conda · jaimergp · May 7, 2025 · Mar 11, 2025 · Mar 11, 2025 · Mar 12, 2025
diff --git a/README.md b/README.md
@@ -36,6 +36,7 @@ for conda's implementation, all major changes should be submitted as
 | [0023](cep-0023.md) | Text spec input files                                                       |
 | [0024](cep-0024.md) | Specification of <code>environment.yml</code> input files                   |
 | [0025](cep-0025.md) | Versioning of Existing conda Standards                                      |
+| [0026](cep-0026.md) | Identifying Packages and Channels in the conda Ecosystem                    |
 
 ## References
 

diff --git a/cep-0026.md b/cep-0026.md
@@ -0,0 +1,239 @@
+# CEP 26 - Identifying Packages and Channels in the conda Ecosystem
+
+<table>
+<tr><td> Title </td><td> CEP 26 - Identifying Packages and Channels in the conda Ecosystem </td>
+<tr><td> Status </td><td> Approved </td></tr>
+<tr><td> Author(s) </td><td>
+  Jaime Rodríguez-Guerra &lt;jaime.rogue@gmail.com&gt; <br />
+  Matthew R. Becker &lt;becker.mr@gmail.com&gt; <br />
+  Cheng H. Lee &lt;clee@anaconda.com&gt;
+</td></tr>
+<tr><td> Created </td><td> Mar 11, 2025</td></tr>
+<tr><td> Updated </td><td> Apr 17, 2025</td></tr>
+<tr><td> Discussion </td><td> https://github.com/conda/ceps/pull/116 </td></tr>
+<tr><td> Implementation </td><td> N/A </td></tr>
+</table>
+
+## Abstract
+
+This CEP aims to standardize names and other strings used to identify packages, artifacts and
+channels in the conda ecosystem.
+
+## Specification
+
+The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
+"RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as
+described in [RFC2119][RFC2119] when, and only when, they appear in all capitals, as shown here.
+
+More specifically, violations of a MUST or MUST NOT rule MUST result in an error. Violations of the
+rules specified by any of the other all-capital terms MAY result in a warning, at discretion of the
+implementation.
+
+### Identifying package artifacts
+
+The conda ecosystem distinguishes between two types of packages:
+
+- Distributable package names: represented by a concrete, downloadable, extractable conda artifact.
+- Virtual package names: not backed by any concrete artifact. They only exist on the client side.
+
+#### Package names
+
+A distributable package name MUST only consist of lowercase ASCII letters, numbers, hyphens,
+periods and underscores. It MUST start with a letter, a number, or a single underscore. It MUST NOT
+include two consecutive separators (hyphen, period, underscore).
+
+Virtual package names MUST only consist of lowercase ASCII letters, numbers, hyphens, periods and
+underscores. They MUST NOT use two consecutive separators, with one exception: they MUST start with
+two underscores.
+
+Distributable package names MUST match the following case-insensitive regex:
+`^(([a-z0-9])|([a-z0-9_](?!_)))[._-]?([a-z0-9]+(\.|-|_|$))*$`.
+
+Virtual package names MUST follow this regex: `^__[a-z0-9][._-]?([a-z0-9]+(\.|-|_|$))*$`.
+
+In all cases, the maximum length of a package name MUST NOT exceed 64 characters.
+
+#### Version strings
+
+Version strings MUST only consist of digits, periods, lowercase ASCII letters, underscores, plus
+symbols, and exclamation marks. Additional rules apply but are out of scope in this CEP and will be
+discussed separately.
+
+The maximum length of a version string MUST NOT exceed 64 characters.
+
+#### Build strings
+
+Builds strings MUST only consist of ASCII letters, numbers, periods, plus symbols, and underscores.
+They MUST match this regex `^[a-zA-Z0-9_\.+]+$`.
+
+The maximum length of a build string MUST NOT exceed 64 characters.
