概览2</div><div>2019-05-13</div></div>
<div class="news-item"><h3 class="title">标题3标题3</h3><div>新闻概览3新闻概览3</div><div>2019-05-14</div></div>
<div class="related-list"><h3 class="title">标题-不需要的2</h3></div><h3 class="title">标题-不需要的3</h3>`

document.querySelector('.preview-wrapper').innerText = sourceData


hightLight()

function hightLight() {
    let regRecord = new RegExp(record, 'g')
    let regField = new RegExp(field)

    let str = ''
    let records = sourceData.match(regRecord)
    let splitedSourceList = sourceData.split(regRecord)

    splitedSourceList.forEach((item, index) => {
        console.log('item:', item, index)
        if (index % 2 === 0) {
            str = str + escape(item)

            if (index !== splitedSourceList.length - 1) {
                console.log(index)
                let r = records[index / 2]
                console.log('r:', r)
                let result = regField.exec(r)[1]
                let list = r.split(result)
                list.forEach((item, index) => {
                    str = str + escape(item)
                    if (index !== list.length - 1) {
                        str = str + '<span class="hight-light-field">' + escape(result) + '</span>'
                    }
                })
            }
        }
    })

    document.querySelector('.preview-wrapper').innerHTML = str
    console.log(str)
}

<div class="preview-wrapper"></div>

.hight-light-field{
    background: yellow;
}