Matches documents that have fields matching a wildcard expression (not analyzed). Supported wildcards are *, which matches any character sequence (including the empty one), and ?, which matches any single character. Note that this query can be slow, as it needs to iterate over many terms. In order to prevent extremely slow wildcard queries, a wildcard term should not start with one of the wildcards * or ?. The wildcard query maps to Lucene WildcardQuery.

GET /_search
{
    "query": {
        "wildcard" : { "user" : "ki*y" }
    }
}

A boost can also be associated with the query:

GET /_search
{
    "query": {
        "wildcard" : { "user" : { "value" : "ki*y", "boost" : 2.0 } }
    }
}

Or :

GET /_search
{
    "query": {
        "wildcard" : { "user" : { "wildcard" : "ki*y", "boost" : 2.0 } }
    }
}

This multi term query allows to control how it gets rewritten using the rewrite parameter.