diff --git a/src/Day01.kt b/src/Day01.kt
index 644d3fd..1edadd1 100644
--- a/src/Day01.kt
+++ b/src/Day01.kt
@@ -33,11 +33,9 @@ fun main() {
         }
     }
 
-    val testInput1 = readInput("Day01_test_part1")
-    check(part1(testInput1) == 11L)
-
-    val testInput2 = readInput("Day01_test_part2")
-    check(part2(testInput2) == 31L)
+    val testInput = readInput("Day01_test")
+    check(part1(testInput) == 11L)
+    check(part2(testInput) == 31L)
 
     val input = readInput("Day01")
     part1(input).println()
diff --git a/src/Day02.kt b/src/Day02.kt
index eeb3dcd..07c057d 100644
--- a/src/Day02.kt
+++ b/src/Day02.kt
@@ -58,11 +58,9 @@ fun main() {
         }
     }
 
-    val testInput1 = readInput("Day02_test_part1")
-    check(part1(testInput1) == 2)
-
-    val testInput2 = readInput("Day02_test_part2")
-    check(part2(testInput2) == 4)
+    val testInput = readInput("Day02_test")
+    check(part1(testInput) == 2)
+    check(part2(testInput) == 4)
 
     val input = readInput("Day02")
     part1(input).println()
diff --git a/src/Day03.kt b/src/Day03.kt
new file mode 100644
index 0000000..dadecd7
--- /dev/null
+++ b/src/Day03.kt
@@ -0,0 +1,58 @@
+fun main() {
+
+    data class Multiplication(
+        val a: Int,
+        val b: Int
+    ) {
+        constructor(a: String, b: String) : this(a.toInt(), b.toInt())
+
+        fun mul() = a * b
+    }
+
+    fun String.extractEnabledMuls(): List<Multiplication> {
+        val regex = Regex("mul\\((\\d{1,3}),(\\d{1,3})\\)|do\\(\\)|don't\\(\\)")
+        var isEnabled = true
+        return buildList {
+            regex.findAll(this@extractEnabledMuls).forEach {
+                val isDo = it.value == "do()"
+                val isDonot = it.value == "don't()"
+                when {
+                    isDo -> isEnabled = true
+                    isDonot -> isEnabled = false
+                    else -> {
+                        if (isEnabled) {
+                            add(Multiplication(it.groups[1]!!.value, it.groups[2]!!.value))
+                        }
+                    }
+                }
+            }
+        }
+    }
+
+    fun String.extractMuls(): List<Multiplication> {
+        val regex = Regex("mul\\((\\d{1,3}),(\\d{1,3})\\)")
+        return buildList {
+            regex.findAll(this@extractMuls).forEach {
+                add(Multiplication(it.groups[1]!!.value, it.groups[2]!!.value))
+            }
+        }
+    }
+
+    fun part1(input: List<String>): Int {
+        val oneBigLine = input.joinToString(separator = "")
+        return oneBigLine.extractMuls().sumOf { it.mul() }
+    }
+
+    fun part2(input: List<String>): Int {
+        val oneBigLine = input.joinToString(separator = "")
+        return oneBigLine.extractEnabledMuls().sumOf { it.mul() }
+    }
+
+    val testInput1 = readInput("Day03_test")
+    check(part1(testInput1) == 161)
+    check(part2(testInput1) == 48)
+
+    val input = readInput("Day03")
+    part1(input).println()
+    part2(input).println()
+}