diff --git a/src/bin/19.rs b/src/bin/19.rs
new file mode 100644
index 000000000..f313a27
--- /dev/null
+++ b/src/bin/19.rs
@@ -0,0 +1,72 @@
+use hashbrown::{HashMap, HashSet};
+use itertools::Itertools;
+use regex::Regex;
+
+pub fn part_one(input: &str) -> Option<u32> {
+    let (replacements, molecule) = input.split_once("\n\n")?;
+    let mappings = replacements
+        .split('\n')
+        .map(|x| x.split_once(" => ").unwrap())
+        .collect_vec();
+
+    let mut molecules = HashSet::new();
+
+    for (a, b) in mappings.iter() {
+        let split = molecule.split(a).collect_vec();
+        for i in 1..split.len() {
+            let x = split[..i].join(a);
+            let y = split[i..].join(a);
+            molecules.insert(format!("{x}{b}{y}"));
+        }
+    }
+
+    Some(molecules.len() as u32)
+}
+
+// https://www.reddit.com/r/adventofcode/comments/3xflz8/comment/cy4etju/
+pub fn part_two(input: &str) -> Option<u32> {
+    let (r, m) = input.split_once("\n\n")?;
+    let mut molecule: String = m.chars().rev().collect();
+
+    let mappings = r
+        .split('\n')
+        .map(|x| x.split_once(" => ").unwrap())
+        .map(|(a, b)| {
+            (
+                b.chars().rev().collect::<String>(),
+                a.chars().rev().collect::<String>(),
+            )
+        })
+        .collect::<HashMap<String, String>>();
+
+    let re = Regex::new(&mappings.iter().map(|(k, _)| k.as_str()).join("|")).unwrap();
+
+    let mut count = 0;
+    while molecule != "e" {
+        let re_range = re.find(&molecule)?.range();
+        let repl = &molecule[re_range];
+        molecule = molecule.replacen(repl, mappings.get(repl)?, 1);
+        count += 1;
+    }
+
+    Some(count)
+}
+fn main() {
+    let input = &aoc::read_file("inputs", 19);
+    aoc::solve!(1, part_one, input);
+    aoc::solve!(2, part_two, input);
+}
+#[cfg(test)]
+mod tests {
+    use super::*;
+    #[test]
+    fn test_part_one() {
+        let input = aoc::read_file("test_inputs", 19);
+        assert_eq!(part_one(&input.trim()), Some(4));
+    }
+    #[test]
+    fn test_part_two() {
+        let input = aoc::read_file("test_inputs", 19);
+        assert_eq!(part_two(&input.trim()), Some(3));
+    }
+}
diff --git a/src/test_inputs/19.txt b/src/test_inputs/19.txt
new file mode 100644
index 000000000..f483a37
--- /dev/null
+++ b/src/test_inputs/19.txt
@@ -0,0 +1,7 @@
+e => H
+e => O
+H => HO
+H => OH
+O => HH
+
+HOH