The Matches API will give you this information - it’s still likely to be fairly 
slow, but it’s a lot easier to use than trying to parse Explain output.

Query q = ….;
Weight w = searcher.createWeight(searcher.rewrite(query), 
ScoreMode.COMPLETE_NO_SCORES, 1.0f);

Matches m = w.matches(context, doc);
List<String> matchingFields = new ArrayList();
for (String field : m) {
 matchingFields.add(field);
}

Bear in mind that `matches` doesn’t maintain any state between calls, so 
calling it for every matching document is likely to be slow; for those cases 
Shai’s suggestion of using a Collector and examining low-level scorers will 
perform better, but it won’t work for every query type.


> On 25 Jun 2022, at 04:14, Yichen Sun <yiche...@bu.edu> wrote:
> 
> Hello!
> 
> I’m a MSCS student from BU and learning to use Lucene. Recently I try to 
> output matched fields by one query. For example, for one document, there are 
> 10 fields and 2 of them match the query. I want to get the name of these 
> fields.
> 
> I have tried using explain() method and getting description then regex. 
> However it cost so much time.
> 
> I wonder what is the efficient way to get the matched fields. Would you 
> please offer some help? Thank you so much!
> 
> Best regards,
> Yichen Sun


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