diff --git a/src/builtins/strings.rs b/src/builtins/strings.rs
index 0204554..ee8a42b 100644
--- a/src/builtins/strings.rs
+++ b/src/builtins/strings.rs
@@ -5,7 +5,7 @@ use crate::ast::Expr;
 use crate::builtins;
 use crate::builtins::utils::{ensure_args_count, ensure_string, ensure_string_collection};
 use crate::lexer::Span;
-use crate::value::Value;
+use crate::value::{Float, Value};
 
 use std::collections::HashMap;
 
@@ -14,6 +14,12 @@ use anyhow::Result;
 pub fn register(m: &mut HashMap<&'static str, builtins::BuiltinFcn>) {
     m.insert("concat", concat);
     m.insert("contains", contains);
+    m.insert("endswith", endswith);
+    m.insert("indexof", indexof);
+    m.insert("indexof_n", indexof_n);
+    m.insert("lower", lower);
+    m.insert("startswith", startswith);
+    m.insert("upper", upper);
 }
 
 fn concat(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
@@ -31,3 +37,63 @@ fn contains(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
     let s2 = ensure_string(name, &params[1], &args[1])?;
     Ok(Value::Bool(s1.contains(&s2)))
 }
+
+fn endswith(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "endswith";
+    ensure_args_count(span, name, params, args, 2)?;
+    let s1 = ensure_string(name, &params[0], &args[0])?;
+    let s2 = ensure_string(name, &params[1], &args[1])?;
+    Ok(Value::Bool(s1.ends_with(&s2)))
+}
+
+fn indexof(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "indexof";
+    ensure_args_count(span, name, params, args, 2)?;
+    let s1 = ensure_string(name, &params[0], &args[0])?;
+    let s2 = ensure_string(name, &params[1], &args[1])?;
+    Ok(Value::from_float(match s1.find(&s2) {
+        Some(pos) => pos as i64,
+        _ => -1,
+    } as Float))
+}
+
+fn indexof_n(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "indexof_n";
+    ensure_args_count(span, name, params, args, 2)?;
+    let s1 = ensure_string(name, &params[0], &args[0])?;
+    let s2 = ensure_string(name, &params[1], &args[1])?;
+
+    let mut positions = vec![];
+    let mut idx = 0;
+    while idx < s1.len() {
+        if let Some(pos) = s1.find(&s2) {
+            positions.push(Value::from_float(pos as Float));
+            idx = pos + 1;
+        } else {
+            break;
+        }
+    }
+    Ok(Value::from_array(positions))
+}
+
+fn lower(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "lower";
+    ensure_args_count(span, name, params, args, 1)?;
+    let s = ensure_string(name, &params[0], &args[0])?;
+    Ok(Value::String(s.to_lowercase()))
+}
+
+fn startswith(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "startswith";
+    ensure_args_count(span, name, params, args, 2)?;
+    let s1 = ensure_string(name, &params[0], &args[0])?;
+    let s2 = ensure_string(name, &params[1], &args[1])?;
+    Ok(Value::Bool(s1.starts_with(&s2)))
+}
+
+fn upper(span: &Span, params: &[Expr], args: &[Value]) -> Result<Value> {
+    let name = "upper";
+    ensure_args_count(span, name, params, args, 1)?;
+    let s = ensure_string(name, &params[0], &args[0])?;
+    Ok(Value::String(s.to_uppercase()))
+}
diff --git a/tests/interpreter/cases/builtins/strings/concat.yaml b/tests/interpreter/cases/builtins/strings/concat.yaml
index 67d6f2d..ddcd383 100644
--- a/tests/interpreter/cases/builtins/strings/concat.yaml
+++ b/tests/interpreter/cases/builtins/strings/concat.yaml
@@ -9,16 +9,16 @@ cases:
         package test
 
