diff --git a/src/data/cel.rs b/src/data/cel.rs
index fcce88b..09e1aba 100644
--- a/src/data/cel.rs
+++ b/src/data/cel.rs
@@ -44,7 +44,7 @@ impl Expression {
     }
 
     pub fn eval(&self) -> Value {
-        let mut ctx = Context::default();
+        let mut ctx = create_context();
         let Map { map } = self.build_data_map();
 
         ctx.add_function("getHostProperty", get_host_property);
@@ -103,6 +103,23 @@ fn get_host_property(This(this): This<Value>) -> ResolveResult {
     }
 }
 
+fn create_context<'a>() -> Context<'a> {
+    let mut ctx = Context::default();
+    ctx.add_function("charAt", strings::char_at);
+    ctx.add_function("indexOf", strings::index_of);
+    ctx.add_function("join", strings::join);
+    ctx.add_function("lastIndexOf", strings::last_index_of);
+    ctx.add_function("lowerAscii", strings::lower_ascii);
+    ctx.add_function("upperAscii", strings::upper_ascii);
+    ctx.add_function("trim", strings::trim);
+    ctx.add_function("replace", strings::replace);
+    ctx.add_function("split", strings::split);
+    ctx.add_function("substring", strings::substring);
+    ctx
+}
+
+mod strings;
+
 #[derive(Clone, Debug)]
 pub struct Predicate {
     expression: Expression,
diff --git a/src/data/cel/strings.rs b/src/data/cel/strings.rs
new file mode 100644
index 0000000..88fdca4
--- /dev/null
+++ b/src/data/cel/strings.rs
@@ -0,0 +1,366 @@
+use cel_interpreter::extractors::{Arguments, This};
+use cel_interpreter::{ExecutionError, ResolveResult, Value};
+use std::sync::Arc;
+
+pub fn char_at(This(this): This<Arc<String>>, arg: i64) -> ResolveResult {
+    match this.chars().nth(arg as usize) {
+        None => Err(ExecutionError::FunctionError {
+            function: "String.charAt".to_owned(),
+            message: format!("No index {arg} on `{this}`"),
+        }),
+        Some(c) => Ok(c.to_string().into()),
+    }
+}
+
+pub fn index_of(
+    This(this): This<Arc<String>>,
+    arg: Arc<String>,
+    Arguments(args): Arguments,
+) -> ResolveResult {
+    match args.len() {
+        1 => match this.find(&*arg) {
+            None => Ok((-1).into()),
+            Some(idx) => Ok((idx as u64).into()),
+        },
+        2 => {
+            let base = match args[1] {
+                Value::Int(i) => i as usize,
+                Value::UInt(u) => u as usize,
+                _ => {
+                    return Err(ExecutionError::FunctionError {
+                        function: "String.indexOf".to_owned(),
+                        message: format!(
+                            "Expects 2nd argument to be an Integer, got `{:?}`",
+                            args[1]
+                        ),
+                    })
+                }
+            };
+            if base >= this.len() {
+                return Ok((-1).into());
+            }
+            match this[base..].find(&*arg) {
+                None => Ok((-1).into()),
+                Some(idx) => Ok(Value::UInt((base + idx) as u64)),
+            }
+        }
+        _ => Err(ExecutionError::FunctionError {
+            function: "String.indexOf".to_owned(),
+            message: format!("Expects 2 arguments at most, got `{args:?}`!"),
+        }),
+    }
+}
+
+pub fn last_index_of(
+    This(this): This<Arc<String>>,
+    arg: Arc<String>,
+    Arguments(args): Arguments,
+) -> ResolveResult {
+    match args.len() {
+        1 => match this.rfind(&*arg) {
+            None => Ok((-1).into()),
+            Some(idx) => Ok((idx as u64).into()),
+        },
+        2 => {
+            let base = match args[1] {
+                Value::Int(i) => i as usize,
+                Value::UInt(u) => u as usize,
+                _ => {
+                    return Err(ExecutionError::FunctionError {
+                        function: "String.lastIndexOf".to_owned(),
+                        message: format!(
+                            "Expects 2nd argument to be an Integer, got `{:?}`",
+                            args[1]
+                        ),
+                    })
+                }
+            };
+            if base >= this.len() {
+                return Ok((-1).into());
+            }
+            match this[base..].rfind(&*arg) {
+                None => Ok((-1).into()),
+                Some(idx) => Ok(Value::UInt(idx as u64)),
+            }
+        }
+        _ => Err(ExecutionError::FunctionError {
+            function: "String.lastIndexOf".to_owned(),
+            message: format!("Expects 2 arguments at most, got `{args:?}`!"),
+        }),
+    }
+}
+
+pub fn join(This(this): This<Arc<Vec<Value>>>, Arguments(args): Arguments) -> ResolveResult {
+    let separator = args
+        .first()
+        .map(|v| match v {
+            Value::String(s) => Ok(s.as_str()),
+            _ => Err(ExecutionError::FunctionError {
+                function: "List.join".to_owned(),
+                message: format!("Expects seperator to be a String, got `{v:?}`!"),
+            }),
+        })
+        .unwrap_or(Ok(""))?;
+    Ok(this
+        .iter()
+        .map(|v| match v {
+            Value::String(s) => Ok(s.as_str().to_string()),
+            _ => Err(ExecutionError::FunctionError {
+                function: "List.join".to_owned(),
+                message: "Expects a list of String values!".to_owned(),
+            }),
+        })
+        .collect::<Result<Vec<_>, _>>()?
