diff --git a/example.mlc b/example.mlc
index e0c3385..f1776a4 100644
--- a/example.mlc
+++ b/example.mlc
@@ -1,3 +1,4 @@
+#with mathlib.mlc
 variable:=-3; c := (a+b- 3) * 23 + variable; d := c - a;Natural : Number (n) := {n >= 0};faculty : Natural (n) -> Natural := if n = 0 then 1 else faculty (n-1) * n end;
 String Natural (n) := {Character * n};hello_word -> String := "Hello World!";
 first_letter -> Character := 'a';
diff --git a/language.toml b/language.toml
index 43c5085..559e2d5 100644
--- a/language.toml
+++ b/language.toml
@@ -11,6 +11,7 @@ comments = ["^--.*", ""]
 with = ["^#with ([\\w./]+)", "cat $1"]
 date = ["#date_now", "date"]
 user = ["#user", "user"]
+test = ["#test", "cat ./mathlib.mlc"]
 
 # Describes tokens to be replaced by identifiers and then later swapped back in after the tokenizer.
 # All special tokens are treated as constants
@@ -28,7 +29,7 @@ operands = [":=", "->", "<=", ">=", "<", ">", "!", "+", "-", "/", "*", "(", ")",
 terminator = [";"]
 
 [semantics]
-keywords = ["if", "then", "else", "end", "with"]
+keywords = ["if", "then", "else", "end"]
 
 [constants]
 number = "(?:0b[01]+|0x[0-9a-fA-F]+|0[0-7]+|[1-9][0-9]*)"
diff --git a/mathlib.mlc b/mathlib.mlc
new file mode 100644
index 0000000..0ff0e56
--- /dev/null
+++ b/mathlib.mlc
@@ -0,0 +1,4 @@
+Sigma -> Array := {0, 1, 2};
+N -> Array := {3};
+P -> Array := {3 -> 012};
+S -> Number := 3;
diff --git a/src/main.rs b/src/main.rs
index a9fa082..20c7894 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -6,15 +6,24 @@ mod tokenizer;
 use tokenizer::*;
 
 fn main() {
+    // Preprocessor
     let sample_code: String = std::fs::read_to_string("example.mlc").unwrap();
     let mut example_tokenizer: Tokenizer = Tokenizer::new();
     let mut meta_rules: crate::preprocessor::MetaRules =
         crate::preprocessor::MetaRules::new("./language.toml");
     let processed_sample_code: String = meta_rules.process(sample_code.to_owned());
+
+    // tokenizing
     example_tokenizer.read_configuration_from_file("./language.toml");
     example_tokenizer.eat(processed_sample_code.as_str());
     example_tokenizer.identify_tokens();
+    //  Insert meta tokens into token list
+    let mut token_index: usize = 0;
+    for meta_token in meta_rules.special_tokens.iter() {
+        println!("Token: {:?}", meta_token.0);
+    }
 
+    // Semantic analysis
     let mut example_identifier: identification::Identifier =
         identification::Identifier::new(example_tokenizer.tokens);
     example_identifier.load_criteria_from_configuration(example_tokenizer.configuration);
diff --git a/src/preprocessor.rs b/src/preprocessor.rs
index e6644fc..e8cbdc6 100644
--- a/src/preprocessor.rs
+++ b/src/preprocessor.rs
@@ -1,5 +1,5 @@
 use crate::tokenizer::Token;
-use regex::{Captures, Regex};
+use regex::{Captures, Match, Regex};
 use toml::{Table, Value};
 
 // MetaRules
@@ -8,7 +8,7 @@ pub struct MetaRules {
     replacement_rules: Vec<(String, (String, String))>,
     interpolation_rules: Vec<(String, (String, String))>,
     token_rules: Vec<(String, String)>,
-    special_tokens: Vec<Token>,
+    pub special_tokens: Vec<(String, Token)>,
 }
 
 // Implementation of MetaRules
@@ -24,7 +24,7 @@ impl MetaRules {
         let mut replacements: Vec<(String, (String, String))> = vec![];
         let mut interpolation: Vec<(String, (String, String))> = vec![];
         let mut meta_token_rules: Vec<(String, String)> = vec![];
-        let meta_tokens: Vec<Token> = vec![];
+        let meta_tokens: Vec<(String, Token)> = vec![];
         let configuration = gtoml::parse(configuration_content.as_str())
             .expect("[ERROR] TOML invalid in preprocessor!");
         let configuration_unpacked: Table = Table::try_from(configuration).unwrap();
@@ -125,22 +125,15 @@ impl MetaRules {
             println!("[INFO] Applying rule {}", rule.0);
             let base_pattern: Regex = Regex::new((rule.1 .0).as_str()).unwrap();
             let processed_code_replacement = processed_code.clone();
-            let parameter = if &base_pattern
-                .captures(processed_code_replacement.as_str())
-                .unwrap()
-                .len()
-                > 0
-            {
-                &base_pattern
-                    .captures(processed_code_replacement.as_str())
-                    .unwrap()[0]
-            } else {
-                &base_pattern
-                    .captures(processed_code_replacement.as_str())
-                    .unwrap()
+
+            let captures: Option<Captures> =
+                base_pattern.captures(processed_code_replacement.as_str());
+            let directive: String;
+            match captures {
+                Some(n) => directive = n.get(0).map_or("", |m| m.as_str()).to_string(),
+                None => continue,
             };
-            let command: &str = &base_pattern.replace(parameter, rule.1 .1.as_str());
-            println!("{:?}", &command);
+            let command: &str = &base_pattern.replace(directive.as_str(), rule.1 .1.as_str());
             let subprocess = std::process::Command::new("/bin/bash")
                 .arg("-c")
                 .arg(String::from("echo \"$(") + command + ")\"")
@@ -154,6 +147,43 @@ impl MetaRules {
                 .to_string();
         }
 
