Add optional regex constraints

doc/manual/souce/design/features.md

@@ -0,0 +1,34 @@
+# Features
+
+Decodal keeps the embedded core small by making heavy functionality opt-in.
+
+## Core defaults
+
+`decodal-core` defaults to `std` only.
+
+```toml
+[features]
+default = ["std"]
+std = []
+regex = ["std", "dep:regex"]
+```
+
+Building `decodal-core` with `--no-default-features` keeps the core in `no_std + alloc` mode and avoids optional dependencies.
+
+## Regex
+
+Regex constraints are implemented behind the `regex` feature.
+When the feature is disabled, regex constraints parse and compose, but validating a concrete value against them returns an unsupported feature diagnostic.
+
+```sh
+cargo run -q -p decodal --features regex -- examples/regex/main.dcdl
+```
+
+Regex constraints are accumulated during `&` composition.
+The implementation does not try to prove whether the intersection of two regex constraints is empty.
+Concrete strings must match every regex constraint attached to the abstract value.
+
+## CLI features
+
+`decodal-cli` exposes a matching `regex` feature that enables `decodal-core/regex`.
+The feature is not enabled by default so the default CLI binary remains small.

doc/manual/souce/design/index.md

@@ -16,3 +16,4 @@ bytecode VM や JIT ではなく、AST を demand-driven に評価すること
 4. [Composition and Materialization](./composition-and-materialization.md)
 5. [Diagnostics and Fallback](./diagnostics-and-fallback.md)
 6. [Embedding API](./embedding-api.md)
+7. [Features](./features.md)