         # Arrays
-        a1 = concat(", ", ["Hello", "world"]) 
+        a1 = concat(", ", ["Hello", "world"])
         a2 = concat("", ["Hello", "world"]) # empty delimiter
         a3 = concat(",", []) # empty array
-        a4 = concat("", []) # empty array and delimiter        
-        
+        a4 = concat("", []) # empty array and delimiter
+
         # Sets
         s1 = concat(", ", {"world", "Hello"})
         s2 = concat("", {"world", "Hello"}) # empty delimiter
         s3 = concat(",", set()) # empty set
-        s4 = concat("", set()) # empty set and delimiter        
+        s4 = concat("", set()) # empty set and delimiter
 
     query: data.test
     want_result:
@@ -40,7 +40,7 @@ cases:
         y = concat(x, [])
     query: data.test
     want_result: {}
-    
+
   - note: undefined-collection
     data: {}
     modules:
@@ -50,7 +50,7 @@ cases:
         y = concat(",", x)
     query: data.test
     want_result: {}
-    
+
   - note: invalid-null-delimiter
     data: {}
     modules: ["package test\nx=concat(null, [])"]
@@ -74,7 +74,7 @@ cases:
     modules: ["package test\nx=concat([], [])"]
     query: data.test
     error: "`concat` expects string argument."
-    
+
   - note: invalid-set-delimiter
     data: {}
     modules: ["package test\nx=concat(set(), [])"]
@@ -86,4 +86,34 @@ cases:
     modules: ["package test\nx=concat({}, [])"]
     query: data.test
     error: "`concat` expects string argument."
-    
+
+
+  - note: invalid-null-collection
+    data: {}
+    modules: ["package test\nx=concat(\"\", null)"]
+    query: data.test
+    error: "`concat` expects array/set of strings."
+
+  - note: invalid-bool-collection
+    data: {}
+    modules: ["package test\nx=concat(\"\", true)"]
+    query: data.test
+    error: "`concat` expects array/set of strings."
+
+  - note: invalid-number-collection
+    data: {}
+    modules: ["package test\nx=concat(\"\", 1)"]
+    query: data.test
+    error: "`concat` expects array/set of strings."
+
+  - note: invalid-string-collection
+    data: {}
+    modules: ["package test\nx=concat(\"\", \"\")"]
+    query: data.test
+    error: "`concat` expects array/set of strings."
+
+  - note: invalid-object-collection
+    data: {}
+    modules: ["package test\nx=concat(\"\", {})"]
+    query: data.test
+    error: "`concat` expects array/set of strings."
diff --git a/tests/interpreter/cases/builtins/strings/endswith.yaml b/tests/interpreter/cases/builtins/strings/endswith.yaml
new file mode 100644
index 0000000..2d8c1d9
--- /dev/null
+++ b/tests/interpreter/cases/builtins/strings/endswith.yaml
@@ -0,0 +1,116 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+cases:
+  - note: all
+    data: {}
+    modules:
+      - |
+        package test
+
+        v1 = endswith("Hello, world", "ld")
+        v2 = endswith("Hello world", "") # empty substring
+        v3 = endswith("", "ld") # empty string
+        v4 = endswith("", "") # empty substring and string
+
+    query: data.test
+    want_result:
+      v1: true
+      v2: true
+      v3: false
+      v4: true
+
+  - note: undefined-string
+    data: {}
+    modules:
+      - |
+        package test
+        x { false }
+        y = endswith(x, "")
+    query: data.test
+    want_result: {}
+
+  - note: undefined-substring
+    data: {}
+    modules:
+      - |
+        package test
+        x { false }
+        y = endswith(",", x)
+    query: data.test
+    want_result: {}
+
+  - note: invalid-null-string
+    data: {}
+    modules: ["package test\nx=endswith(null, ``)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-bool-string
+    data: {}
+    modules: ["package test\nx=endswith(true, ``)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-number-string
+    data: {}
+    modules: ["package test\nx=endswith(1, ``)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-array-string
+    data: {}
+    modules: ["package test\nx=endswith([], ``)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-set-string
+    data: {}
+    modules: ["package test\nx=endswith(set(), ``)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-object-string
+    data: {}
+    modules: ["package test\nx=endswith({}, ``)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+
+
+
+  - note: invalid-null-substring
+    data: {}
+    modules: ["package test\nx=endswith(``, null)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-bool-substring
+    data: {}
+    modules: ["package test\nx=endswith(``, true)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-number-substring
+    data: {}
+    modules: ["package test\nx=endswith(``, 1)"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-array-substring
+    data: {}
+    modules: ["package test\nx=endswith(``, [])"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-set-substring
+    data: {}
+    modules: ["package test\nx=endswith(``, set())"]
+    query: data.test
+    error: "`endswith` expects string argument."
+
+  - note: invalid-object-substring
+    data: {}
+    modules: ["package test\nx=endswith(``, {})"]
+    query: data.test
+    error: "`endswith` expects string argument."