diff --git a/src/lib.rs b/src/lib.rs
index d7a323a..4a92cb3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,32 +1,9 @@
 use std::error::Error;
+use crate::parser::tokenizer::tokenize;
+use crate::parser::sanitizer::sanitize_tokens;
+use crate::parser::ast_builder::build_ast;
 
-#[derive(Debug, PartialEq, Clone)]
-enum Token {
-    Number(String),
-    Variable(String),
-    ImaginaryUnit(),
-    Addition(),
-    Multiplication(),
-    Substraction(),
-    Division(),
-    Modulo(),
-    Exponentiation(),
-    Equal(),
-    OpenParenthesis(),
-    CloseParenthesis(),
-}
-
-#[derive(Debug, PartialEq)]
-enum TokenType {
-    Number(),
-    Variable(),
-    ImaginaryUnit(),
-    Operator(),
-    Substraction(),
-    OpenParenthesis(),
-    CloseParenthesis(),
-    Equal(),
-}
+pub mod parser;
 
 struct _Rational {
     numerator: i128,
@@ -38,232 +15,13 @@ struct _GaussianRational {
     imaginary: _Rational,
 }
 
-fn check_number(my_string: String, i: usize) -> Result<Token, String> {
-    println!("Checking number at index {i}");
-    if my_string.as_str() == "." {
-        return Err(format!("unexpected token: `.` at position {i}"));
-    }
-    let find = my_string.find(".");
-    if let Some(j) = find {
-        let find = my_string[j+1..].find(".");
-        if let Some(_) = find {
-            return Err(format!("unexpected token: `{my_string}` at position {i}"))
-        }
-    }
-    Ok(Token::Number(my_string))
-}
-
-fn tokenize(query: &str) -> Result<Vec<Token>, String> {
-    let mut tokens: Vec<Token> = vec![];
-    let mut my_string = String::new();
-    let mut is_last_number = false;
-    let mut is_last_variable = false;
-
-    for (i, token) in query.chars().enumerate() {
-        let mut is_still_number = false;
-        let mut is_still_variable = false;
-        let mut is_pop_needed = true;
-        match token {
-            '+' => tokens.push(Token::Addition()),
-            '-' => tokens.push(Token::Substraction()),
-            '*' => tokens.push(Token::Multiplication()),
-            '/' => tokens.push(Token::Division()),
-            '%' => tokens.push(Token::Modulo()),
-            '^' => tokens.push(Token::Exponentiation()),
-            '=' => tokens.push(Token::Equal()),
-            '(' => tokens.push(Token::OpenParenthesis()),
-            ')' => tokens.push(Token::CloseParenthesis()),
-            'a'..='z' | 'A'..='Z' => {
-                if is_last_number == true {
-                    let mut tmp_token = None;
-                    if is_pop_needed {
-                        tmp_token = tokens.pop();
-                    }
-                    tokens.push(check_number(my_string, i)?);
-                    if let Some(tok) = tmp_token {
-                        tokens.push(tok);
-                    }
-                    is_last_number = false;
-                    my_string = String::new();
-                }
-                if is_last_variable == false {
-                    is_last_variable = true;
-                }
-                is_still_variable = true;
-                my_string += &token.to_string();
-            },
-            '0'..='9' | '.' => {
-                if is_last_variable == true {
-                    let mut tmp_token = None;
-                    if is_pop_needed {
-                        tmp_token = tokens.pop();
-                    }
-                    match my_string.as_str() {
-                        "i" => tokens.push(Token::ImaginaryUnit()),
-                        _ => tokens.push(Token::Variable(my_string)),
-                    }
-                    if let Some(tok) = tmp_token {
-                        tokens.push(tok);
-                    }
-                    is_last_variable = false;
-                    my_string = String::new();
-                }
-                if is_last_number == false {
-                    is_last_number = true;
-                }
-                is_still_number = true;
-                my_string += &token.to_string();
-            },
-            ' ' => is_pop_needed = false,
-            _ => return Err(format!("unexpected token: `{token}` at position {i}")),
-        };
-        if is_last_variable && !is_still_variable {
-            let mut tmp_token = None;
-            if is_pop_needed {
-                tmp_token = tokens.pop();
-            }
-            match my_string.as_str() {
-                "i" => tokens.push(Token::ImaginaryUnit()),
-                _ => tokens.push(Token::Variable(my_string)),
-            }
-            if let Some(tok) = tmp_token {
-                tokens.push(tok);
-            }
-            is_last_variable = false;
-            my_string = String::new();
-        }
-        else if is_last_number && !is_still_number {
-            let mut tmp_token = None;
-            if is_pop_needed {
-                tmp_token = tokens.pop();
-            }
-            tokens.push(check_number(my_string, i)?);
-            if let Some(tok) = tmp_token {
-                tokens.push(tok);
-            }
-            is_last_number = false;
-            my_string = String::new();
-        }
-    }
-    if is_last_variable {
-        match my_string.as_str() {
-            "i" => tokens.push(Token::ImaginaryUnit()),
-            _ => tokens.push(Token::Variable(my_string)),
-        }
-    }
-    else if is_last_number {
-        tokens.push(check_number(my_string, query.len())?);
-    }
-
-    Ok(tokens)
-}
-
-fn get_token_type(token: &Token) -> TokenType {
-    match token {
-        Token::Number(_) => TokenType::Number(),
-        Token::Variable(_) => TokenType::Variable(),
-        Token::ImaginaryUnit() => TokenType::ImaginaryUnit(),
-        Token::OpenParenthesis() => TokenType::OpenParenthesis(),
-        Token::CloseParenthesis() => TokenType::CloseParenthesis(),
-        Token::Substraction() => TokenType::Substraction(),
-        Token::Equal() => TokenType::Equal(),
-        _ => TokenType::Operator(),
-    }
-}
-
-fn sanitize_tokens(tokens: Vec<Token>) -> Result<Vec<Token>, String> {
-    let mut sanitized_tokens: Vec<Token> = vec![];
-    let mut last_token_type = None;
-    let mut open_close_equal_count = (0, 0, 0);
-
-    for token in tokens {
-        let token_type = get_token_type(&token);
-        match token_type {
-            TokenType::OpenParenthesis() => open_close_equal_count.0 += 1,
-            TokenType::CloseParenthesis() => {
-                open_close_equal_count.1 += 1;
-                if open_close_equal_count.1 > open_close_equal_count.0 {
-                    return Err(format!("Error at token {:?}, closing parenthesis that was never opened", token));
