# MSC2758: Common grammar for textual identifiers
The matrix specification uses textual identifiers for a wide range of
concepts. Examples include "event types" and "room versions".
In the past, these identifiers have often lacked a formal grammar, leaving
servers and clients to make assumptions about questions such as which
characters are permitted, minimum and maximum lengths, etc.
This proposal suggests a common grammar which can be used as a basis for
*future* identifier types, to reduce the work involved in future specification
work.
No attempt is made here to bring existing identifiers into line; however
examples of identifiers which might have benefitted from such a grammar in the
past include:
* [`capabilities` ](https://matrix.org/docs/spec/client_server/r0.6.0#get-matrix-client-r0-capabilities )
identifiers.
* authentication types for the [User-Interactive Authentication mechanism ](https://matrix.org/docs/spec/client_server/r0.6.0#user-interactive-authentication-api ).
* login types for [`/_matrix/client/r0/login` ](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-login ).
* event types
* [`m.room.message` `msgtypes` ](https://matrix.org/docs/spec/client_server/r0.6.0#m-room-message-msgtypes )
* `app_id` for [`POST /_matrix/client/r0/pushers/set` ](https://matrix.org/docs/spec/client_server/r0.6.0#post-matrix-client-r0-pushers-set ).
* `rule_ids` , `actions` and `tweaks` for [push rules ](https://matrix.org/docs/spec/client_server/r0.6.0#push-rules ).
* [E2E messaging algorithm names ](https://matrix.org/docs/spec/client_server/r0.6.0#messaging-algorithm-names ).
## Proposal
We define a "common namespaced identifier grammar". This can then be referenced
by other parts of the grammar, in much the same way as [Unpadded
Base64](https://matrix.org/docs/spec/appendices#unpadded-base64) is defined
today.
The grammar is defined as follows:
* An identifier may not be less than one character or more than 255 characters
in length.
* Identifiers must start with one of the characters `[a-z]` , and be entirely
composed of the characters `[a-z]` , `[0-9]` , `-` , `_` and `.` .
* Identifiers starting with the characters `m.` are reserved for use by the
formal matrix specification.
* Implementations wishing to implement unspecified identifiers should follow
the Java Package Naming convention of starting with a reversed domain
name (with a dot after the domain name part). For example, for the
organisation `example.com` , a valid identifier would be
`com.example.identifier` .
This grammar is intended for use entirely by internal identifiers, and *not*
for user-visible strings.
### Rationale
* Avoiding non-ascii characters sidesteps any issues with homoglyphs or
altenative encodings of the same characters.
* Avoiding upper-case character sidesteps any concerns over case-sensitivity.
