How does a regexp query work in Elasticsearch?

When a regexp query is executed in Elasticsearch, it searches for documents that contain terms that match a specified regular expression. The regexp query searches the inverted index for terms that match the specified regular expression and returns any documents that contain a matching term.

The regular expression can include a variety of features, including character classes, quantifiers, and capturing groups. The regular expression syntax used in Elasticsearch is based on the Java regular expression syntax.

Here’s an example of a regexp query in Elasticsearch:

GET /my_index/_search
{
  "query": {
    "regexp": {
      "title": {
        "value": "elasticsearch.*",
        "flags": "ALL"
      }
    }
  }
}

In this example, we are searching the `title` field in the `my_index` index for terms that match the regular expression “elasticsearch.*”, which matches any text that starts with “elasticsearch”. The `flags` parameter is set to “ALL” to enable all available regular expression features.

The regexp query can also be used in combination with other query types, such as the bool query, to construct more complex search queries. By allowing for searching for terms that match a specified regular expression, the regexp query provides a powerful and flexible way to search for text in Elasticsearch.

However, it’s important to note that regexp queries can be computationally expensive and may not be suitable for large datasets or high-traffic applications. Additionally, regexp queries can be less precise than other query types, such as the term or match query, because they match a broader range of terms. Therefore, it’s important to carefully consider the use case and performance implications before using regexp queries in Elasticsearch.