diff --git a/Cargo.lock b/Cargo.lock
index 6f1a9af..5615831 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -11,16 +11,17 @@ dependencies = [
 
 [[package]]
 name = "anyhow"
-version = "1.0.34"
+version = "1.0.36"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "bf8dcb5b4bbaa28653b647d8c77bd4ed40183b48882e130c1f1ffb73de069fd7"
+checksum = "68803225a7b13e47191bab76f2687382b60d259e8cf37f6e1893658b84bb9479"
 
 [[package]]
 name = "aoc2020"
 version = "0.1.0"
 dependencies = [
  "anyhow",
- "lazy_static",
+ "once_cell",
+ "peg",
  "regex",
 ]
 
@@ -36,6 +37,57 @@ version = "2.3.4"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0ee1c47aaa256ecabcaea351eae4a9b01ef39ed810004e298d2511ed284b1525"
 
+[[package]]
+name = "once_cell"
+version = "1.5.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "13bd41f508810a131401606d54ac32a467c97172d74ba7662562ebba5ad07fa0"
+
+[[package]]
+name = "peg"
+version = "0.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9f76678828272f177ac33b7e2ac2e3e73cc6c1cd1e3e387928aa69562fa51367"
+dependencies = [
+ "peg-macros",
+ "peg-runtime",
+]
+
+[[package]]
+name = "peg-macros"
+version = "0.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "636d60acf97633e48d266d7415a9355d4389cea327a193f87df395d88cd2b14d"
+dependencies = [
+ "peg-runtime",
+ "proc-macro2",
+ "quote",
+]
+
+[[package]]
+name = "peg-runtime"
+version = "0.6.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9555b1514d2d99d78150d3c799d4c357a3e2c2a8062cd108e93a06d9057629c5"
+
+[[package]]
+name = "proc-macro2"
+version = "1.0.24"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1e0704ee1a7e00d7bb417d0770ea303c1bccbabf0ef1667dae92b5967f5f8a71"
+dependencies = [
+ "unicode-xid",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "991431c3519a3f36861882da93630ce66b52918dcf1b8e2fd66b397fc96f28df"
+dependencies = [
+ "proc-macro2",
+]
+
 [[package]]
 name = "regex"
 version = "1.4.2"
@@ -62,3 +114,9 @@ checksum = "d40c6d1b69745a6ec6fb1ca717914848da4b44ae29d9b3080cbee91d72a69b14"
 dependencies = [
  "lazy_static",
 ]
+
+[[package]]
+name = "unicode-xid"
+version = "0.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f7fe0bb3479651439c9112f72b6c505038574c9fbb575ed1bf3b797fa39dd564"
diff --git a/Cargo.toml b/Cargo.toml
index 67b3e35..f019297 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -9,4 +9,5 @@ edition = "2018"
 [dependencies]
 anyhow = "1.0"
 once_cell = "1.5.2"
+peg = "0.6.3"
 regex = "1"
\ No newline at end of file
diff --git a/data/input-18.txt b/data/input-18.txt
new file mode 100644
index 0000000..11b56b1
--- /dev/null
+++ b/data/input-18.txt
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diff --git a/data/output-18.txt b/data/output-18.txt
new file mode 100644
index 0000000..7bdb331
--- /dev/null
+++ b/data/output-18.txt
@@ -0,0 +1,2 @@
+31142189909908
+323912478287549
diff --git a/src/day18.rs b/src/day18.rs
new file mode 100644
index 0000000..3ff55fa
--- /dev/null
+++ b/src/day18.rs
@@ -0,0 +1,47 @@
+// ==== day 18
+
+use crate::utils;
+use anyhow::{anyhow, Result};
+
+peg::parser! {
+    grammar arithmetic_parser() for str {
+        rule ws() = quiet!{[' ' | '\n' | '\t']+}
+
+        rule number() -> i64
+            = n:$(['0'..='9']+) { n.parse().unwrap() }
+
+        pub rule arith_equal_precedence() -> i64 = precedence!{
+            x:(@) "+" y:@ { x + y }
+            x:(@) "*" y:@ { x * y }
+            --
+            ws()* n:number() ws()* { n }
+            ws()* "(" ws()* e:arith_equal_precedence() ws()* ")" ws()* { e }
+        }
+
+        pub rule arith_addition_precedence() -> i64 = precedence!{
+            x:(@) "*" y:@ { x * y }
+            --
+            x:(@) "+" y:@ { x + y }
+            --
+            ws()* n:number() ws()* { n }
+            ws()* "(" ws()* e:arith_addition_precedence() ws()* ")" ws()* { e }
+        }
+    }
+}
+
+// main
+pub fn run() -> Result<(i64, i64)> {
+    let data = utils::read_file("data/input-18.txt")?;
+    let answers = utils::read_i64s("data/output-18.txt")?;
+
+    let p1 = data.lines().fold(0, |sum, line| {
+        sum + arithmetic_parser::arith_equal_precedence(line).unwrap()
+    });
+    let p2 = data.lines().fold(0, |sum, line| {
+        sum + arithmetic_parser::arith_addition_precedence(line).unwrap()
+    });
+
+    assert_eq!(p1, answers[0]);
+    assert_eq!(p2, answers[1]);
+    Ok((p1, p2))
+}
diff --git a/src/main.rs b/src/main.rs
index 1c1dcb8..dad0b4d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -16,16 +16,9 @@ mod day13;
 mod day14;
 mod day15;
 mod day16;
+mod day18;
 mod utils;
 
-// TODO
-// ====
-// * clean up error handling (move to `?` inside closures)
-// * templatize read_i64s()
-// * build out a bit of a harness for the runner
-// * add criterion benchmarking
-// * figure out nicer way to organize into modules
-
 // ==== main()
 
 fn main() -> Result<()> {