One way to achieve this is to use the html_strip char filter while analyzing preview_html field.
This would ensure that while matches would not occur on html markup and hence highlighting would ignore it to as shown in the example below.

Example:

put test{   "settings": {      "index": {         "analysis": {            "char_filter": {               "my_html": {                  "type": "html_strip"               }            },            "analyzer": {               "my_html": {                  "tokenizer": "standard",                  "char_filter": [                     "my_html"                  ],                  "type": "custom"               }            }         }      }   }}put test/test/_mapping{   "properties": {      "preview_html": {         "type": "string",         "analyzer": "my_html",         "search_analyzer": "standard"      }   }}put test/test/1{    "preview_html": "<p> p </p>"}post test/test/_search{   "query": {      "match": {         "preview_html": "p"      }   },   "highlight": {      "fields": {         "preview_html": {}      }   }}

Results

 "hits": [         {            "_index": "test",            "_type": "test",            "_id": "1",            "_score": 0.30685282,            "_source": {               "preview_html": "<p> p </p>"            },            "highlight": {               "preview_html": [                  "<p> <em>p</em> </p>"               ]            }         }      ]