URLschemes

URL schemes, commonly referred to as URI schemes, are the initial component of a Uniform Resource Identifier that indicates the protocol or handler to be used to access the resource. In the generic URI syntax, the scheme appears before the colon (scheme:). The scheme name is case-insensitive and must start with a letter, followed by letters, digits, plus, period, or hyphen. The scheme guides software on how to interpret and retrieve the remainder of the URI, which may include authority, path, query, and fragment.

Common schemes include http and https for the web, ftp for file transfer, mailto for email, file for local files, and tel for telephone numbers. Many platforms support custom application schemes, such as myapp:// or appname://, which allow a program to be launched or a specific action to be performed within an app. On mobile devices, apps register their schemes or use platform mechanisms to enable deep linking.

Handling and registration: When a URI with a known scheme is activated, the operating system or browser consults a registry of handlers to locate the responsible application. Web browsers typically navigate standard schemes like http/https, while custom schemes invoke local applications. Servers do not own schemes; rather, app manifests or OS registries declare supported schemes.

Security and privacy: Because schemes can trigger external applications, improper handling may lead to phishing, unintended actions, or denial of service. Validation and restrictions are important, especially for user-provided or third-party URIs. Some platforms provide alternatives to custom schemes, such as universal or app links over HTTPS, to improve safety and user experience.

Encoding and interoperability: The scheme portion is ASCII and case-insensitive; the remainder of the URI may be percent-encoded. RFC 3986 defines the formal grammar for URIs. Examples: http://example.org, https://secure.example, mailto:[e-mail], myapp://open?id=123.