Could you paste the regex portion of it please? Possibly some efficiencies to be gained there. You could also split the file into smaller chunks and then check the boundaries of the chunks.
The regex is dead simple: /Authorization: Basic (.*)\ngrant_type=refresh_token/ "." does not match newline, so I'm basically looking two lines that conform to a template.
Specific cases can be transformed with some grep/awk magic, but IMO the concept of pattern matching against a stream is interesting regardless.