What format is this data structure?

Looks like JSON but with BNF symbol at the beginning and single line entries: https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pubmed&id=22368089&tool=my_tool&email=my_email@example.com

I've tried "application/json" as an Accept header but it did not work. I can add " &retmode=xml" which works but I'm just curious what format the data is in.