-                }
-            },
-            TokenType::Equal() => {
-                open_close_equal_count.2 += 1;
-                if open_close_equal_count.0 != open_close_equal_count.1 {
-                    return Err(format!("Error, query has a different number of opening and closing parentheses"));
-                }
-                open_close_equal_count.0 = 0;
-                open_close_equal_count.1 = 0;
-            },
-            _ => (),
-        }
-        if let Some(last_token_type) = last_token_type {
-            match (&last_token_type, &token_type) {
-                (TokenType::Number(), TokenType::Variable()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::Number(), TokenType::ImaginaryUnit()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::Number(), TokenType::OpenParenthesis()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::Variable(), TokenType::Number()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::Variable(), TokenType::ImaginaryUnit()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::Variable(), TokenType::OpenParenthesis()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::ImaginaryUnit(), TokenType::Number()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::ImaginaryUnit(), TokenType::Variable()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::ImaginaryUnit(), TokenType::OpenParenthesis()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::CloseParenthesis(), TokenType::OpenParenthesis()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::CloseParenthesis(), TokenType::Number()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::CloseParenthesis(), TokenType::Variable()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::CloseParenthesis(), TokenType::ImaginaryUnit()) => sanitized_tokens.push(Token::Multiplication()),
-                (TokenType::OpenParenthesis(), TokenType::Operator()) => return Err(format!("Error at token {:?}, operator forbidden directly after opening parenthesis", token)),
-                (TokenType::Operator(), TokenType::CloseParenthesis()) => return Err(format!("Error at token {:?}, all operators are forbidden directly before closing parenthesis", token)),
-                (TokenType::Substraction(), TokenType::CloseParenthesis()) => return Err(format!("Error at token {:?}, all operators are forbidden directly before closing parenthesis", token)),
-                (TokenType::OpenParenthesis(), TokenType::CloseParenthesis()) => return Err(format!("Error at token {:?}, empty parentheses", token)),
-                (TokenType::Substraction(), TokenType::Equal()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
-                (TokenType::Substraction(), TokenType::Operator()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
-                (TokenType::Operator(), TokenType::Equal()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
-                (TokenType::Operator(), TokenType::Substraction()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
-                (TokenType::Equal(), TokenType::Operator()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
-                (TokenType::OpenParenthesis(), TokenType::OpenParenthesis()) => (),
-                (TokenType::CloseParenthesis(), TokenType::CloseParenthesis()) => (),
-                _ => {
-                    if token_type == last_token_type {
-                        return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token));
-                    }
-                },
-            }
-        } else if token_type == TokenType::Operator() || token_type == TokenType::Equal() {
-            return Err(format!("Error at token {:?}, query can't start with this token", token));
-        }
-        sanitized_tokens.push(token);
-
-        last_token_type = Some(token_type);
-    }
-    if open_close_equal_count.0 != open_close_equal_count.1 {
-        return Err(format!("Error, query has a different number of opening and closing parentheses"));
-    }
-    if open_close_equal_count.2 > 1 {
-        return Err(format!("Error, query shouldn't have more than 1 equals sign"));
-    }
-    let last_token = sanitized_tokens.last().unwrap();
-    match get_token_type(&last_token) {
-        TokenType::Equal() | TokenType::Operator() | TokenType::Substraction() => return Err(format!("Error at token {:?}, query can't end with this token", last_token)),
-        _ => (),
-    }
-    Ok(sanitized_tokens)
-}
-
-struct Node {
-    _lhs: Option<Box<Node>>,
-    _rhs: Option<Box<Node>>,
-}
-
-fn build_ast(_tokens: Vec<Token>) -> Node {
-    Node { _lhs: None, _rhs: None }
-}
-
 pub fn parse(query: &str) -> Result<Vec<f64>, Box<dyn Error>> {
     let tokens = tokenize(query)?;
+    println!("{:?}", tokens);
     let sanitized_tokens = sanitize_tokens(tokens)?;
+    println!("{:?}", sanitized_tokens);
     let _ast = build_ast(sanitized_tokens);
 
-    println!("{:?}", tokenize(query));
 
     Ok(vec![])
 }
@@ -356,539 +114,7 @@ pub fn solve(equation: Vec<f64>) {
 mod tests {
     use super::*;
 
-    mod tokenize {
-        use super::*;
-
-        #[test]
-        fn addition() {
-            let query = "+";
-            let result: Vec<Token> = vec![Token::Addition()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn substraction() {
-            let query = "-";
-            let result: Vec<Token> = vec![Token::Substraction()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn multiplication() {
-            let query = "*";
-            let result: Vec<Token> = vec![Token::Multiplication()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn division() {
-            let query = "/";
-            let result: Vec<Token> = vec![Token::Division()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn modulo() {
-            let query = "%";
-            let result: Vec<Token> = vec![Token::Modulo()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn exponentiation() {
-            let query = "^";
-            let result: Vec<Token> = vec![Token::Exponentiation()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn equal() {
-            let query = "=";
-            let result: Vec<Token> = vec![Token::Equal()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn open_parenthesis() {
-            let query = "(";
-            let result: Vec<Token> = vec![Token::OpenParenthesis()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn close_parenthesis() {
-            let query = ")";
-            let result: Vec<Token> = vec![Token::CloseParenthesis()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn imaginary_unit() {
-            let query = "i";
