Case Converter

UPPERCASE converts every letter to capitals (HELLO WORLD). lowercase converts every letter to small (hello world). Title Case capitalizes the first letter of each word (Hello World). Sentence case capitalizes only the first letter of the whole string (Hello world). camelCase joins words with no separator, lowercasing the first word and capitalizing subsequent words (helloWorld). PascalCase is the same as camelCase but capitalizes the first word too (HelloWorld). snake_case lowercases all letters and joins with underscores (hello_world). kebab-case lowercases all letters and joins with hyphens (hello-world). CONSTANT_CASE uppercases all letters and joins with underscores (HELLO_WORLD). dot.case lowercases and joins with periods (hello.world). Alternating case alternates upper and lower letter by letter (hElLo WoRlD). Each format encodes both a separator rule and a capitalization rule, and our converter handles all combinations of input format to output format.

Both formats join words with no separator. The difference is the first letter — camelCase lowercases it (myVariableName), PascalCase capitalizes it (MyClassName). The convention across modern languages: camelCase for variables, function names, and method names; PascalCase for class names, type names, interface names, enums, and React/Vue/Svelte components. JavaScript example: const userAccount = getCurrentUser() — userAccount and getCurrentUser are camelCase; class UserAccount {} — UserAccount is PascalCase. Java: int totalAmount, void calculateTotal() (camelCase methods/fields), class Calculator (PascalCase classes). C#: more nuanced — public properties and methods are PascalCase (UserName, GetUserById), private fields use camelCase (often with _ prefix). TypeScript follows JavaScript conventions plus PascalCase for types and interfaces. React components must be PascalCase to render correctly — lowercase tag names are interpreted as HTML elements.

snake_case is the dominant convention in dynamic and scripting languages. Python PEP 8 mandates snake_case for function names, variable names, method names, module names, package names: def calculate_total(user_id), total_amount = 0, import data_processor. Constants are CAPS_WITH_UNDERSCORES (MAX_RETRY = 5). Classes are PascalCase (class UserAccount). Ruby uses snake_case throughout — methods, variables, file names — with constants in PascalCase or CONSTANT_CASE. SQL: standard naming for tables and columns is snake_case (user_accounts, created_at, last_login_date) — quoted identifiers preserve case but introduce fragility across databases. JSON: Python-generated and Ruby-generated APIs typically return snake_case field names (Stripe API, Twitter API, Django REST Framework). When transforming between Python backend and JavaScript frontend, choose one and convert at the API boundary (e.g., axios interceptor, Pydantic alias generators).

kebab-case is required in three places: (1) CSS class names — .primary-button, .nav-link-active. JavaScript identifiers cannot contain hyphens, so CSS chose hyphens precisely to be impossible to confuse with JavaScript identifiers. (2) URL slugs — /blog/how-to-calculate-mortgage. Google explicitly recommends hyphens over underscores for word separation: hyphens are treated as word breaks, underscores as connectors, so 'best-running-shoes' indexes as three separate terms while 'best_running_shoes' indexes as one. (3) HTML attribute names — particularly data-* attributes and Web Components custom element names. The Web Components specification requires custom element names to contain a hyphen (my-component, not myComponent) to distinguish them from standard HTML elements. CSS custom properties (variables) also use kebab-case: --primary-color, --max-width. When CSS-in-JS frameworks transform property names, they convert camelCase JavaScript to kebab-case CSS automatically.

CONSTANT_CASE (also called SCREAMING_SNAKE_CASE or UPPER_SNAKE_CASE) signals 'this value never changes' across every major language. Use cases: (1) Language-level constants — JavaScript const PI = 3.14159, Java public static final int MAX = 100, Python ABSOLUTE_PATH = '/etc', Go const MaxRetries = 5 (though Go often uses MixedCase for exported constants). (2) Enum values — TypeScript enum Status { PENDING, IN_PROGRESS, COMPLETED }, Java public enum Status { PENDING, IN_PROGRESS }, Python's StrEnum members. (3) Environment variables — DATABASE_URL, NODE_ENV, AWS_ACCESS_KEY_ID across all OS and frameworks. (4) C/C++ macros — #define BUFFER_SIZE 1024, #define MAX_RETRY 5. (5) Docker compose YAML environment sections, Kubernetes ConfigMaps, GitHub Actions secrets, Vercel/Netlify dashboards. The visual SCREAMING distinction makes constants impossible to mistake for variables at a glance — that's the entire point of the convention.

Title Case capitalizes the first letter of every word; Sentence case capitalizes only the first letter of the whole phrase plus proper nouns. Different style guides diverge on what to capitalize in Title Case. AP Style and APA: capitalize words of 4+ letters, do NOT capitalize short prepositions (in, on, at, of, by, to, for), articles (a, an, the), or coordinating conjunctions (and, but, or, nor) — unless they're the first or last word. Chicago Manual of Style: capitalize all words except articles, coordinating conjunctions, and prepositions regardless of length. New York Times: caps for words of 5+ letters. Modern tech style guides (Google, Apple, Microsoft Manual of Style) prefer Sentence case for UI labels, body copy, and even headings — it reads as less shouty and more conversational. SEO testing (CoSchedule, Backlinko) shows Title Case titles receive 14-20% higher CTR in search results, so use Title Case for blog post titles regardless of which body style you prefer.

