Json_parse_exception in elastic search

{"error":{"root_cause":[{"type":"json_parse_exception","reason":"Unrecognized character escape '&' (code 38)\n at [Source: org.elasticsearch.transport.netty4.ByteBufStreamInput@5c05c49d; line: 7, column: 188]"}]

we are using query structure following below code.

{
"query": {
"multi_match": {
"query": "' . $searchstringmsg . '",
"type": "most_fields",
"fields": [
"post_title",
"post_content",
"custom.cat_name",
"custom.series_name",
"custom.topic_name",
"custom.genre_name",
"post_author.display_name",
"post_meta.wps_subtitle",
"terms.post_tag.name"
],
"fuzziness": "1"
}
},
"aggs": {
"top-sites": {
"terms": {
"field": "custom.pos_format.keyword"
},
"aggs": {
"top_tags_hits": {
"top_hits": {}
}
}
}
}
}

I see nothing wrong with the query you copied so maybe we need more backgroud on when you get the error...
The error message is quite clear and means that on line 7 offset 188, you have invalid json - in this case you have \& to be precise... Here backslash should be escaped to be valid json \\& if you get the error when you try to index a document
This invalid json with non-escaped backslash would give out the same error :

POST test/_doc
{
"myfield" : "test \&"
}

while indexing is not a problem , when user search with special characters getting this type of error.can you please suggest me