How to Use This Tool
Changing case sounds like something a keyboard shortcut should handle, and for upper and lower it is. The two that actually need a tool are title case, where the rules are genuinely non-obvious, and the code naming conventions, where converting by hand is tedious and error-prone.
Title case, done properly
Most online converters capitalise every word. That is not title case — it is a mistake that makes headlines look like they were written by a machine. Real title case leaves articles, short prepositions and conjunctions in lower case unless they open or close the title.
The two dominant style guides disagree on the details. AP uses a length rule: capitalise anything four letters or longer, so "with" is capitalised and "into" is too. Chicago goes by part of speech: all prepositions stay lowercase regardless of length, so "Between" becomes "between". Both are correct; pick the one your publication uses and be consistent. The third option capitalises everything, which is there because sometimes a client insists.
The acronym option
Keep ALL-CAPS words as acronyms is on by default and prevents the most annoying failure mode: "the NASA API" becoming "The Nasa Api". A word already written entirely in capitals is treated as an acronym and left alone in title and sentence case. Turn it off if your source text is entirely upper case, in which case every word would qualify and nothing would convert.
Sentence case
Sentence case capitalises the first letter of each sentence and lowercases the rest. It sounds trivial and is not, because it has to find sentence boundaries. Full stops inside abbreviations, decimals and ellipses are all handled, so "costs $1.50 approx. today" does not gain three capitals it should not have.
Code naming
The second block converts to camelCase, PascalCase, snake_case, kebab-case, CONSTANT_CASE and dot.case. Input can be in any of those forms — paste a snake_case database column and get the camelCase property back. This is the tedious half of renaming things across a codebase, and doing it by hand is where typos come from.
