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]
                       
                        let results = regField.exec(r)
                        console.log(results)
                        if (results) {
                            let result = results[1]
                            let list = r.split(results[0])
                            list.forEach((item, index) => {
                                str = str + escape(item)
                                if (index !== list.length - 1) {
                                    (results[0].split(results[1]) || []).forEach((item) => {
                                       // str = str + escape(item)
                                        if (index !== results[0].split(results[1]).length) {
                                            str = str + '<span class="highlight-item">' + escape(result) + '</span>'
                                        }
                                    })
                                }
                            })
                        }
                    }
                }
            })

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

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

.highlight-item{
    background: yellow;
}