-            let result: Vec<Token> = vec![Token::ImaginaryUnit()];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn variable() {
-            let query = "variable";
-            let result: Vec<Token> = vec![Token::Variable(String::from("variable"))];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn variable_double_i() {
-            let query = "ii";
-            let result: Vec<Token> = vec![Token::Variable(String::from("ii"))];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn number_natural() {
-            let query = "123456";
-            let result: Vec<Token> = vec![Token::Number(String::from("123456"))];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn number_rational() {
-            let query = "123.456";
-            let result: Vec<Token> = vec![Token::Number(String::from("123.456"))];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn number_point_something() {
-            let query = ".123456";
-            let result: Vec<Token> = vec![Token::Number(String::from(".123456"))];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn number_trailing_point() {
-            let query = "123456.";
-            let result: Vec<Token> = vec![Token::Number(String::from("123456."))];
-
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        fn number_variable() {
-            let query = "23x";
-            let result: Vec<Token> = vec![Token::Number(String::from("23")), Token::Variable(String::from("x"))];
-            assert_eq!(tokenize(query).unwrap(), result);
-
-            let query = "23i";
-            let result: Vec<Token> = vec![Token::Number(String::from("23")), Token::ImaginaryUnit()];
-            assert_eq!(tokenize(query).unwrap(), result);
-
-            let query = "x23";
-            let result: Vec<Token> = vec![Token::Variable(String::from("x")), Token::Number(String::from("23"))];
-            assert_eq!(tokenize(query).unwrap(), result);
-
-            let query = "i23";
-            let result: Vec<Token> = vec![Token::ImaginaryUnit(), Token::Number(String::from("23"))];
-            assert_eq!(tokenize(query).unwrap(), result);
-        }
-
-        #[test]
-        #[should_panic]
-        fn number_double_point() {
-            let query = "12.34.56";
-            tokenize(query).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn number_double_point_in_a_row() {
-            let query = "123..456";
-            tokenize(query).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn number_only_point() {
-            let query = ".";
-            tokenize(query).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn invalid_token() {
-            let query = "324*43224+243_+234=234";
-            tokenize(query).unwrap();
-        }
-    }
-
-    mod sanitize_tokens {
-        use super::*;
-
-        fn plus() -> Token {
-            Token::Addition()
-        }
-
-        fn minus() -> Token {
-            Token::Substraction()
-        }
-
-        fn times() -> Token {
-            Token::Multiplication()
-        }
-
-        fn divided_by() -> Token {
-            Token::Division()
-        }
-
-        fn modulo() -> Token {
-            Token::Modulo()
-        }
-
-        fn equals() -> Token {
-            Token::Equal()
-        }
-
-        fn x() -> Token {
-            Token::Variable(String::from("x"))
-        }
-
-        fn i() -> Token {
-            Token::ImaginaryUnit()
-        }
-
-        fn number() -> Token {
-            Token::Number(String::from("123"))
-        }
-
-        fn open() -> Token {
-            Token::OpenParenthesis()
-        }
-
-        fn close() -> Token {
-            Token::CloseParenthesis()
-        }
-
-        #[test]
-        #[should_panic]
-        fn double_number() {
-            let tokens = vec![number(), number()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn double_operator() {
-            let tokens = vec![number(), plus(), divided_by()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn double_variable() {
-            let tokens = vec![x(), x()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn double_imaginary_unit() {
-            let tokens = vec![i(), i()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn sanitize_tokens_starts_with_operator() {
-            let tokens = vec![plus(), number(), divided_by(), x()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        fn starts_with_negative_number() {
-            let tokens = vec![minus(), number(), divided_by(), x()];
-            let results = tokens.clone();
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-        }
-
-        #[test]
-        fn number_operator_variable() {
-            let tokens = vec![number(), divided_by(), x()];
-            let results = tokens.clone();
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-        }
-
-        #[test]
-        fn insert_multiplication() {
-            let tokens = vec![number(), x()];
-            let results = vec![number(), times(), x()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![x(), number()];
-            let results = vec![x(), times(), number()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![number(), i()];
-            let results = vec![number(), times(), i()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![i(), number()];
-            let results = vec![i(), times(), number()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![i(), x()];
-            let results = vec![i(), times(), x()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![x(), i()];
-            let results = vec![x(), times(), i()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-        }
-
-        #[test]
-        fn insert_multiplication_parenthesis() {
-            let tokens = vec![open(), number(), modulo(), number(), close(), open(), i(), times(), x(), close()];
-            let results = vec![open(), number(), modulo(), number(), close(), times(), open(), i(), times(), x(), close()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![number(), open(), number(), modulo(), number(), close()];
-            let results= vec![number(), times(), open(), number(), modulo(), number(), close()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![i(), open(), number(), modulo(), number(), close()];
-            let results= vec![i(), times(), open(), number(), modulo(), number(), close()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![x(), open(), number(), modulo(), number(), close()];
-            let results= vec![x(), times(), open(), number(), modulo(), number(), close()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![open(), number(), modulo(), number(), close(), number()];
