add recipe extension cep

cep-recipe-extensions.md

@@ -0,0 +1,54 @@
+# Extensions to the recipe spec
+
+This document describes extensions to the recipe specification as merged in cep-14.
+
+## The `build.post_process` section
+
+We became aware that the `conda_build_config.yaml` 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 `$$`.
+
+Internally, we use `replace_all` from the `regex` crate. This means that the regex is applied to the entire file, not line by line.
+
+### Example
+
+```yaml
+build:
+  post_process:
+    - files:
+        - "*.txt"
+      regex: "foo"
+      replacement: "bar"
+    - files:
+        - "*.cmake"
+      regex: "/sysroot/"
+      replacement: "$${PREFIX}/"
+```
+
+## Globs, positive and negative
+
+Following some community discussion we would like to extend the recipe format in such a way that anywhere where a list of globs is accepted, we alternatively accept a dictionary with `include` and `exclude` keys. The values of these keys are lists of globs that are included or excluded respectively.
+
+For example:
+
+```yaml
+files:
+  include:
+    - "*.txt"
+  exclude:
+    - "foo.txt"
+```
+
+The evaluation would go as follows:
+
+- first record all matches for the `include` globs
+- then remove all matches for the `exclude` globs
+
+In conda-build, this is discussed here: [Link to PR 5216](https://github.com/conda/conda-build/pull/5216)