+        .join(separator)
+        .into())
+}
+
+pub fn lower_ascii(This(this): This<Arc<String>>) -> ResolveResult {
+    Ok(this.to_ascii_lowercase().into())
+}
+
+pub fn upper_ascii(This(this): This<Arc<String>>) -> ResolveResult {
+    Ok(this.to_ascii_uppercase().into())
+}
+
+pub fn trim(This(this): This<Arc<String>>) -> ResolveResult {
+    Ok(this.trim().into())
+}
+
+pub fn replace(This(this): This<Arc<String>>, Arguments(args): Arguments) -> ResolveResult {
+    match args.len() {
+        count @ 2..=3 => {
+            let from = match &args[0] {
+                Value::String(s) => s.as_str(),
+                _ => Err(ExecutionError::FunctionError {
+                    function: "String.replace".to_owned(),
+                    message: format!(
+                        "First argument of type String expected, got `{:?}`",
+                        args[0]
+                    ),
+                })?,
+            };
+            let to = match &args[1] {
+                Value::String(s) => s.as_str(),
+                _ => Err(ExecutionError::FunctionError {
+                    function: "String.replace".to_owned(),
+                    message: format!(
+                        "Second argument of type String expected, got `{:?}`",
+                        args[1]
+                    ),
+                })?,
+            };
+            if count == 3 {
+                let n = match &args[2] {
+                    Value::Int(i) => *i as usize,
+                    Value::UInt(u) => *u as usize,
+                    _ => Err(ExecutionError::FunctionError {
+                        function: "String.replace".to_owned(),
+                        message: format!(
+                            "Third argument of type Integer expected, got `{:?}`",
+                            args[2]
+                        ),
+                    })?,
+                };
+                Ok(this.replacen(from, to, n).into())
+            } else {
+                Ok(this.replace(from, to).into())
+            }
+        }
+        _ => Err(ExecutionError::FunctionError {
+            function: "String.replace".to_owned(),
+            message: format!("Expects 2 or 3 arguments, got {args:?}"),
+        }),
+    }
+}
+
+pub fn split(This(this): This<Arc<String>>, Arguments(args): Arguments) -> ResolveResult {
+    match args.len() {
+        count @ 1..=2 => {
+            let sep = match &args[0] {
+                Value::String(sep) => sep.as_str(),
+                _ => {
+                    return Err(ExecutionError::FunctionError {
+                        function: "String.split".to_string(),
+                        message: format!(
+                            "Expects a first argument of type String, got `{:?}`",
+                            args[0]
+                        ),
+                    })
+                }
+            };
+            let list = if count == 2 {
+                let pos = match &args[1] {
+                    Value::UInt(u) => *u as usize,
+                    Value::Int(i) => *i as usize,
+                    _ => Err(ExecutionError::FunctionError {
+                        function: "String.split".to_string(),
+                        message: format!(
+                            "Expects a second argument of type Integer, got `{:?}`",
+                            args[1]
+                        ),
+                    })?,
+                };
+                this.splitn(pos, sep)
+                    .map(|s| Value::String(s.to_owned().into()))
+                    .collect::<Vec<Value>>()
+            } else {
+                this.split(sep)
+                    .map(|s| Value::String(s.to_owned().into()))
+                    .collect::<Vec<Value>>()
+            };
+            Ok(list.into())
+        }
+        _ => Err(ExecutionError::FunctionError {
+            function: "String.split".to_owned(),
+            message: format!("Expects at most 2 arguments, got {args:?}"),
+        }),
+    }
+}
+
+pub fn substring(This(this): This<Arc<String>>, Arguments(args): Arguments) -> ResolveResult {
+    match args.len() {
+        count @ 1..=2 => {
+            let start = match &args[0] {
+                Value::Int(i) => *i as usize,
+                Value::UInt(u) => *u as usize,
+                _ => Err(ExecutionError::FunctionError {
+                    function: "String.substring".to_string(),
+                    message: format!(
+                        "Expects a first argument of type Integer, got `{:?}`",
+                        args[0]
+                    ),
+                })?,
+            };
+            let end = if count == 2 {
+                match &args[1] {
+                    Value::Int(i) => *i as usize,
+                    Value::UInt(u) => *u as usize,
+                    _ => Err(ExecutionError::FunctionError {
+                        function: "String.substring".to_string(),
+                        message: format!(
+                            "Expects a second argument of type Integer, got `{:?}`",
+                            args[0]
+                        ),
+                    })?,
+                }
+            } else {
+                this.chars().count()
+            };
+            if end < start {
+                Err(ExecutionError::FunctionError {
+                    function: "String.substring".to_string(),
+                    message: format!("Can't have end be before the start: `{end} < {start}"),
+                })?