-            let results= vec![open(), number(), modulo(), number(), close(), times(), number()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![open(), number(), modulo(), number(), close(), i()];
-            let results= vec![open(), number(), modulo(), number(), close(), times(), i()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![open(), number(), modulo(), number(), close(), x()];
-            let results= vec![open(), number(), modulo(), number(), close(), times(), x()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec![number(), open(), number(), modulo(), number(), close(), x()];
-            let results= vec![number(), times(), open(), number(), modulo(), number(), close(), times(), x()];
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-        }
-
-        #[test]
-        #[should_panic]
-        fn parenthesis_operator() {
-            let tokens = vec!(open(), plus(), number(), close());
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn operator_parenthesis() {
-            let tokens = vec!(open(), number(), modulo(), close());
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn minus_parenthesis() {
-            let tokens = vec!(open(), number(), minus(), close());
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn empty_parentheses() {
-            let tokens = vec!(open(), close());
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        fn parenthesis_minus() {
-            let tokens = vec!(open(), minus(), number(), close());
-            let results = tokens.clone();
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-        }
-
-        #[test]
-        fn double_parentheses() {
-            let tokens = vec!(open(), open(), number(), plus(), number(), close(), plus(), number(), close());
-            let results = tokens.clone();
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec!(open(), number(), plus(), open(), number(), plus(), number(), close(), close());
-            let results = tokens.clone();
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec!(open(), open(), number(), plus(), number(), close(), number(), close());
-            let results = vec!(open(), open(), number(), plus(), number(), close(), times(), number(), close());
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-
-            let tokens = vec!(open(), number(), open(), number(), plus(), number(), close(), close());
-            let results= vec!(open(), number(), times(), open(), number(), plus(), number(), close(), close());
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-        }
-
-        #[test]
-        #[should_panic]
-        fn wrong_parentheses_count() {
-            let tokens = vec![open(), number(), close(), close()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn wrong_parentheses_count_2() {
-            let tokens = vec![open(), open(), number(), close()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn wrong_parentheses_count_hard() {
-            let tokens = vec![open(), open(), number(), close(), equals(), x(), close()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn wrong_equal_count() {
-            let tokens = vec![number(), equals(), number(), equals(), number()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn starts_with_equal() {
-            let tokens = vec![equals(), number(), plus(), number()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn ends_with_equal() {
-            let tokens = vec![number(), plus(), number(), equals()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn ends_with_minus() {
-            let tokens = vec![number(), plus(), number(), minus()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn ends_with_operator() {
-            let tokens = vec![number(), plus(), number(), divided_by()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn ends_with_open_brackets() {
-            let tokens = vec![number(), plus(), number(), open(), number(), close(), open()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn starts_with_close_brackets() {
-            let tokens = vec![close(), number(), plus(), number(), open(), number()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn wrong_brackets_order() {
-            let tokens = vec![number(), close(), number(), plus(), number(), open(), number()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn minus_operator() {
-            let tokens = vec![number(), minus(), plus(), i()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn operator_minus() {
-            let tokens = vec![number(), plus(), minus(), i()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn minus_equals() {
-            let tokens = vec![number(), minus(), equals(), i()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn operator_equals() {
-            let tokens = vec![number(), modulo(), equals(), i()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        #[should_panic]
-        fn equals_operator() {
-            let tokens = vec![number(), equals(), times(), i()];
-            sanitize_tokens(tokens).unwrap();
-        }
-
-        #[test]
-        fn equals_minus() {
-            let tokens = vec![number(), equals(), minus(), x()];
-            let results = tokens.clone();
-            assert_eq!(sanitize_tokens(tokens).unwrap(), results);
-        }
-
-        mod get_token_type {
-            use super::*;
-
-            #[test]
-            fn exhaustive() {
-                assert_eq!(get_token_type(&Token::Number(String::from("123"))), TokenType::Number());
-                assert_eq!(get_token_type(&Token::Variable(String::from("var"))), TokenType::Variable());
-                assert_eq!(get_token_type(&Token::ImaginaryUnit()), TokenType::ImaginaryUnit());
-                assert_eq!(get_token_type(&Token::OpenParenthesis()), TokenType::OpenParenthesis());
-                assert_eq!(get_token_type(&Token::CloseParenthesis()), TokenType::CloseParenthesis());
-                assert_eq!(get_token_type(&Token::Substraction()), TokenType::Substraction());
-                assert_eq!(get_token_type(&Token::Equal()), TokenType::Equal());
-                assert_eq!(get_token_type(&Token::Addition()), TokenType::Operator());
-                assert_eq!(get_token_type(&Token::Multiplication()), TokenType::Operator());
-                assert_eq!(get_token_type(&Token::Division()), TokenType::Operator());
-                assert_eq!(get_token_type(&Token::Modulo()), TokenType::Operator());
-                assert_eq!(get_token_type(&Token::Exponentiation()), TokenType::Operator());
-            }
-        }
-
-    }
-
-#[test]
+    #[test]
     fn parse_degree_0() {
