# 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.