+
+#### Artifact extensions
+
+Artifact extensions MUST only consist of lowercase ASCII letters, numbers and periods. They must
+start and end with a letter or a number. They MUST NOT include two consecutive periods. They MUST
+match this regex `^[a-z0-9](\.?[a-z0-9])*$`.
+
+The maximum length of a file extension MUST NOT exceed 16 characters.
+
+> The conda ecosystem currently recognizes two artifact extensions: `tar.bz2` and `conda`,
+versioned `v1` and `v2` respectively.
+
+#### Distribution strings
+
+A "distribution string" MAY be used to identify a package artifact, without specifying the
+extension or channel. It MUST match the following syntax:
+
+```text
+[<subdir>/]<package name>-<version string>-<build string>
+```
+
+Distribution strings apply to distributable packages. They are used as the name of
+the directories where artifacts are extracted in the package cache, for example.
+
+Virtual packages MAY be also identified by a distribution string, but in those cases a subdir MUST NOT be present.
+
+> Note: Despite the similarity, distribution strings are not `MatchSpec`-like specifiers and MUST
+> NOT be used as such.
+
+#### Filenames
+
+The filename of distributable conda artifacts is obtained by adding the artifact extension to its
+distribution string (without the subdir, if present). It MUST match this syntax:
+
+```text
+<package name>-<version string>-<build string>.<extension>
+```
+
+The maximum length of a filename MUST NOT exceed 211 characters.
+
+Virtual conda packages do not exist on disk and SHOULD NOT need filename standardization.
+
+### Identifying channels
+
+A conda channel is defined as a URL where one can find one or more `repodata.json` files arranged
+in one subdirectory (_subdir_) each. `noarch/repodata.json` MUST be present to consider the parent
+location a channel.
+
+#### Channel base URLs
+
+The base URL for the arbitrary location of a repodata file is defined as:
+
+```text
+<scheme>://[<authority>][/<path>/][/label/<label name>]/<subdir>/repodata.json
+```
+
+with `<scheme>`, `<authority>` and `<path>` defined by [RFC
+3986](https://datatracker.ietf.org/doc/html/rfc3986#section-3.2).
+
+Taken the channel definition above, the base URL without trailing slashes is thus:
+
+```text
+<scheme>://[<authority>][/<path>/][/label/<label name>]
+```
+
+For example, given `https://conda.anaconda.org/conda-forge/noarch/repodata.json`, the part leading
+to `noarch/repodata.json` and thus base URL is `https://conda.anaconda.org/conda-forge`. For local
+repodata such as `file:///home/username/channel/noarch/repodata.json`, the channel base URL is
+`file:///home/username/channel`.
+
+When present, each path component MUST only contain lowercase ASCII letters, numbers, underscores,
+periods, and dashes. They MUST NOT start with a period or a dash. They SHOULD start and end with a
+letter or a number. If present, each path component MUST match this regex:
+
+```re
+^[a-z0-9_][a-z0-9_.-]*$
+```
+
+For `file://`-based channel URLs, the path component rules MAY be understood as recommendations
+only.
+
+The maximum length of an individual path component in a channel base URL MUST NOT exceed 128
+characters. The maximum length of a channel base URL SHOULD NOT exceed 256 characters.
+
+To avoid ambiguous `MatchSpec` grammar, the last path component of a channel base URL SHOULD NOT
+match any `subdir` identifiers. If it does, the behavior in this ambiguous case is not defined
+and implementation dependent.
+
+#### Channel names
+
+For convenience, the channel _name_ is defined as the concatenation of `scheme`, `authority` and
+`path` components of a channel URL. At least one of `authority` or `path` SHOULD be present. In
+their absence, the channel name MUST be considered empty, regardless the scheme. Empty channel
+names SHOULD NOT be used.
+
+When the scheme and authority fields are missing, the full URL can be inferred with these rules:
+
+- If the channel name matches the regex `^\.{0,2}[/\\].*$`, or if it matches the regex
+  `^[A-Z]:([\\/].*)?$` (for Windows drives), it SHOULD be understood as the path component of a
+  `file://` URL.
