pattern capture token filter code

[dishant sharma <di...@gmail.com>]

I am currently making some changes in the default pattern capture group
token filter code to meet my requirement. I am a beginner in JAVA so
finding it a bit hard to fully understand the code and make changes, I have
successfully done my changes in the increment token() method and got the
desired result as per my requirement. But, currently I am stuck while
making changes in the preserve original part of the code.

I only want help regarding understanding the following code and rest I will
do the changes myself. Can anyone please help in understanding the working
of this code like what is happening here?

        if (preserveOriginal) {
            currentMatcher = 0;
//            offsetAtt.setOffset(222, 333);
//            offsetAtt.setOffset(offsetAtt.startOffset() ,
offsetAtt.endOffset());
        } else if (nextCapture()) {
            final int start = matchers[currentMatcher]
                    .start(currentGroup[currentMatcher]);
            final int end = matchers[currentMatcher]
                    .end(currentGroup[currentMatcher]);

            // if we start at 0 we can simply set the length and save the copy
            if (start == 0) {
                charTermAttr.setLength(end);
            } else {
                charTermAttr.copyBuffer(spare.chars(), start, end - start);
            }
            currentGroup[currentMatcher]++;
        }


Link to the entire code file is:

https://www.tabnine.com/web/assistant/code/rs/5c65edff1095a500018f6f23#L141

Link to the functionality that I'm trying to achieve:

https://github.com/elastic/elasticsearch/issues/85947

Any help will be very useful.

Regards,
Dishant