diff --git a/src/abstract_tree/built_in_value.rs b/src/abstract_tree/built_in_value.rs
index bbafcb5..9735548 100644
--- a/src/abstract_tree/built_in_value.rs
+++ b/src/abstract_tree/built_in_value.rs
@@ -4,9 +4,7 @@ use enum_iterator::{all, Sequence};
 use serde::{Deserialize, Serialize};
 
 use crate::{
-    built_in_functions::{
-        fs::fs_functions, json::json_functions, string::string_functions, Callable,
-    },
+    built_in_functions::{fs::fs_functions, json::json_functions, str::string_functions, Callable},
     AbstractTree, BuiltInFunction, Format, Function, List, Map, Result, SyntaxNode, Type, Value,
 };
 
diff --git a/src/built_in_functions/mod.rs b/src/built_in_functions/mod.rs
index 8fff517..45eb7a3 100644
--- a/src/built_in_functions/mod.rs
+++ b/src/built_in_functions/mod.rs
@@ -1,6 +1,6 @@
 pub mod fs;
 pub mod json;
-pub mod string;
+pub mod str;
 
 use std::fmt::{self, Display, Formatter};
 
@@ -9,7 +9,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::{Error, Format, Map, Result, Type, Value};
 
-use self::{fs::Fs, json::Json, string::StrFunction};
+use self::{fs::Fs, json::Json, str::StrFunction};
 
 pub trait Callable {
     fn name(&self) -> &'static str;
diff --git a/src/built_in_functions/string.rs b/src/built_in_functions/str.rs
similarity index 100%
rename from src/built_in_functions/string.rs
rename to src/built_in_functions/str.rs
diff --git a/tests/built_in_string_functions.rs b/tests/built_in_string_functions.rs
index 6296814..095e396 100644
--- a/tests/built_in_string_functions.rs
+++ b/tests/built_in_string_functions.rs
@@ -13,3 +13,37 @@ fn as_bytes() {
         ])))
     );
 }
+
+#[test]
+fn ends_with() {
+    let result = interpret("str:ends_with('abc', 'c')");
+
+    assert_eq!(result, Ok(Value::Boolean(true)));
+
+    let result = interpret("str:ends_with('abc', 'b')");
+
+    assert_eq!(result, Ok(Value::Boolean(false)));
+}
+
+#[test]
+fn find() {
+    let result = interpret("str:find('abc', 'a')");
+
+    assert_eq!(result, Ok(Value::Option(Some(Box::new(Value::Integer(0))))));
+
+    let result = interpret("str:find('foobar', 'b')");
+
+    assert_eq!(result, Ok(Value::Option(Some(Box::new(Value::Integer(3))))));
+}
+
+#[test]
+fn insert() {
+    assert_eq!(
+        interpret("str:insert('ac', 1, 'b')"),
+        Ok(Value::String("abc".to_string()))
+    );
+    assert_eq!(
+        interpret("str:insert('foo', 3, 'bar')"),
+        Ok(Value::String("foobar".to_string()))
+    );
+}