         let query = "5 * X^0 = 3 * X^0";
         let result: Vec<f64> = vec![2.];
@@ -896,7 +122,7 @@ mod tests {
         assert_eq!(parse(query).unwrap(), result);
     }
 
-#[test]
+    #[test]
     fn parse_degree_1() {
         let query = "5 * X^0 + 3 * X^1 = 3 * X^0";
         let result: Vec<f64> = vec![2., 3.];
@@ -904,7 +130,7 @@ mod tests {
         assert_eq!(parse(query).unwrap(), result);
     }
 
-#[test]
+    #[test]
     fn parse_degree_2() {
         let query = "5 * X^0 + 6 * X^1 + 8 * X^2 = 3 * X^0 - 2 * X^2";
         let result = vec![2., 6., 10.];
@@ -912,7 +138,7 @@ mod tests {
         assert_eq!(parse(query).unwrap(), result);
     }
 
-#[test]
+    #[test]
     fn parse_random_order() {
         let query = "9.3 * X^3 + 4.3 * X^0 + 3.4 * X^2 - 1.5 * X^3 - 13.12 * X^1 = 1.4 * X^2 - 5.1 * X^3 + 1.4 * X^1 -6.3 * X^0";
         let result = vec![10.6, -14.52, 2., 12.9];
diff --git a/src/parser.rs b/src/parser.rs
new file mode 100644
index 0000000..6f45358
--- /dev/null
+++ b/src/parser.rs
@@ -0,0 +1,19 @@
+pub mod tokenizer;
+pub mod sanitizer;
+pub mod ast_builder;
+
+#[derive(Debug, PartialEq, Clone)]
+pub enum Token {
+    Number(String),
+    Variable(String),
+    ImaginaryUnit(),
+    Addition(),
+    Multiplication(),
+    Substraction(),
+    Division(),
+    Modulo(),
+    Exponentiation(),
+    Equal(),
+    OpenParenthesis(),
+    CloseParenthesis(),
+}
\ No newline at end of file
diff --git a/src/parser/ast_builder.rs b/src/parser/ast_builder.rs
new file mode 100644
index 0000000..ffd67ec
--- /dev/null
+++ b/src/parser/ast_builder.rs
@@ -0,0 +1,10 @@
+use super::Token;
+
+pub struct Node {
+    _lhs: Option<Box<Node>>,
+    _rhs: Option<Box<Node>>,
+}
+
+pub fn build_ast(_tokens: Vec<Token>) -> Node {
+    Node { _lhs: None, _rhs: None }
+}
diff --git a/src/parser/sanitizer.rs b/src/parser/sanitizer.rs
new file mode 100644
index 0000000..696ab81
--- /dev/null
+++ b/src/parser/sanitizer.rs
@@ -0,0 +1,457 @@
+use super::Token;
+
+#[derive(Debug, PartialEq)]
+enum TokenType {
+    Number(),
+    Variable(),
+    ImaginaryUnit(),
+    Operator(),
+    Substraction(),
+    OpenParenthesis(),
+    CloseParenthesis(),
+    Equal(),
+}
+
+fn get_token_type(token: &Token) -> TokenType {
+    match token {
+        Token::Number(_) => TokenType::Number(),
+        Token::Variable(_) => TokenType::Variable(),
+        Token::ImaginaryUnit() => TokenType::ImaginaryUnit(),
+        Token::OpenParenthesis() => TokenType::OpenParenthesis(),
+        Token::CloseParenthesis() => TokenType::CloseParenthesis(),
+        Token::Substraction() => TokenType::Substraction(),
+        Token::Equal() => TokenType::Equal(),
+        _ => TokenType::Operator(),
+    }
+}
+
+pub fn sanitize_tokens(tokens: Vec<Token>) -> Result<Vec<Token>, String> {
+    let mut sanitized_tokens: Vec<Token> = vec![];
+    let mut last_token_type = None;
+    let mut open_close_equal_count = (0, 0, 0);
+
+    for token in tokens {
+        let token_type = get_token_type(&token);
+        match token_type {
+            TokenType::OpenParenthesis() => open_close_equal_count.0 += 1,
+            TokenType::CloseParenthesis() => {
+                open_close_equal_count.1 += 1;
+                if open_close_equal_count.1 > open_close_equal_count.0 {
+                    return Err(format!("Error at token {:?}, closing parenthesis that was never opened", token));
+                }
+            },
+            TokenType::Equal() => {
+                open_close_equal_count.2 += 1;
+                if open_close_equal_count.0 != open_close_equal_count.1 {
+                    return Err(format!("Error, query has a different number of opening and closing parentheses"));
+                }
+                open_close_equal_count.0 = 0;
+                open_close_equal_count.1 = 0;
+            },
+            _ => (),
+        }
+        if let Some(last_token_type) = last_token_type {
+            match (&last_token_type, &token_type) {
+                (TokenType::Number(), TokenType::Variable()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::Number(), TokenType::ImaginaryUnit()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::Number(), TokenType::OpenParenthesis()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::Variable(), TokenType::Number()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::Variable(), TokenType::ImaginaryUnit()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::Variable(), TokenType::OpenParenthesis()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::ImaginaryUnit(), TokenType::Number()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::ImaginaryUnit(), TokenType::Variable()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::ImaginaryUnit(), TokenType::OpenParenthesis()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::CloseParenthesis(), TokenType::OpenParenthesis()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::CloseParenthesis(), TokenType::Number()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::CloseParenthesis(), TokenType::Variable()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::CloseParenthesis(), TokenType::ImaginaryUnit()) => sanitized_tokens.push(Token::Multiplication()),
+                (TokenType::OpenParenthesis(), TokenType::Operator()) => return Err(format!("Error at token {:?}, operator forbidden directly after opening parenthesis", token)),
+                (TokenType::Operator(), TokenType::CloseParenthesis()) => return Err(format!("Error at token {:?}, all operators are forbidden directly before closing parenthesis", token)),
+                (TokenType::Substraction(), TokenType::CloseParenthesis()) => return Err(format!("Error at token {:?}, all operators are forbidden directly before closing parenthesis", token)),
+                (TokenType::OpenParenthesis(), TokenType::CloseParenthesis()) => return Err(format!("Error at token {:?}, empty parentheses", token)),
+                (TokenType::Substraction(), TokenType::Equal()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
+                (TokenType::Substraction(), TokenType::Operator()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
+                (TokenType::Operator(), TokenType::Equal()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
+                (TokenType::Operator(), TokenType::Substraction()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
+                (TokenType::Equal(), TokenType::Operator()) => return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token)),
+                (TokenType::OpenParenthesis(), TokenType::OpenParenthesis()) => (),
