In Elasticsearch, how do I search for an arbitrary substring? - elasticsearch

Question

In Elasticsearch, how do I search for an arbitrary substring, perhaps including spaces? (Searching for part of a word isn't quite enough; I want to search any substring of an entire field.)
I imagine it has to be in a keyword field, rather than a text field.
Suppose I have only a few thousand documents in my Elasticsearch index, and I try:
"query": {
"wildcard" : { "description" : "*plan*" }
}
That works as expected--I get every item where "plan" is in the description, even ones like "supplantation".
Now, I'd like to do
"query": {
"wildcard" : { "description" : "*plan is*" }
}
...so that I might match documents with "Kaplan isn't" among many other possibilities.
It seems this isn't possible with wildcard, match prefix, or any other query type I might see. How do I simply search on any substring? (In SQL, I would just do description LIKE '%plan is%')
(I am aware any such query would be slow or perhaps even impossible for large data sets.)

Answer 1

Have you tried the regxp query in elasticsearch? It sure does sound like something you might be interested in.

Answer 2

I was hoping there might be something built-in for this Elasticsearch, given that this simple substring search seems like a very basic capability (Thinking about it, it is implemented as strstr() in C, LIKE '%%' in SQL, Ctrl+F in most text editors, String.IndexOf in C#, etc.), but this seems not to be the case. Note that the regexp query doesn't support case insensitivity, so I also needed to pair it with this custom analyzer, so that the index matches all-lowercase. Then I can convert my search string to lowercase as well.
{
"settings": {
"analysis": {
"analyzer": {
"lowercase_keyword": {
"type": "custom",
"tokenizer": "keyword",
"filter": [ "lowercase" ]
}
}
}
},
"mappings": {
...
"description": {"type": "text", "analyzer": "lowercase_keyword"},
}
}
Example query:
"query": {
"regexp" : { "description" : ".*plan is.*" }
}
Thanks to Jai Sharma for leading me; I just wanted to provide more detail.