Case Converter

Every case conversion starts with the same problem: working out where the words are. That is easy for "hello world" and much harder for `parseHTMLDocument`, where the boundaries are implied by capitalisation and there is a run of consecutive capitals in the middle. The splitter used here handles three cases explicitly — a lowercase or digit followed by a capital starts a new word, a run of capitals followed by a capital-then-lowercase splits before the last capital, and any punctuation or separator character ends a word. So `parseHTMLDocument` correctly becomes parse / HTML / Document, and `user_first_name`, `user-first-name` and `User First Name` all produce the same three words.

That is what makes conversion between conventions work rather than just conversion from prose. Pasting `userFirstName` gives you `user_first_name` and `USER_FIRST_NAME` correctly, which a naive lowercase-and-replace-spaces implementation cannot do.

The conventions themselves map to ecosystems. camelCase is standard for JavaScript variables and JSON keys. PascalCase names classes, React components and type definitions. snake_case is the Python and Ruby convention and the norm for SQL columns. CONSTANT_CASE marks environment variables and compile-time constants. kebab-case is required in URLs and CSS class names, since neither tolerates underscores well, and it is the convention for filenames in most modern projects. dot.case appears in configuration keys and namespaces.

Title Case has a subtlety the others do not: it is not simply capitalising every word. Standard English style keeps articles, short prepositions and conjunctions lowercase unless they begin the string — so "The Lord of the Rings", not "The Lord Of The Rings". That rule is applied here. Sentence case capitalises the first letter after each sentence-ending punctuation mark rather than collapsing your text into one long sentence.

Everything runs in your browser as pure string manipulation — no upload, no network request.