Numeric fulltext search query for the field type String

I am having hard time with fulltext search for number search. Below is the mentioned scenario.

Mappings :

"licenseNum": {
"type": "string",
"analyzer": "keyword"

Sample data which is stored in licenseNum field are AX101, 450CB, 9999, etc. I am able to do fulltext search for AX101 & 450CB but not able to search 9999.

I have tried this with not analyzed & numeric_detection : false , but had no luck.

Please help me to solve this problem.