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+# Extensions to the recipe spec
+
+This document describes extensions to the recipe specification as merged in
+cep-14.
+
+## The `build.post_process` section
+
+The `conda_build_config.yaml` configuration file has ways to specify
+post-processing steps. These are regex replacements that are performed on any
+new files.
+
+Instead of adding these instructions to the `conda_build_config.yaml`, we
+decided to add a new section to the recipe spec: `build.post_process`. This
+section is a list of dictionaries, each with the following keys:
+
+- `files`: globs to select files to process
+- `regex`: the regex to apply
+- `replacement`: the replacement string
+
+The regex specification follows Rust `regex` syntax. Most notably, the
+replacement string can refer to capture groups with `$1`, `$2`, etc. This also
+means that replacement strings need to escape `$` with `$$`.
+
+Internally, we use `replace_all` from the `regex` crate. This means that the
+regex is applied to the entire file, not line by line.
+
+### Example for `post_process`
+
+```yaml
+build:
+  post_process:
+    - files:
+        - "*.txt"
+      regex: "foo"
+      replacement: "bar"
+    - files:
+        - "*.cmake"
+      regex: "/sysroot/"
+      replacement: "$${PREFIX}/"
+```
+
+## Files selector in the `build` section
+
+A `files` key is added to the `build` section to simplify the inclusion /
+exclusion of files in the package.
+
+This key accepts a list of globs that are used to select files to include in the
+package.
+
+```yaml
+build:
+  files:
+    - "*.txt"
+    - "*.md"
+```
+
+This is especially useful when building multiple outputs, as the same build
+script can be used for all outputs, but the files to include can be different.
+The globs are evaluated against any new files in the prefix, after the build
+script has been run.
+
+Files that are not selected by the globs are excluded from the package.
+
+## Update to the "globs" definition
+
+Anywhere the recipe format accepts a list of globs is extended to accept:
+
+- a single glob (`files: foo/*.txt`)
+- a list of globs (`files: [foo/*.txt, bar/*.txt]`)
+- a dictionary with `include` and `exclude` keys (`files: {include: [foo/*.txt],
+  exclude: [foo/bar.txt]}`), where `include` and `exclude` are lists of globs.
+
+The lists are "conditional" lists and can contain `if` and `else` statements.
+
+A single glob or a list of globs is equivalent to a dictionary with only an
+`include` key.
+
+The evaluation of the globs is done in two steps:
+
+- first record all matches for the `include` globs. If no `include` globs are
+  given but an `exclude` glob is provided, all files are included.
+- Any file that matched an `include` glob is tested against the `exclude` globs.
+  If it matches an `exclude` glob, it is removed from the list of files to
+  include. If no exclude globs are given, no files are removed.
+
+### Example for `glob` definition
+
+For example:
+
+```yaml
+# single glob
+files: foo/*.txt
+
+# list of globs
+files:
+  - foo/*.txt
+  - bar/*.txt
+  - if: win
+    then:
+      - win.txt
+
+# dictionary with include and exclude keys
+files:
+  include:
+    - *.txt
+  exclude:
+    - foo.txt
+```
+
+In conda-build, this is discussed here: [Link to PR
+5216](https://github.com/conda/conda-build/pull/5216)
+
+## Compression level
+
+There are use cases where it is interesting for a output to set the compression
+level - for example when packaging pre-compressed data. We would like to extend
+the recipe format so that each output is able to set a "default" compression
+level with the following syntax:
+
+```yaml
+build:
+  compression_level: 1-10 # (integer)
+```
+
+The compression level maps to the following levels in the `.zstd` and `bz2`
+formats:
+
+- `zstd`: 1-19 (1 = 1, 10 = 19)
+- `bz2`: 1-9 (1 = 1, 10 = 9)
+
+Note that this compression level can still be overridden by the command line. It
+also does not define wether a `.conda` or `.tar.bz2` file is created.
+
+## System requirements
+
+This is a new section in the `build` section that defines the lower bound system
+requirements for a given package.
+
+In this section you can define the "virtual" packages that will be injected in
+the `host` sections of the specs:
+
+```yaml
+build:
+  system_requirements:
+    # this one is used on `osx-*` platforms
+    - osx: 11.0 # (or should we call this / alias it to `macos`?)
+    # the following are used on `linux-*` platforms
+    - glibc: 2.17
+    - linux: 3.10
+    # the following is used on `win-*` platforms
+    - win: 10.1
+```
+
+The system requirements are used to inject virtual packages in the `host`
+section of the recipe. They will also automatically add a dependency in the
+`run` section of the recipe, e.g. `__glibc >=2.17`, or `__osx >=11.0`.