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15_WildcardRegexp.asciidoc

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wildcard and regexp queries

The wildcard query is a low-level term-based query similar in nature to the prefix query, but it allows you to specify a pattern instead of just a prefix. It uses the standard shell wildcards where ? matches any character, and * matches zero or more characters.

This query would match the documents containing W1F 7HW and W2F 8HW:

GET /my_index/address/_search
{
    "query": {
        "wildcard": {
            "postcode": "W?F*HW" (1)
        }
    }
}

  1. The ? will match the 1 and the 2, while the * matches the space and the 7 and 8.

Imagine now that you wanted to match all postcodes just in the W area. A prefix match would also include postcodes starting with WC, and you would have a similar problem with a wildcard match. We only want to match postcodes that begin with a W, followed by a number. The regexp query allows you to write these more complicated patterns:

GET /my_index/address/_search
{
    "query": {
        "regexp": {
            "postcode": "W[0-9].+" (1)
        }
    }
}

  1. The regular expression says that the term must begin with a W, followed by any number from 0 to 9, followed by 1 or more other characters.

The wildcard and regexp queries work in exactly the same way as the prefix query. They also have to scan the list of terms in the inverted index to find all matching terms, and gather document IDs term-by-term. The only difference between them and the prefix query is that they support more complex patterns.

This means that the same caveats apply. Running these queries on a field with many unique terms can be very resource intensive indeed. Avoid using a pattern that starts with a wildcard, eg "*foo" or, as a regexp, ".*foo".

While prefix matching can be made more efficient by preparing your data at index time, wildcard and regular expression matching can only really be done at query time. These queries have their place, but should be used sparingly.

Caution

The prefix, wildcard and regexp queries operate on terms. If you use them to query an analyzed field then they will examine each term in the field, not the field as a whole.

For instance, let’s say that our title field contains `Quick brown box'' which produces the terms: `quick, brown and fox.

This query would match:

{ "regexp": { "title": "br.*" }}

But neither of these queries would match:

{ "regexp": { "title": "Qu.*" }} (1)
{ "regexp": { "title": "quick br*" }} (2)

  1. The term in the index is quick, not Quick.

  2. quick and brown are separate terms.