+                (TokenType::CloseParenthesis(), TokenType::CloseParenthesis()) => (),
+                _ => {
+                    if token_type == last_token_type {
+                        return Err(format!("Error at token {:?}, two incompatible tokens type in a row", token));
+                    }
+                },
+            }
+        } else if token_type == TokenType::Operator() || token_type == TokenType::Equal() {
+            return Err(format!("Error at token {:?}, query can't start with this token", token));
+        }
+        sanitized_tokens.push(token);
+
+        last_token_type = Some(token_type);
+    }
+    if open_close_equal_count.0 != open_close_equal_count.1 {
+        return Err(format!("Error, query has a different number of opening and closing parentheses"));
+    }
+    if open_close_equal_count.2 > 1 {
+        return Err(format!("Error, query shouldn't have more than 1 equals sign"));
+    }
+    let last_token = sanitized_tokens.last().unwrap();
+    match get_token_type(&last_token) {
+        TokenType::Equal() | TokenType::Operator() | TokenType::Substraction() => return Err(format!("Error at token {:?}, query can't end with this token", last_token)),
+        _ => (),
+    }
+    Ok(sanitized_tokens)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn plus() -> Token {
+        Token::Addition()
+    }
+
+    fn minus() -> Token {
+        Token::Substraction()
+    }
+
+    fn times() -> Token {
+        Token::Multiplication()
+    }
+
+    fn divided_by() -> Token {
+        Token::Division()
+    }
+
+    fn modulo() -> Token {
+        Token::Modulo()
+    }
+
+    fn equals() -> Token {
+        Token::Equal()
+    }
+
+    fn x() -> Token {
+        Token::Variable(String::from("x"))
+    }
+
+    fn i() -> Token {
+        Token::ImaginaryUnit()
+    }
+
+    fn number() -> Token {
+        Token::Number(String::from("123"))
+    }
+
+    fn open() -> Token {
+        Token::OpenParenthesis()
+    }
+
+    fn close() -> Token {
+        Token::CloseParenthesis()
+    }
+
+    #[test]
+    #[should_panic]
+    fn double_number() {
+        let tokens = vec![number(), number()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn double_operator() {
+        let tokens = vec![number(), plus(), divided_by()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn double_variable() {
+        let tokens = vec![x(), x()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn double_imaginary_unit() {
+        let tokens = vec![i(), i()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn sanitize_tokens_starts_with_operator() {
+        let tokens = vec![plus(), number(), divided_by(), x()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    fn starts_with_negative_number() {
+        let tokens = vec![minus(), number(), divided_by(), x()];
+        let results = tokens.clone();
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+    }
+
+    #[test]
+    fn number_operator_variable() {
+        let tokens = vec![number(), divided_by(), x()];
+        let results = tokens.clone();
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+    }
+
+    #[test]
+    fn insert_multiplication() {
+        let tokens = vec![number(), x()];
+        let results = vec![number(), times(), x()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![x(), number()];
+        let results = vec![x(), times(), number()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![number(), i()];
+        let results = vec![number(), times(), i()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![i(), number()];
+        let results = vec![i(), times(), number()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![i(), x()];
+        let results = vec![i(), times(), x()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![x(), i()];
+        let results = vec![x(), times(), i()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+    }
+
+    #[test]
+    fn insert_multiplication_parenthesis() {
+        let tokens = vec![open(), number(), modulo(), number(), close(), open(), i(), times(), x(), close()];
+        let results = vec![open(), number(), modulo(), number(), close(), times(), open(), i(), times(), x(), close()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![number(), open(), number(), modulo(), number(), close()];
+        let results= vec![number(), times(), open(), number(), modulo(), number(), close()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![i(), open(), number(), modulo(), number(), close()];
+        let results= vec![i(), times(), open(), number(), modulo(), number(), close()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![x(), open(), number(), modulo(), number(), close()];
+        let results= vec![x(), times(), open(), number(), modulo(), number(), close()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![open(), number(), modulo(), number(), close(), number()];
+        let results= vec![open(), number(), modulo(), number(), close(), times(), number()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![open(), number(), modulo(), number(), close(), i()];
+        let results= vec![open(), number(), modulo(), number(), close(), times(), i()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![open(), number(), modulo(), number(), close(), x()];
+        let results= vec![open(), number(), modulo(), number(), close(), times(), x()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec![number(), open(), number(), modulo(), number(), close(), x()];
+        let results= vec![number(), times(), open(), number(), modulo(), number(), close(), times(), x()];
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+    }
+
+    #[test]
+    #[should_panic]
+    fn parenthesis_operator() {
+        let tokens = vec!(open(), plus(), number(), close());
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn operator_parenthesis() {
+        let tokens = vec!(open(), number(), modulo(), close());
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn minus_parenthesis() {
+        let tokens = vec!(open(), number(), minus(), close());
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn empty_parentheses() {
+        let tokens = vec!(open(), close());
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    fn parenthesis_minus() {
+        let tokens = vec!(open(), minus(), number(), close());
+        let results = tokens.clone();
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+    }
+
+    #[test]
+    fn double_parentheses() {
+        let tokens = vec!(open(), open(), number(), plus(), number(), close(), plus(), number(), close());
+        let results = tokens.clone();
