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The syntax

RFC 3986 defines one generic syntax for all URIs:

scheme ":" ["//" authority] path ["?" query] ["#" fragment]

• scheme — names the interpretation (http, https, mailto, file, urn, data…). Required; begins with a letter.
• authority — optional; typically userinfo@host:port, introduced by //.
• path — always present (possibly empty); the hierarchical part identifying the resource within the scheme/authority.
• query — optional, after ?; non-hierarchical parameters.
• fragment — optional, after #; a reference to a secondary part of the resource, resolved by the client.

Characters outside the permitted set are percent-encoded as %HH. References may be absolute (complete) or relative (resolved against a base URI) — the mechanism that lets documents link without repeating a common prefix.

IRI

The Internationalized Resource Identifier (RFC 3987) extends URIs to the full Unicode repertoire, so identifiers can use non-ASCII characters directly; an IRI maps to a URI by percent-encoding.

For the exhaustive grammar, see RFC 3986.

Sources

• RFC 3986 / STD 66 — the generic URI syntax in full.
• RFC 3987 — IRIs.