Sentence case converter

Lowercases everything, then capitalises the first letter of each sentence. It is the fix for text that arrived in block capitals or in inconsistent case.

Sentences are detected by their punctuation — a full stop, question mark or exclamation mark followed by a space, plus the start of each line. That works for ordinary prose and has one predictable failure: proper nouns are lowercased along with everything else, so "I went to Paris in March" comes back as "I went to paris in march". There is no way around it without knowing which words are names, so treat this as a first pass and fix the proper nouns by hand. The other known limitation is abbreviations: "e.g. this" gets an unwanted capital because the full stop looks like a sentence ending.