Add AnyRegexOutput.as(_:).

Sources/_StringProcessing/RegexDSL/AnyRegexOutput.swift

@@ -56,6 +56,21 @@ extension AnyRegexOutput {
     fatalError("FIXME: Not implemented")
     // self.init(input: match.input, _elements: <elements of output tuple>)
   }
+
+  /// Returns a typed output by converting the underlying value to the specified
+  /// type.
+  /// - Parameter type: The expected output type.
+  /// - Returns: The output, if the underlying value can be converted to the
+  ///   output type, or nil otherwise.
+  public func `as`<Output>(_ type: Output.Type) -> Output? {
+    let elements = _elements.map {
+      StructuredCapture(
+        optionalCount: $0.optionalDepth,
+        storedCapture: .init(range: $0.bounds)
+      ).existentialOutputComponent(from: input[...])
+    }
+    return TypeConstruction.tuple(of: elements) as? Output
+  }
 }
 
 extension AnyRegexOutput {

Tests/RegexTests/RegexDSLTests.swift

@@ -572,6 +572,12 @@ class RegexDSLTests: XCTestCase {
       XCTAssertTrue(output[1].substring == "A6F0")
       XCTAssertTrue(output[2].substring == "A6F1")
       XCTAssertTrue(output[3].substring == "Extend")
+      let typedOutput = try XCTUnwrap(output.as(
+        (Substring, Substring, Substring?, Substring).self))
+      XCTAssertEqual(typedOutput.0, line[...])
+      XCTAssertTrue(typedOutput.1 == "A6F0")
+      XCTAssertTrue(typedOutput.2 == "A6F1")
+      XCTAssertTrue(typedOutput.3 == "Extend")
     }
   }