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec!(open(), number(), plus(), open(), number(), plus(), number(), close(), close());
+        let results = tokens.clone();
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec!(open(), open(), number(), plus(), number(), close(), number(), close());
+        let results = vec!(open(), open(), number(), plus(), number(), close(), times(), number(), close());
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+
+        let tokens = vec!(open(), number(), open(), number(), plus(), number(), close(), close());
+        let results= vec!(open(), number(), times(), open(), number(), plus(), number(), close(), close());
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+    }
+
+    #[test]
+    #[should_panic]
+    fn wrong_parentheses_count() {
+        let tokens = vec![open(), number(), close(), close()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn wrong_parentheses_count_2() {
+        let tokens = vec![open(), open(), number(), close()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn wrong_parentheses_count_hard() {
+        let tokens = vec![open(), open(), number(), close(), equals(), x(), close()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn wrong_equal_count() {
+        let tokens = vec![number(), equals(), number(), equals(), number()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn starts_with_equal() {
+        let tokens = vec![equals(), number(), plus(), number()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn ends_with_equal() {
+        let tokens = vec![number(), plus(), number(), equals()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn ends_with_minus() {
+        let tokens = vec![number(), plus(), number(), minus()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn ends_with_operator() {
+        let tokens = vec![number(), plus(), number(), divided_by()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn ends_with_open_brackets() {
+        let tokens = vec![number(), plus(), number(), open(), number(), close(), open()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn starts_with_close_brackets() {
+        let tokens = vec![close(), number(), plus(), number(), open(), number()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn wrong_brackets_order() {
+        let tokens = vec![number(), close(), number(), plus(), number(), open(), number()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn minus_operator() {
+        let tokens = vec![number(), minus(), plus(), i()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn operator_minus() {
+        let tokens = vec![number(), plus(), minus(), i()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn minus_equals() {
+        let tokens = vec![number(), minus(), equals(), i()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn operator_equals() {
+        let tokens = vec![number(), modulo(), equals(), i()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn equals_operator() {
+        let tokens = vec![number(), equals(), times(), i()];
+        sanitize_tokens(tokens).unwrap();
+    }
+
+    #[test]
+    fn equals_minus() {
+        let tokens = vec![number(), equals(), minus(), x()];
+        let results = tokens.clone();
+        assert_eq!(sanitize_tokens(tokens).unwrap(), results);
+    }
+
+    mod get_token_type {
+        use super::*;
+
+        #[test]
+        fn exhaustive() {
+            assert_eq!(get_token_type(&Token::Number(String::from("123"))), TokenType::Number());
+            assert_eq!(get_token_type(&Token::Variable(String::from("var"))), TokenType::Variable());
+            assert_eq!(get_token_type(&Token::ImaginaryUnit()), TokenType::ImaginaryUnit());
+            assert_eq!(get_token_type(&Token::OpenParenthesis()), TokenType::OpenParenthesis());
+            assert_eq!(get_token_type(&Token::CloseParenthesis()), TokenType::CloseParenthesis());
+            assert_eq!(get_token_type(&Token::Substraction()), TokenType::Substraction());
+            assert_eq!(get_token_type(&Token::Equal()), TokenType::Equal());
+            assert_eq!(get_token_type(&Token::Addition()), TokenType::Operator());
+            assert_eq!(get_token_type(&Token::Multiplication()), TokenType::Operator());
+            assert_eq!(get_token_type(&Token::Division()), TokenType::Operator());
+            assert_eq!(get_token_type(&Token::Modulo()), TokenType::Operator());
+            assert_eq!(get_token_type(&Token::Exponentiation()), TokenType::Operator());
+        }
+    }
+}
diff --git a/src/parser/tokenizer.rs b/src/parser/tokenizer.rs
new file mode 100644
index 0000000..7ba18c3
--- /dev/null
+++ b/src/parser/tokenizer.rs
@@ -0,0 +1,301 @@
+use super::Token;
+
+fn check_number(my_string: String, i: usize) -> Result<Token, String> {
+    println!("Checking number at index {i}");
+    if my_string.as_str() == "." {
+        return Err(format!("unexpected token: `.` at position {i}"));
+    }
+    let find = my_string.find(".");
+    if let Some(j) = find {
+        let find = my_string[j+1..].find(".");
+        if let Some(_) = find {
+            return Err(format!("unexpected token: `{my_string}` at position {i}"))
+        }
+    }
+    Ok(Token::Number(my_string))
+}
+
+pub fn tokenize(query: &str) -> Result<Vec<Token>, String> {
+    let mut tokens: Vec<Token> = vec![];
+    let mut my_string = String::new();
+    let mut is_last_number = false;
+    let mut is_last_variable = false;
+
+    for (i, token) in query.chars().enumerate() {
+        let mut is_still_number = false;
+        let mut is_still_variable = false;
+        let mut is_pop_needed = true;
+        match token {
+            '+' => tokens.push(Token::Addition()),
+            '-' => tokens.push(Token::Substraction()),
+            '*' => tokens.push(Token::Multiplication()),
+            '/' => tokens.push(Token::Division()),
+            '%' => tokens.push(Token::Modulo()),
+            '^' => tokens.push(Token::Exponentiation()),
+            '=' => tokens.push(Token::Equal()),
+            '(' => tokens.push(Token::OpenParenthesis()),
+            ')' => tokens.push(Token::CloseParenthesis()),
+            'a'..='z' | 'A'..='Z' => {
+                if is_last_number == true {
+                    let mut tmp_token = None;
+                    if is_pop_needed {
+                        tmp_token = tokens.pop();
+                    }
+                    tokens.push(check_number(my_string, i)?);
+                    if let Some(tok) = tmp_token {
+                        tokens.push(tok);
+                    }
+                    is_last_number = false;
+                    my_string = String::new();
