How to prevent ngram omits words that have length < ngram.minLength

Hi all,
Is there anyway to keep the words that shorter than min length of ngram ?
For example:
the data is "abc x" ==> ngram(2,4) will generates: ab, bc, abc. It remove
"x" since "x" has length=1. I want to keep "x" because it still a keyword.
What filters I need to use in order to keep it? I'm using ES 0.90 ?
I think ngram should have an option to keep shorter words like this case.

Thanks,
Duc

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