diff --git a/src/tokenize.rs b/src/tokenize.rs
new file mode 100644
index 0000000..944d931
--- /dev/null
+++ b/src/tokenize.rs
@@ -0,0 +1,113 @@
+#[derive(Debug)]
+pub enum Token {
+	Negative,
+	StartGroup,
+	EndGroup,
+	Number(String),
+	Operator(String),
+	Word(String),
+}
+
+/// Turn a string into a set of tokens.
+/// Does not check syntax. Fails if `input` contains an invalid character.
+//
+// # Arguments:
+// `input`: A string like `(-3*2.2)/3`
+//
+// # Returns:
+// * `Ok(Vec<token>)` if we were successful.
+// * `Err(())` if we couldn't tokenize this string.
+pub fn tokenize(input: &String) -> Result<Vec<Token>, ()> {
+	let mut v: Vec<Token> = Vec::new();
+	let mut t: Option<Token> = None;
+	for c in input.chars() {
+		match c {
+			// Minus sign can be both a Negative and an Operator.
+			// Needs special treatment.
+			'-' => {
+				if t.is_some() { v.push(t.unwrap()); t = None; }
+				match v.last() {
+					// If previous token was any of the following,
+					// this is the "minus" operator
+					Some(Token::Number(_)) |
+					Some(Token::EndGroup) |
+					Some(Token::Word(_)) => {
+						v.push(Token::Operator(String::from(c)));
+					},
+
+					// Otherwise, this is a negative sign.
+					_ => { t = Some(Token::Negative); }
+				};
+			},
+
+			// Number.
+			// Commas act just like dots.
+			',' | '.' | '0'..='9' => {
+				match &mut t {
+					// If we're already building a number,
+					// append.
+					Some(Token::Number(val)) => {
+						val.push(if c == ',' {'.'} else {c});
+					},
+
+					// If we're not building a number, finalize
+					// previous token and start one.
+					_ => {
+						if t.is_some() { v.push(t.unwrap()); }
+						t = Some(Token::Number(String::from(c)));
+					}
+				};
+			},
+
+			// Word
+			'A'..='Z' |
+			'a'..='z' => {
+				match &mut t {
+					// If we're already building a number,
+					// append.
+					Some(Token::Word(val)) => {
+						val.push(c);
+					},
+
+					// If we're not building a number, finalize
+					// previous token and start one.
+					_ => {
+						if t.is_some() { v.push(t.unwrap()); }
+						t = Some(Token::Word(String::from(c)));
+					}
+				};
+			},
+
+
+			// Operation
+			// Always one character
+			'+' | '*' | '/' | '^' => {
+				// Finalize previous token
+				if t.is_some() { v.push(t.unwrap()); t = None; }
+				v.push(Token::Operator(String::from(c)));
+			}
+			
+			// Groups
+			// Always one character
+			'(' => {
+				if t.is_some() { v.push(t.unwrap()); t = None; }
+				v.push(Token::StartGroup);
+			},
+			')' => {
+				if t.is_some() { v.push(t.unwrap()); t = None; }
+				v.push(Token::EndGroup);
+			},
+
+			// Space. Basic seperator.
+			' ' => {
+				if t.is_some() { v.push(t.unwrap()); t = None; }
+			}
+
+			// Invalid token
+			_ => { return Err(()); }
+		};
+	}
+
+	if t.is_some() { v.push(t.unwrap()); }
+	return Ok(v);
+}
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