Query string "OR" on a field

[espogian]

Hi, I'm facing an odd situation.

I have a text field (string_value) in a index (let's call it search_index) which can get the value "OR" (in caps, without quotes).
I created a Elasticsearch filter section in Logstash to query this index:

elasticsearch {
	hosts => ["elasticsearch:9200"]
	index => ["logstash-search_index-*"]
	query => "string_value.keyword:%{[string]}"
	result_size => 1
	fields => {"other_field" => "other_field"}
}

What this is basically doing, assuming that string = OR, is to query Elasticsearch in this way:

GET logstash-search_index-*/_search?q=string_value.keyword:OR

However, in this particular case I get an error from Elasticseach:

...
"type": "parse_exception",
"reason": "Cannot parse 'string_value.keyword:OR': Encountered \" <OR> \"OR \"\" at line 1, column 24.\nWas expecting one of:\n    <BAREOPER> ...\n    \"(\" ...\n    \"*\" ...\n    <QUOTED> ...\n    <TERM> ...\n    <PREFIXTERM> ...\n    <WILDTERM> ...\n    <REGEXPTERM> ...\n    \"[\" ...\n    \"{\" ...\n    <NUMBER> ...\n    ",
...

Other string combinations are working correctly.
How to avoid this?

Edit: apparently the way to address the issue is to generate the following query:

GET logstash-search_index-*/_search?q=string_value.keyword:"OR"

But the question is: how to address this on Logstash configuration? Custom query template?