+        for token_style in self.token_rules.iter() {
+            println!("[INFO] Searching meta tokens of style {}", token_style.0);
+
+            // Search all occurrences
+            let token_pattern: Regex =
+                Regex::new(token_style.1.as_str()).expect("Could not assign pattern.");
+            let match_list: Match;
+            match_list = match token_pattern.find(processed_code.as_str()) {
+                Some(n) => n,
+                None => continue,
+            };
+
+            // Create id for each occurrence
+            let meta_id: String = String::from("meta_token_")
+                + match_list.start().to_string().as_str()
+                + "__"
+                + match_list.end().to_string().as_str();
+
+            // Replace token by id
+            let meta_value: String = match_list.as_str().to_string();
+            let value_regex: Regex =
+                Regex::new(meta_value.as_str()).expect("Could not create pattern.");
+            processed_code = value_regex
+                .replace(processed_code.as_str(), meta_id.as_str())
+                .to_string();
+            println!("Replace {} with {}.", meta_value, meta_id);
+
+            // Safe id and token
+            self.special_tokens.push((
+                meta_id,
+                Token {
+                    token: meta_value,
+                    token_type: crate::TokenType::IDENTIFIER,
+                },
+            ));
+        }
+
         return processed_code;
     }
 }
diff --git a/src/testcases.rs b/src/testcases.rs
index 092d231..0986760 100644
--- a/src/testcases.rs
+++ b/src/testcases.rs
@@ -1,9 +1,61 @@
 #[cfg(test)]
 mod tests {
-    use super::*;
+
+    // preprocessor
+    #[test]
+    fn test_replacements() {
+        let mut ruleset: crate::preprocessor::MetaRules =
+            crate::preprocessor::MetaRules::new("./language.toml");
+        let sut: String = ruleset.process(String::from("-- Comment to remove"));
+        let verify: String = String::from("");
+        let case_comment_at_end: String =
+            ruleset.process(String::from("This -- comment is not removed."));
+        let case_comment_at_end_verify: String = String::from("This -- comment is not removed.");
+        assert_eq!(sut, verify);
+        assert_eq!(case_comment_at_end, case_comment_at_end_verify);
+    }
 
     #[test]
-    fn dummy_test() {
-        assert_eq!(2, 2);
+    fn test_interpolation() {
+        let mut ruleset: crate::preprocessor::MetaRules =
+            crate::preprocessor::MetaRules::new("./language.toml");
+        let run_with_interpolation_test: String = ruleset.process(String::from("#test"));
+        let interpolation_verification: String = std::fs::read_to_string("./mathlib.mlc").unwrap();
+
+        assert_eq!(run_with_interpolation_test, interpolation_verification);
+    }
+
+    #[test]
+    fn test_meta_token() {
+        let mut ruleset: crate::preprocessor::MetaRules =
+            crate::preprocessor::MetaRules::new("./language.toml");
+        let meta_token_test_string: String = ruleset.process(String::from("\"sample\""));
+        let meta_token_sample_string: String = String::from("\"sample\"");
+        let meta_token_verify: Vec<crate::tokenizer::Token> = vec![crate::tokenizer::Token {
+            token: meta_token_sample_string,
+            token_type: crate::tokenizer::TokenType::IDENTIFIER,
+        }];
+        assert_eq!(meta_token_verify.len(), ruleset.special_tokens.len());
+        assert_eq!(
+            meta_token_verify[0].token,
+            ruleset.special_tokens[0].1.token
+        );
+        assert_eq!(meta_token_test_string, "meta_token_0__8");
+    }
+
+    // Tokenizer
+    #[test]
+    fn test_eat() {
+        let mut sample: crate::tokenizer::Tokenizer = crate::tokenizer::Tokenizer::new();
+        sample.read_configuration_from_file("./language.toml");
+        sample.eat("faculty : Natural n := if n = 0 then 1 else n * faculty (n - 1);");
+
+        assert_eq!(
+            sample.token_list,
+            vec![
+                "faculty", ":", "Natural", "n", ":=", "if", "n", "=", "0", "then", "1", "else",
+                "n", "*", "faculty", "(", "n", "-", "1", ")", ";"
+            ]
+        )
     }
 }
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index 598e8be..db28f33 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -212,6 +212,15 @@ impl Tokenizer {
             }
         }
         self.token_list.append(&mut new_tokens);
+
+        // Clean up token list
+        let mut cleaned_token_list: Vec<String> = vec![];
+        for token in self.token_list.iter() {
+            if token.as_str() != "" {
+                cleaned_token_list.push(token.to_string());
+            }
+        }
+        self.token_list = cleaned_token_list;
     }
 
     // @name identify_tokens