diff --git a/Cargo.lock b/Cargo.lock
index 8769182..288230e 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2,6 +2,15 @@
 # It is not intended for manual editing.
 version = 4
 
+[[package]]
+name = "aho-corasick"
+version = "1.1.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ddd31a130427c27518df266943a5308ed92d4b226cc639f5a8f1002816174301"
+dependencies = [
+ "memchr",
+]
+
 [[package]]
 name = "decodal"
 version = "0.1.0"
@@ -12,3 +21,41 @@ dependencies = [
 [[package]]
 name = "decodal-core"
 version = "0.1.0"
+dependencies = [
+ "regex",
+]
+
+[[package]]
+name = "memchr"
+version = "2.8.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "88904434abc2901f197fe8cc55f0445e7ded921dba5911dad2e2b39b48e663c4"
+
+[[package]]
+name = "regex"
+version = "1.12.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f1292b7759ae1cb9ec195452d1390a074f0cd8541ab7a5a8c31cd6db45d4a6ba"
+dependencies = [
+ "aho-corasick",
+ "memchr",
+ "regex-automata",
+ "regex-syntax",
+]
+
+[[package]]
+name = "regex-automata"
+version = "0.4.14"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6e1dd4122fc1595e8162618945476892eefca7b88c52820e74af6262213cae8f"
+dependencies = [
+ "aho-corasick",
+ "memchr",
+ "regex-syntax",
+]
+
+[[package]]
+name = "regex-syntax"
+version = "0.8.11"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d6f6ff9a378485b298a5286656da665ba74413d36db0979633275d2e708145d4"
diff --git a/crates/decodal-cli/Cargo.toml b/crates/decodal-cli/Cargo.toml
index faee5fb..4fb5122 100644
--- a/crates/decodal-cli/Cargo.toml
+++ b/crates/decodal-cli/Cargo.toml
@@ -3,5 +3,9 @@ name = "decodal"
 version = "0.1.0"
 edition = "2024"
 
+[features]
+default = []
+regex = ["decodal-core/regex"]
+
 [dependencies]
 decodal-core = { path = "../decodal-core" }
diff --git a/crates/decodal-core/Cargo.toml b/crates/decodal-core/Cargo.toml
index da71e4d..436ebe3 100644
--- a/crates/decodal-core/Cargo.toml
+++ b/crates/decodal-core/Cargo.toml
@@ -6,6 +6,7 @@ edition = "2024"
 [features]
 default = ["std"]
 std = []
-regex = []
+regex = ["std", "dep:regex"]
 
 [dependencies]
+regex = { version = "1.10", default-features = false, features = ["std", "unicode-perl"], optional = true }
diff --git a/crates/decodal-core/src/eval.rs b/crates/decodal-core/src/eval.rs
index a3201b5..6f29db2 100644
--- a/crates/decodal-core/src/eval.rs
+++ b/crates/decodal-core/src/eval.rs
@@ -707,11 +707,7 @@ impl<L: SourceLoader> Engine<L> {
                         "value does not satisfy comparison constraint",
                     )
                 }),
-            Constraint::Regex(_) => Err(Diagnostic::new(
-                DiagnosticKind::UnsupportedFeature,
-                span,
-                "regex constraints require a future regex feature",
-            )),
+            Constraint::Regex(pattern) => satisfies_regex(value, pattern, span),
             Constraint::BuiltinPredicate(name) => Err(Diagnostic::new(
                 DiagnosticKind::UnsupportedFeature,
                 span,
@@ -967,6 +963,42 @@ fn value_matches_primitive(value: &RuntimeValue, primitive: PrimitiveType) -> bo
     )
 }
 
