Given foo.txt:
redpen -r json foo.txt produces:
[{"document":"foo.txt","errors":[{"sentence":"I Dockerfile","endPosition":{"offset":14,"lineNum":1},"validator":"Spelling","lineNum":1,"sentenceStartColumnNum":0,"message":"Found possibly misspelled word "Dockerfile".","startPosition":{"offset":2,"lineNum":1}}]}]
The offset with startPosition is 2 here, but should be 3.
It seems like the backtick caused the offset error here.
Also the offset for endPosition seems to be off.