+                }
+                if is_last_variable == false {
+                    is_last_variable = true;
+                }
+                is_still_variable = true;
+                my_string += &token.to_string();
+            },
+            '0'..='9' | '.' => {
+                if is_last_variable == true {
+                    let mut tmp_token = None;
+                    if is_pop_needed {
+                        tmp_token = tokens.pop();
+                    }
+                    match my_string.as_str() {
+                        "i" => tokens.push(Token::ImaginaryUnit()),
+                        _ => tokens.push(Token::Variable(my_string)),
+                    }
+                    if let Some(tok) = tmp_token {
+                        tokens.push(tok);
+                    }
+                    is_last_variable = false;
+                    my_string = String::new();
+                }
+                if is_last_number == false {
+                    is_last_number = true;
+                }
+                is_still_number = true;
+                my_string += &token.to_string();
+            },
+            ' ' => is_pop_needed = false,
+            _ => return Err(format!("unexpected token: `{token}` at position {i}")),
+        };
+        if is_last_variable && !is_still_variable {
+            let mut tmp_token = None;
+            if is_pop_needed {
+                tmp_token = tokens.pop();
+            }
+            match my_string.as_str() {
+                "i" => tokens.push(Token::ImaginaryUnit()),
+                _ => tokens.push(Token::Variable(my_string)),
+            }
+            if let Some(tok) = tmp_token {
+                tokens.push(tok);
+            }
+            is_last_variable = false;
+            my_string = String::new();
+        }
+        else if is_last_number && !is_still_number {
+            let mut tmp_token = None;
+            if is_pop_needed {
+                tmp_token = tokens.pop();
+            }
+            tokens.push(check_number(my_string, i)?);
+            if let Some(tok) = tmp_token {
+                tokens.push(tok);
+            }
+            is_last_number = false;
+            my_string = String::new();
+        }
+    }
+    if is_last_variable {
+        match my_string.as_str() {
+            "i" => tokens.push(Token::ImaginaryUnit()),
+            _ => tokens.push(Token::Variable(my_string)),
+        }
+    }
+    else if is_last_number {
+        tokens.push(check_number(my_string, query.len())?);
+    }
+
+    Ok(tokens)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn addition() {
+        let query = "+";
+        let result: Vec<Token> = vec![Token::Addition()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn substraction() {
+        let query = "-";
+        let result: Vec<Token> = vec![Token::Substraction()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn multiplication() {
+        let query = "*";
+        let result: Vec<Token> = vec![Token::Multiplication()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn division() {
+        let query = "/";
+        let result: Vec<Token> = vec![Token::Division()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn modulo() {
+        let query = "%";
+        let result: Vec<Token> = vec![Token::Modulo()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn exponentiation() {
+        let query = "^";
+        let result: Vec<Token> = vec![Token::Exponentiation()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn equal() {
+        let query = "=";
+        let result: Vec<Token> = vec![Token::Equal()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn open_parenthesis() {
+        let query = "(";
+        let result: Vec<Token> = vec![Token::OpenParenthesis()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn close_parenthesis() {
+        let query = ")";
+        let result: Vec<Token> = vec![Token::CloseParenthesis()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn imaginary_unit() {
+        let query = "i";
+        let result: Vec<Token> = vec![Token::ImaginaryUnit()];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn variable() {
+        let query = "variable";
+        let result: Vec<Token> = vec![Token::Variable(String::from("variable"))];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn variable_double_i() {
+        let query = "ii";
+        let result: Vec<Token> = vec![Token::Variable(String::from("ii"))];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn number_natural() {
+        let query = "123456";
+        let result: Vec<Token> = vec![Token::Number(String::from("123456"))];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn number_rational() {
+        let query = "123.456";
+        let result: Vec<Token> = vec![Token::Number(String::from("123.456"))];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn number_point_something() {
+        let query = ".123456";
+        let result: Vec<Token> = vec![Token::Number(String::from(".123456"))];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn number_trailing_point() {
+        let query = "123456.";
+        let result: Vec<Token> = vec![Token::Number(String::from("123456."))];
+
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    fn number_variable() {
+        let query = "23x";
+        let result: Vec<Token> = vec![Token::Number(String::from("23")), Token::Variable(String::from("x"))];
+        assert_eq!(tokenize(query).unwrap(), result);
+
+        let query = "23i";
+        let result: Vec<Token> = vec![Token::Number(String::from("23")), Token::ImaginaryUnit()];
+        assert_eq!(tokenize(query).unwrap(), result);
+
+        let query = "x23";
+        let result: Vec<Token> = vec![Token::Variable(String::from("x")), Token::Number(String::from("23"))];
+        assert_eq!(tokenize(query).unwrap(), result);
+
+        let query = "i23";
+        let result: Vec<Token> = vec![Token::ImaginaryUnit(), Token::Number(String::from("23"))];
+        assert_eq!(tokenize(query).unwrap(), result);
+    }
+
+    #[test]
+    #[should_panic]
+    fn number_double_point() {
+        let query = "12.34.56";
+        tokenize(query).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn number_double_point_in_a_row() {
+        let query = "123..456";
+        tokenize(query).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn number_only_point() {
+        let query = ".";
+        tokenize(query).unwrap();
+    }
+
+    #[test]
+    #[should_panic]
+    fn invalid_token() {
+        let query = "324*43224+243_+234=234";
+        tokenize(query).unwrap();
+    }
+}