diff --git a/2015/Sources/AdventOfCode.swift b/2015/Sources/AdventOfCode.swift
index 896eef1..7b20b1d 100644
--- a/2015/Sources/AdventOfCode.swift
+++ b/2015/Sources/AdventOfCode.swift
@@ -20,6 +20,7 @@ let allChallenges: [any AdventDay] = [
   Day16(),
   Day17(),
   Day18(),
+  Day19(),
 ]
 
 @main
diff --git a/2015/Sources/Day19.swift b/2015/Sources/Day19.swift
new file mode 100644
index 0000000..ccada8a
--- /dev/null
+++ b/2015/Sources/Day19.swift
@@ -0,0 +1,33 @@
+import Foundation
+
+struct Day19: AdventDay {
+  let data: String
+
+  private var replacements: [String: [String]] {
+    data.split(separator: /\n\n/)[0]
+      .matches(of: /^(?'source'[A-Za-z]{1}[a-z]{,1}) => (?'replacement'[A-Za-z]+)$/.anchorsMatchLineEndings())
+      .map {
+        (String($0.output.source), String($0.output.replacement))
+      }
+      .reduce(into: [String:[String]]()) { result, mapping in
+        result[mapping.0, default: []].append(mapping.1)
+      }
+  }
+  private var molecole: String {
+    String(data.split(separator: /\n\n/)[1])
+  }
+
+  func part1() -> Int {
+    let all = replacements.flatMap { key, values in
+      values.flatMap { value in
+        molecole.matches(of: try! Regex(key)).map { match in
+          var molecole = molecole
+          molecole.replaceSubrange(match.range, with: value)
+          return molecole
+        }
+      }
+    }.uniqued()
+
+    return Set(all).count
+  }
+}
diff --git a/2015/Tests/Day19Tests.swift b/2015/Tests/Day19Tests.swift
new file mode 100644
index 0000000..fb50c72
--- /dev/null
+++ b/2015/Tests/Day19Tests.swift
@@ -0,0 +1,17 @@
+import Testing
+
+@testable import AoC2015
+
+struct Day19Tests {
+  private let testData = """
+    H => HO
+    H => OH
+    O => HH
+
+    HOH
+  """.replacing(/(  )+/.anchorsMatchLineEndings(), with: "")
+
+  @Test func testPart1() {
+    #expect(Day19(data: testData).part1() == 4)
+  }
+}