diff --git a/day-4/go.mod b/day-4/go.mod
new file mode 100644
index 0000000..78166bc
--- /dev/null
+++ b/day-4/go.mod
@@ -0,0 +1,5 @@
+module github.com/onyx-and-iris/aoc2023/day-4
+
+go 1.20
+
+require github.com/go-playground/assert v1.2.1
diff --git a/day-4/go.sum b/day-4/go.sum
new file mode 100644
index 0000000..80dcf13
--- /dev/null
+++ b/day-4/go.sum
@@ -0,0 +1,2 @@
+github.com/go-playground/assert v1.2.1 h1:ad06XqC+TOv0nJWnbULSlh3ehp5uLuQEojZY5Tq8RgI=
+github.com/go-playground/assert v1.2.1/go.mod h1:Lgy+k19nOB/wQG/fVSQ7rra5qYugmytMQqvQ2dgjWn8=
diff --git a/day-4/one.go b/day-4/one.go
new file mode 100644
index 0000000..d2299ec
--- /dev/null
+++ b/day-4/one.go
@@ -0,0 +1,61 @@
+package main
+
+import (
+	"regexp"
+)
+
+type M map[string]interface{}
+
+var allCards []M
+
+var _regex_winning_numbers = regexp.MustCompile(
+	`^Card (?P<card>[\s\d]+):\s` +
+		`(?P<winning>[0-9\s]+)\s` +
+		`\|\s` +
+		`(?P<mynums>[0-9\s]+)$`)
+
+var _regex_nums = regexp.MustCompile("[0-9]+")
+
+// compareLists returns the number of matching elements between winning and mynums
+func compareLists(winningStr string, mynumsStr string) (int, error) {
+	x, err := numStrToArr(winningStr, []int{})
+	if err != nil {
+		return 0, err
+	}
+	winning := x
+	x, err = numStrToArr(mynumsStr, []int{})
+	if err != nil {
+		return 0, err
+	}
+	mynums := x
+
+	n := 0
+	for _, num := range mynums {
+		if contains(winning, num) {
+			n += 1
+		}
+	}
+	return n, nil
+}
+
+// one computes points based on matching numbers
+func one(lines []string) (int, error) {
+	for _, line := range lines {
+		parseAllCards(line)
+	}
+	cards = make([]Card, len(lines))
+
+	sum := 0
+	for x, card := range allCards {
+		m, err := compareLists(card["winning"].(string), card["mynums"].(string))
+		if err != nil {
+			return 0, err
+		}
+		cards[x].matches = m
+		if cards[x].matches > 0 {
+			sum += (pow(2, cards[x].matches-1))
+		}
+	}
+
+	return sum, nil
+}
diff --git a/day-4/one_test.go b/day-4/one_test.go
new file mode 100644
index 0000000..0eab921
--- /dev/null
+++ b/day-4/one_test.go
@@ -0,0 +1,25 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/go-playground/assert/v2"
+)
+
+func TestCompareLists(t *testing.T) {
+	//t.Skip("skipping test")
+	t.Run("Should find 4 matches", func(t *testing.T) {
+		res, _ := compareLists("41 48 83 86 17", "83 86  6 31 17  9 48 53")
+		assert.Equal(t, 4, res)
+	})
+
+	t.Run("Should find 2 matches", func(t *testing.T) {
+		res, _ := compareLists(" 1 21 53 59 44 ", "69 82 63 72 16 21 14  1")
+		assert.Equal(t, 2, res)
+	})
+
+	t.Run("Should find 0 matches", func(t *testing.T) {
+		res, _ := compareLists("87 83 26 28 32", "88 30 70 12 93 22 82 36")
+		assert.Equal(t, 0, res)
+	})
+}
diff --git a/day-4/run.sh b/day-4/run.sh
new file mode 100755
index 0000000..3f6776a
--- /dev/null
+++ b/day-4/run.sh
@@ -0,0 +1,5 @@
+#!/usr/bin/env bash
+
+INPUT="input.txt"
+
+cat $INPUT | go run .
\ No newline at end of file
diff --git a/day-4/solution.go b/day-4/solution.go
new file mode 100644
index 0000000..3ea39e6
--- /dev/null
+++ b/day-4/solution.go
@@ -0,0 +1,22 @@
+package main
+
+import (
+	"fmt"
+	"log"
+)
+
+func main() {
+	lines := readlines()
+
+	ans, err := one(lines)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("solution one: %d\n", ans)
+
+	ans, err = two(lines)
+	if err != nil {
+		log.Fatal(err)
+	}
+	fmt.Printf("solution two: %d\n", ans)
+}
diff --git a/day-4/two.go b/day-4/two.go
new file mode 100644
index 0000000..8732ca5
--- /dev/null
+++ b/day-4/two.go
@@ -0,0 +1,27 @@
+package main
+
+// Cards represents a single card.
+// it tracks its matching numbers and occurrences
+type Card struct {
+	matches     int
+	occurrences int
+}
+
+var cards = []Card{}
+
+// two returns the total number of occurrences for all cards
+func two(lines []string) (int, error) {
+	var sum = 0
+
+	for i := range cards {
+		cards[i].occurrences++
+
+		for j := i + 1; j <= i+cards[i].matches; j++ {
+			cards[j].occurrences += cards[i].occurrences
+		}
+
+		sum += cards[i].occurrences
+	}
+
+	return sum, nil
+}
diff --git a/day-4/util.go b/day-4/util.go
new file mode 100644
index 0000000..29fbc79
--- /dev/null
+++ b/day-4/util.go
@@ -0,0 +1,63 @@
+package main
+
+import (
+	"bufio"
+	"log"
+	"math"
+	"os"
+	"strconv"
+)
+
+// readlines reads lines from stdin.
+// Then it returns them as an array of strings
+func readlines() []string {
+	lines := []string{}
+
+	scanner := bufio.NewScanner(os.Stdin)
+	for scanner.Scan() {
+		lines = append(lines, scanner.Text())
+	}
+
+	if err := scanner.Err(); err != nil {
+		log.Fatal(err)
+	}
+
+	return lines
+}
+
+func parseAllCards(line string) {
+	groups := M{}
+	res := _regex_winning_numbers.FindStringSubmatch(line)
+	names := _regex_winning_numbers.SubexpNames()
+	for i := range res {
+		groups[names[i]] = res[i]
+	}
+	allCards = append(allCards, groups)
+}
+
+// numStrToArr converts a string of numbers or an array of numbers
+func numStrToArr(nums string, arr []int) ([]int, error) {
+	for _, num := range _regex_nums.FindAllString(nums, -1) {
+		n, err := strconv.Atoi(num)
+		if err != nil {
+			return []int{}, err
+		}
+		arr = append(arr, n)
+	}
+	return arr, nil
+}
+
+// contains returns true if an slice of elements contains a given element
+func contains[T comparable](elems []T, v T) bool {
+	for _, s := range elems {
+		if v == s {
+			return true
+		}
+	}
+	return false
+}
+
+// pow returns the value of x to the n
+func pow(x, n int) int {
+	return int(math.Pow(float64(x), float64(n)))
+}