Fediverse ID Syntax

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Currently there is no canonical syntax for a Fediverse ID. However, we can infer a syntax for the Fediverse ID given that it has to be used with WebFinger.

De Facto

Currently, the syntax of a Fediverse ID is whatever each Fediverse software supports. Which could be different from each other! And could even change over time!

The final say for what is and is not a valid Fediverse ID are the many implementations of WebFinger in-the-wild for all the different Fediverse software out there. Which again could be different from each other! And again could even change over time!

The reason being because, from a technical point-of-view, a Fediverse ID is turned into an acct URI and then resolved with WebFinger.

But, having said that, we can infer a minimum syntax and a maximum syntax for what should be considered a valid Fediverse ID. I.e., the syntax that any implmentation for Fediverse IDs should support. But, to say it explicitly, an implemention for Fediverse IDs could support more than just what is given by thes syntaxes.

Perhaps oddly, the maximal syntax is simpler than the minimal syntax.

Maximal Fediverse ID Syntax

An acct URI has some constraints according to the IETF RFC-7565 (The 'acct' URI Scheme) specification. But — software, such as Fediverse software, don't have to implement those constraints. I.e., they can accept more than just what IETF RFC-7565 (The 'acct' URI Scheme) specifies.

Given that, here is a more maximal syntax for a Fediverse ID: 


fediverseID = "@" [ actor ] "@" host

Where, given that ‘NOTATSIGN’ (i.e., not at-sign) is any Unicode character that isn't the U+0040 ("@") at-signs —

‘actor’ is defined as: 


actor       = NOTATSIGN *NOTATSIGN      ; one or more characters that are not an at-sign ("@") (U+0040)

And ‘host’ is defined as: 


host        = NOTATSIGN *NOTATSIGN      ; one or more characters that are not an at-sign ("@") (U+0040)

Note in this maximal Fediverse ID syntax ‘actor’ is optional. But when ‘actor’ isn't included there still needs to be 2 U+0040 ("@") at-signs.

Here are some Fediverse IDs that fit this maximal Fediverse ID syntax:

acct URI

Part of the process of resolving a Fediverse ID transfomrs a Fediverse ID into an acct URI. (And then using that acct URI version of the Fediverse ID makes a request to WebFinger.)

Here is an example of transforming a Fediverse ID into an acct URI: 


     at sign
       ↓
       @joeblow@example.com ← Fediverse ID

   acct:joeblow@example.com ← acct URI
       ↑
  no at sign

A client would then make a request to: 


https://example.com/.well-known/host-meta

To discover what the (template) URL for WebFinger is. And then, for example, if it is at: 


https://example.com/.well-known/webfinger?resource={uri}

Then make a request to: 


https://example.com/.well-known/webfinger?resource=acct:joeblow@example.com

This WebFinger implementation could accept whatever it wants. It doesn't have to restrict itself to what is specified by the IETF RFC-7565 (The 'acct' URI Scheme) specification. BUT —

Minimal Fediverse ID Syntax

Let us consider what a minimal Fediverse ID syntax would be if we constrained ourselves to what the IETF RFC-7565 (The 'acct' URI Scheme) specificaiton says.

Here is what we get: 


fediverseID = "@" actor "@" host

Where “actor” is defined as: 


actor       = unreserved / sub-delims 0*( unreserved / pct-encoded / sub-delims )

unreserved  = ALPHA / DIGIT / "-" / "." / "_" / "~"

sub-delims  = "!" / "$" / "&" / "'" / "(" / ")" / "*" / "+" / "," / ";" / "="

pct-encoded = "%" HEXDIG HEXDIG

ALPHA       =  %x41-5A / %x61-7A   ; A-Z / a-z

DIGIT       =  %x30-39             ; 0-9

HEXDIG      =  DIGIT / "A" / "B" / "C" / "D" / "E" / "F"

(Again note that an implementation could support more characters for “actor” than what is given by this definition.)

And “host” is defined as 


host        = IP-literal / IPv4address / reg-name

IP-literal  = "[" ( IPv6address / IPvFuture  ) "]"

IPv6address =                            6( h16 ":" ) ls32
            /                       "::" 5( h16 ":" ) ls32
            / [               h16 ] "::" 4( h16 ":" ) ls32
            / [ *1( h16 ":" ) h16 ] "::" 3( h16 ":" ) ls32
            / [ *2( h16 ":" ) h16 ] "::" 2( h16 ":" ) ls32
            / [ *3( h16 ":" ) h16 ] "::"    h16 ":"   ls32
            / [ *4( h16 ":" ) h16 ] "::"              ls32
            / [ *5( h16 ":" ) h16 ] "::"              h16
            / [ *6( h16 ":" ) h16 ] "::"

h16         = 1*4HEXDIG

ls32        = ( h16 ":" h16 ) / IPv4address

IPvFuture   = "v" 1*HEXDIG "." 1*( unreserved / sub-delims / ":" )

IPv4address = dec-octet "." dec-octet "." dec-octet "." dec-octet

dec-octet   = DIGIT                 ; 0-9
            / %x31-39 DIGIT         ; 10-99
            / "1" 2DIGIT            ; 100-199
            / "2" %x30-34 DIGIT     ; 200-249
            / "25" %x30-35          ; 250-255

reg-name    = *( unreserved / pct-encoded / sub-delims )