+#[cfg(feature = "regex")]
+fn satisfies_regex(value: &RuntimeValue, pattern: &str, span: Span) -> Result<()> {
+    let RuntimeValue::Concrete(ConcreteValue::String(value)) = value else {
+        return Err(Diagnostic::new(
+            DiagnosticKind::ConstraintViolation,
+            span,
+            "regex constraints require a string value",
+        ));
+    };
+    let regex = regex::Regex::new(pattern).map_err(|error| {
+        Diagnostic::new(
+            DiagnosticKind::Syntax,
+            span,
+            format!("invalid regex constraint `{pattern}`: {error}"),
+        )
+    })?;
+    if regex.is_match(value) {
+        Ok(())
+    } else {
+        Err(Diagnostic::new(
+            DiagnosticKind::ConstraintViolation,
+            span,
+            "value does not satisfy regex constraint",
+        ))
+    }
+}
+
+#[cfg(not(feature = "regex"))]
+fn satisfies_regex(_value: &RuntimeValue, _pattern: &str, span: Span) -> Result<()> {
+    Err(Diagnostic::new(
+        DiagnosticKind::UnsupportedFeature,
+        span,
+        "regex constraints require the regex feature",
+    ))
+}
+
 fn compare_value(value: &RuntimeValue, op: CompareOp, expected: &LiteralValue) -> bool {
     match (value, expected) {
         (RuntimeValue::Concrete(ConcreteValue::Int(actual)), LiteralValue::Int(expected)) => {
@@ -1169,4 +1201,22 @@ mod tests {
         let Data::Object(fields) = data else { panic!() };
         assert_eq!(fields[1].value, Data::String(String::from("local")));
     }
+
+    #[cfg(feature = "regex")]
+    #[test]
+    fn regex_constraint_matches_concrete_string() {
+        let data = eval_data(r#"value = String & /^api-[0-9]+$/ default "api-123";"#);
+        let Data::Object(fields) = data else { panic!() };
+        assert_eq!(fields[0].value, Data::String(String::from("api-123")));
+    }
+
+    #[cfg(feature = "regex")]
+    #[test]
+    fn regex_constraint_rejects_non_matching_string() {
+        let parsed =
+            crate::parse_source(r#"value = String & /^api-[0-9]+$/ default "web-123";"#).unwrap();
+        let mut engine = Engine::from_parse(parsed.ast, parsed.root);
+        let value = engine.eval_root().unwrap();
+        assert!(engine.materialize(&value).is_err());
+    }
 }
diff --git a/doc/manual/souce/design/features.md b/doc/manual/souce/design/features.md
new file mode 100644
index 0000000..014a91e
--- /dev/null
+++ b/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.
diff --git a/doc/manual/souce/design/index.md b/doc/manual/souce/design/index.md
index 31ae7c6..f6da3d6 100644
--- a/doc/manual/souce/design/index.md
+++ b/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)
diff --git a/doc/manual/souce/index.md b/doc/manual/souce/index.md
index 385281a..a67d239 100644
--- a/doc/manual/souce/index.md
+++ b/doc/manual/souce/index.md
@@ -42,4 +42,5 @@ Decodal は Deferred Constraint Data Language、略称 DCDL のプロジェク
    4. [Composition and Materialization](./design/composition-and-materialization.md)
    5. [Diagnostics and Fallback](./design/diagnostics-and-fallback.md)
    6. [Embedding API](./design/embedding-api.md)
+   7. [Features](./design/features.md)
 4. [Open Issues](./open-issues.md)
diff --git a/examples/regex/README.md b/examples/regex/README.md
new file mode 100644
index 0000000..933b4f5
--- /dev/null
+++ b/examples/regex/README.md
@@ -0,0 +1,9 @@
+# Regex example
+
+Regex constraints are optional. Run this example with the `regex` feature enabled:
+
+```sh
+cargo run -q -p decodal --features regex -- examples/regex/main.dcdl
+```
+
+Without the feature, regex validation returns an unsupported feature diagnostic.
diff --git a/examples/regex/main.dcdl b/examples/regex/main.dcdl
new file mode 100644
index 0000000..61638d8
--- /dev/null
+++ b/examples/regex/main.dcdl
@@ -0,0 +1,7 @@
+let
+  Identifier = String & /^[A-Za-z_][A-Za-z0-9_]*$/;
+  ApiName = Identifier & /^api_/;
+in
+{
+  service = ApiName default "api_gateway";
+}