diff --git a/src/parsing.rs b/src/parsing.rs
index 0d108c1..ca8781d 100644
--- a/src/parsing.rs
+++ b/src/parsing.rs
@@ -30,7 +30,7 @@ impl BackingFunction {
 }
 
 lazy_static::lazy_static! {
-    static ref VALID_VARIABLES: Vec<char> = "xeπ".chars().collect();
+    static ref VALID_VARIABLES: Vec<char> = "xXEπ".chars().collect();
     static ref LETTERS: Vec<char> = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"
         .chars()
         .collect();
@@ -130,6 +130,29 @@ pub fn test_func(function_string: &str) -> Option<String> {
     }
 }
 
+// Used for testing: passes function to `add_asterisks` before running `test_func`
+#[cfg(test)]
+fn test_func_helper(function_string: &str) -> Option<String> {
+    test_func(&add_asterisks(function_string.to_string()))
+}
+
+#[test]
+fn test_func_test() {
+    // These shouldn't fail
+    assert!(test_func_helper("x^2").is_none());
+    assert!(test_func_helper("2x").is_none());
+    // assert!(test_func_helper("e^x").is_none()); // need to fix!!! PR to exmex
+    assert!(test_func_helper("E^x").is_none());
+    assert!(test_func_helper("log10(x)").is_none());
+    assert!(test_func_helper("xxxxx").is_none());
+
+    // Expect these to fail
+    assert!(test_func_helper("a").is_some());
+    assert!(test_func_helper("l^2").is_some());
+    assert!(test_func_helper("log222(x)").is_some());
+    assert!(test_func_helper("abcdef").is_some());
+}
+
 // Tests to make sure my cursed function works as intended
 #[test]
 fn asterisk_test() {