Each language has codified conventions either in an official style guide or by community consensus. JavaScript/TypeScript: camelCase variables and functions, PascalCase classes/types/components, CONSTANT_CASE constants. Per Airbnb, Google and Standard style guides. Python: snake_case variables/functions/methods/modules, PascalCase classes, CAPS_WITH_UNDERSCORES constants. Mandated by PEP 8. Java: camelCase variables/methods, PascalCase classes/interfaces, CAPS_WITH_UNDERSCORES constants. Per Google Java Style Guide. Go: MixedCaps for both exported (capital first letter) and unexported (lowercase first letter) — gofmt enforces. Rust: snake_case for functions/variables/modules, PascalCase for types/traits, SCREAMING_SNAKE_CASE for constants. Per official Rust style guide. C#: PascalCase for everything public (methods, properties, classes), camelCase for parameters and private fields (often with _ prefix). Ruby: snake_case throughout, except classes/modules (PascalCase) and constants (PascalCase or CAPS_WITH_UNDERSCORES). Always match the surrounding code's convention rather than imposing your preferred style.

URL slugs should be lowercase kebab-case for four reasons. (1) Google word-separation: hyphens are treated as word separators in URLs, underscores are treated as connectors. /best-running-shoes ranks for 'best', 'running', 'shoes' independently; /best_running_shoes is treated as one compound term. (2) Case sensitivity: Apache, nginx, and other servers may treat /About and /about as different URLs by default — duplicate content penalty risk. Always use lowercase. (3) User-typed reliability: capital letters cause typos; lowercase URLs are easier to share verbally or write in print. (4) Standards alignment: RFC 3986 treats paths as case-sensitive, but the de facto web convention is lowercase. Best practice: strip diacritics (café → cafe), transliterate non-ASCII characters, drop articles (the/a/an) to shorten, target 60-70 character slugs for optimal SEO display in SERPs. Avoid: dates in slugs (date them in publish metadata), session IDs, query parameters embedded in slugs.

Acronyms are the trickiest part of programming case conventions because two competing approaches exist. Approach A (preserve acronyms): HTTPRequest, HTMLParser, UserID, JSONResponse, URLBuilder. Older C, C++, Java code from the 1990s-2000s used this. Easier to identify acronyms at a glance. Approach B (treat as words): HttpRequest, HtmlParser, UserId, JsonResponse, UrlBuilder. Microsoft .NET, Google Java Style, modern TypeScript and JavaScript favor this. Easier to read multi-acronym names like HtmlHttpClient vs HTMLHTTPClient. Microsoft's rule: two-letter acronyms keep both letters capitalized (IOStream, IPAddress) but three+ letter acronyms become Pascal (HtmlParser, not HTMLParser). Google Java Style: always treat as words (HtmlParser, not HTMLParser, even for 3-letter HTML). Our converter applies Approach B by default. When working in an existing codebase, always match the established style — introducing a new acronym convention causes inconsistency that lingers for years.

Different contexts have different case sensitivity rules. URLs: technically case-sensitive (RFC 3986) but most web servers treat paths case-insensitively by default — always lowercase for safety and SEO. HTML: tag and attribute NAMES are case-insensitive (DIV = div = Div), but attribute VALUES are case-sensitive (class names, ID values must match exactly when referenced from CSS/JS). JSON: fully case-sensitive — {"userName": ...} and {"username": ...} are different fields. SQL: depends on database — MySQL on Windows is case-insensitive for table names, MySQL on Linux is case-sensitive; PostgreSQL folds unquoted identifiers to lowercase but preserves quoted ones; SQLite is case-insensitive by default. File systems: Windows NTFS and macOS APFS (default) are case-insensitive but case-preserving; Linux ext4 and macOS APFS (case-sensitive option) are case-sensitive. Git on case-insensitive file systems can cause silent issues when collaborators on case-sensitive systems pull. Best practice: enforce one case per system via lint rules, IDE settings, or git config (core.ignorecase).

For single variables: paste into our converter, get the result, paste back. For bulk renames within a file or project, use the safest tool available. IDE rename refactor (F2 in VS Code, Shift-F6 in JetBrains, Cmd-R-R in Xcode): safest option — updates all references including imports, comments (optional), and dynamic strings (often). Project-wide find-replace with regex: faster but risky — manually verify each substitution and run tests after. Command-line: ripgrep with --pcre2 plus sed pipeline for systematic renames across many files. AST-based tools: jscodeshift for JavaScript/TypeScript, libcst for Python, OpenRewrite for Java — safest for complex changes involving thousands of files with semantic understanding. For converting an entire JSON API response from snake_case to camelCase, use a transformation library at the API boundary (camelCase-keys for Node, humps for JS, Pydantic alias generators for Python). Never run bulk renames on a dirty git working tree — branch, refactor, test, then merge.

Naive case conversion fails on several edge cases. (1) Acronyms — covered above, requires explicit convention choice. (2) Numbers in identifiers — should '2024Q4Revenue' become '2024_q4_revenue' or '2024_q_4_revenue'? Most converters split letter-digit transitions but not digit-letter; ours preserves digit groups. (3) Unicode case mapping — German ß lowercases to 'ss' but uppercases to ẞ (2017 official) or 'SS' (legacy). Turkish dotted/undotted i has locale-specific rules. Greek final sigma (ς) vs medial (σ) doesn't survive round-trip case operations. (4) Brand names with internal capitalization — iPhone, eBay, YouTube — should be preserved exactly, never converted. Add brand names to your editor's case-exception list. (5) Initialisms vs acronyms — NASA is an acronym (pronounced as a word), FBI is an initialism (pronounced as letters) — style guides treat them identically but they read differently. (6) Mixed-language text — converting CJK characters mixed with Latin has no defined behavior; most tools pass CJK through unchanged. Our converter handles standard Unicode for Latin/Cyrillic scripts and passes CJK characters through; for production multilingual systems, use ICU-based libraries (icu4j, PyICU) for locale-aware case operations.