Made the x_1 notation possible.

.gitignore

@@ -1,2 +1,3 @@
 */target
 kalk/Cargo.lock
+kalk_cli/test

kalk/src/lexer.rs

@@ -140,12 +140,22 @@ impl<'a> Lexer<'a> {
     fn next_identifier(&mut self) -> Token {
         let start = self.index;
         let mut end = start;
-        let letter_reg = regex::Regex::new(r"[A-z']").unwrap();
+        let letter_reg = regex::Regex::new(r"[A-z'_]").unwrap();
         let mut value = String::new();
 
         while is_valid_identifier(self.peek()) {
             let c = *self.peek().unwrap();
 
+            // If the current character is an underscore, expect a number next.
+            // This is to allow the notation like the following: x_1
+            if c == '_' {
+                self.advance();
+                let num = self.next_number_literal().value;
+                value.push('_');
+                value.push_str(&num.trim_end()); // Trim, since the number_literal function allows whitespace, which identifiers should not contain.
+                break;
+            }
+
             // Only allow identifiers with a special character to have *one* character. No more.
             // Break the loop if it isn't the first run and the current character is a special character.
             if end - start > 0 && !letter_reg.is_match(&c.to_string()) {

kalk_cli/src/repl.rs

@@ -54,9 +54,9 @@ impl Highlighter for LineHighlighter {
     fn highlight<'l>(&self, line: &'l str, _: usize) -> Cow<'l, str> {
         let mut coloured = line.to_string();
 
-        let reg = Regex::new(r"([A-z]+|[\+-/\*\^!])").unwrap();
+        let reg = Regex::new(r"(([^0-9\.,\(\)=\+-/\*\^!_]+(_\d+)?)|[\+-/\*\^!])").unwrap();
         let unit = Regex::new(r"(deg|rad)").unwrap();
-        let identifier = Regex::new(r"[^0-9\.,\(\)=\+-/\*\^!]+").unwrap();
+        let identifier = Regex::new(r"[^0-9\.,\(\)=\+-/\*\^!_]+(_\d+)?").unwrap();
         let op = Regex::new(r"[\+-/\*\^!]+").unwrap();
 
         coloured = reg