Allow case-insensitive keywords

src/lexer.rs

     static ref WHITESPACE_REGEX: Regex = Regex::new(r"\s").unwrap();
     static ref MATCHERS: Vec<Matcher> = vec![
         Matcher::new(r#"^"(.*)""#, TokenType::String),
-        Matcher::new(r#"^SELECT"#, TokenType::Select),
-        Matcher::new(r#"^FROM"#, TokenType::From),
-        Matcher::new(r#"^WHERE"#, TokenType::Where),
+        Matcher::new(r#"(?i)^SELECT"#, TokenType::Select),
+        Matcher::new(r#"(?i)^FROM"#, TokenType::From),
+        Matcher::new(r#"(?i)^WHERE"#, TokenType::Where),
         Matcher::new(r#"^[a-z][a-zA-Z_\-.*]*"#, TokenType::Identfiier),
         Matcher::new(r#"^[0-9]+"#, TokenType::Number),
         Matcher::new(r#"^\*"#, TokenType::Star),
     }
 
     #[test]
-    fn it_scans_keywords() {
+    fn it_scans_uppercase_keywords() {
         assert_eq!(
             scan("SELECT FROM WHERE").unwrap(),
             vec![
         )
     }
 
+    #[test]
+    fn it_scans_lowercase_keywords() {
+        assert_eq!(
+            scan("select from where").unwrap(),
+            vec![
+                Token::new(TokenType::Select, "select"),
+                Token::new(TokenType::From, "from"),
+                Token::new(TokenType::Where, "where"),
+            ]
+        )
+    }
+
     #[test]
     fn it_scans_an_identifier() {
         assert_eq!(