+- Otherwise, the tool SHOULD provide a user-configurable mechanism to use a default scheme and
+  authority, with the provided channel name taken as the rest of the path component. At the time of
+  this CEP's writing, most tools assume the default URL scheme and authority to be
+  `https://conda.anaconda.org`.
+
+#### Subdir names
+
+Channel subdir names MUST either be the literal `noarch` or a string following the syntax
+`{os}-{arch}`, where `{os}` and `{arch}` MUST only consist of lowercase ASCII letters and numbers.
+Non-`noarch` subdirs MUST match this regex: `^[a-z0-9]+-[a-z0-9]+$`.
+
+The maximum length of a subdir name MUST NOT exceed 32 characters.
+
+#### Label names
+
+Channel label names MUST only consist of ASCII letters, digits, underscores, hyphens, forward
+slashes, periods, and whitespace. They MUST start with a letter. They MUST match this regex:
+`^[a-zA-Z][0-9a-zA-Z_\-\./]*$`. The last `/`-delimited component of a label
+SHOULD NOT match any `subdir` identifier. If it does, the behavior in this ambiguous
+case is undefined and implementation dependent.
+
+The label `nolabel` is reserved and MUST only be used for conda packages which have no other
+labels. In other words, in the space of labels, the empty set is represented by the labels
+`nolabel`.
+
+A URL for a package, repodata, etc. without a label component MUST be assumed to have the default
+label `main`.
+
+The maximum length of a label name MUST NOT exceed 128 characters.
+
+## Backwards compatibility
+
+The conda subdir and package name regexes are backwards compatible with the current `conda`
+implementation (25.3) and all existing packages on the `defaults` and `conda-forge` channels,
+except for the `__anaconda_core_depends` package on the `defaults` channel, which was [deprecated
+in April 2025][anaconda-core-depends-depr]. See [this
+comment](https://github.com/conda/ceps/pull/116#discussion_r1992234677).
+
+The regex for labels was pulled from an anaconda.org error message describing the set of valid
+labels.
+
+As of 2025-03-12T19:00Z, of the ~1.9M channel names on anaconda.org:
+
+- 7,219 violate the regex `^[a-z0-9]+((-|_|.)[a-z0-9]+)*$`;
+- 98 violate the regex `^[a-z0-9][a-z0-9_.-]*$` (allowing channel names to end with `_`, `.`, or
+  `-`); and
+- 6 violate `^[a-z0-9_][a-z0-9_.-]*$` (allowing channel names to start with `_`). Of those six,
+  five start with `.`, and the other starts with `~`.
+
+See [this comment](https://github.com/conda/ceps/pull/116#discussion_r1992154574) for more details.
+The authors have excluded the channel names in the last case that start with `.` or `~` given
+possible security implications. A low percentage, ~0.4%, of channels do not match the
+recommendations for channel names above, but are allowed.
+
+The maximum lengths allowed for the different fields have been chosen so the resulting path
+components (directory names, filenames) comfortably fit in a the 255-char maximum limit some
+filesystems impose. As of 2025-03-01T13:00Z, there are no violations of these limits in any of the
+packages published for `conda-forge`, `bioconda` and `defaults`. See [this
+comment](https://github.com/conda/ceps/pull/116#issuecomment-2763392999) and [this
+comment](https://github.com/conda/ceps/pull/116#issuecomment-2759130187) for more details.
+
+## Copyright
+
+All CEPs are explicitly [CC0 1.0 Universal](https://creativecommons.org/publicdomain/zero/1.0/).
+
+<!-- links -->
+
+[RFC2119]: https://www.ietf.org/rfc/rfc2119.txt
+
+[anaconda-core-depends-depr]: https://conda.zulipchat.com/#narrow/channel/457607-general/topic/conda-oci.20incubation/near/512865665