diff --git a/day4/common.py b/day4/common.py
new file mode 100644
index 0000000..1e6a340
--- /dev/null
+++ b/day4/common.py
@@ -0,0 +1,3 @@
+def parse(filename: str) -> list[str]:
+    with open(filename) as f:
+        return [line.strip() for line in f.readlines()]
diff --git a/day4/part1.py b/day4/part1.py
new file mode 100755
index 0000000..72d199f
--- /dev/null
+++ b/day4/part1.py
@@ -0,0 +1,34 @@
+#!/usr/bin/env python
+from collections.abc import Iterable
+
+import regex as re
+from common import parse
+
+
+def transposed(
+    matrix: list[str],
+) -> Iterable:  # see https://docs.python.org/3/library/itertools.html for the base "transpose" function
+    return map(lambda x: "".join(x), zip(*matrix, strict=True))
+
+
+def solve(input: list[str]) -> int:
+    diagonal = 0
+    horizontal = sum([len(re.findall(r"(XMAS|SAMX)", line, overlapped=True)) for line in input])  # Horizontal
+    vertical = sum([len(re.findall(r"(XMAS|SAMX)", line, overlapped=True)) for line in transposed(input)])  # Vertical
+
+    for y in range(len(input) - 3):
+        for x in range(len(input) - 3):
+            # print(y, x, "\\", "".join([input[y + i][x + i] for i in range(4)]))
+            diagonal += 1 if "".join([input[y + i][x + i] for i in range(4)]) in ["XMAS", "SAMX"] else 0
+            # print(y + 3, x, "/", "".join([input[y + 3 - i][x + i] for i in range(4)]))
+            diagonal += 1 if "".join([input[y + 3 - i][x + i] for i in range(4)]) in ["XMAS", "SAMX"] else 0
+    # print(diagonal, horizontal, vertical)
+    return diagonal + horizontal + vertical
+
+
+def main():
+    print(solve(parse("input")))
+
+
+if __name__ == "__main__":
+    main()
diff --git a/day4/requirements.txt b/day4/requirements.txt
new file mode 100644
index 0000000..4f9256d
--- /dev/null
+++ b/day4/requirements.txt
@@ -0,0 +1 @@
+regex
diff --git a/requirements.txt b/dev_requirements.txt
similarity index 100%
rename from requirements.txt
rename to dev_requirements.txt