Elastic Search Regex are not working as expected

I have go the problem in making elastic search regex work. I have a document that looks like this:

{"content": "keySyAtUXpd8JxrpUH2Sd"}

I have trying the following regex key[0-9A-Za-z_]{18} which perfectly matches with the string in regexer.com but when I query the request from elastic search it doesn't show any hits.

Here's the request that i'm using:

curl -XGET 'https://localhost:9200/_search?pretty' -H 'Content-Type: application/json' -H 'Authorization: Basic redacted' -k -d '{
  "query": { "regexp": { "content": "key[0-9A-Za-z_]{18}" } }
}'

I have also tried the regex with .*key[0-9A-Za-z_]{18}.*, tried to escape - as \\- but it doesn't seems to be working as well.

You should add -H "Content-Type: application/json"

Try this:

curl -u elastic:password -XPOST "https://localhost:9200/test_regex/_bulk?refresh" -H "Content-Type: application/json" -d'
{"index":{"_id":"1"}}
{"content": "keySyAtUXpd8JxrpUH2Sd"}
'

curl -u elastic:password -XGET "https://localhost:9200/_search?pretty" -H "Content-Type: application/json" -d'
{
  "query": { "regexp": { "content": "key[0-9A-Za-z_]{18}" } }
}'

You need to run the regexp query against the content.keyword field

curl -XGET 'https://localhost:9200/_search?pretty' -H 'Content-Type: application/json' -H 'Authorization: Basic redacted' -k -d '{
  "query": { "regexp": { "content.keyword": "key[0-9A-Za-z_]{18}" } }
}'

PS: easier to test and provide feedback with real content and real queries