+            }
+            Ok(this
+                .chars()
+                .skip(start)
+                .take(end - start)
+                .collect::<String>()
+                .into())
+        }
+        _ => Err(ExecutionError::FunctionError {
+            function: "String.substring".to_owned(),
+            message: format!("Expects at most 2 arguments, got {args:?}"),
+        }),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::data::Expression;
+    use cel_interpreter::Value;
+
+    #[test]
+    fn extended_string_fn() {
+        let e = Expression::new("'abc'.charAt(1)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "b".into());
+
+        let e = Expression::new("'hello mellow'.indexOf('')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), 0.into());
+        let e = Expression::new("'hello mellow'.indexOf('ello')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), 1.into());
+        let e = Expression::new("'hello mellow'.indexOf('jello')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), (-1).into());
+        let e = Expression::new("'hello mellow'.indexOf('', 2)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), 2.into());
+        let e =
+            Expression::new("'hello mellow'.indexOf('ello', 20)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), (-1).into());
+
+        let e = Expression::new("'hello mellow'.lastIndexOf('')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), 12.into());
+        let e =
+            Expression::new("'hello mellow'.lastIndexOf('ello')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), 7.into());
+        let e =
+            Expression::new("'hello mellow'.lastIndexOf('jello')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), (-1).into());
+        let e = Expression::new("'hello mellow'.lastIndexOf('ello', 6)")
+            .expect("This must be valid CEL");
+        assert_eq!(e.eval(), 1.into());
+        let e = Expression::new("'hello mellow'.lastIndexOf('ello', 20)")
+            .expect("This must be valid CEL");
+        assert_eq!(e.eval(), (-1).into());
+
+        let e = Expression::new("['hello', 'mellow'].join()").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "hellomellow".into());
+        let e = Expression::new("[].join()").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "".into());
+        let e = Expression::new("['hello', 'mellow'].join(' ')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "hello mellow".into());
+
+        let e = Expression::new("'TacoCat'.lowerAscii()").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "tacocat".into());
+        let e = Expression::new("'TacoCÆt Xii'.lowerAscii()").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "tacocÆt xii".into());
+
+        let e = Expression::new("'TacoCat'.upperAscii()").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "TACOCAT".into());
+        let e = Expression::new("'TacoCÆt Xii'.upperAscii()").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "TACOCÆT XII".into());
+
+        let e = Expression::new("'  \ttrim\n    '.trim()").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "trim".into());
+
+        let e =
+            Expression::new("'hello hello'.replace('he', 'we')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "wello wello".into());
+        let e = Expression::new("'hello hello'.replace('he', 'we', -1)")
+            .expect("This must be valid CEL");
+        assert_eq!(e.eval(), "wello wello".into());
+        let e = Expression::new("'hello hello'.replace('he', 'we', 1)")
+            .expect("This must be valid CEL");
+        assert_eq!(e.eval(), "wello hello".into());
+        let e = Expression::new("'hello hello'.replace('he', 'we', 0)")
+            .expect("This must be valid CEL");
+        assert_eq!(e.eval(), "hello hello".into());
+        let e = Expression::new("'hello hello'.replace('', '_')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "_h_e_l_l_o_ _h_e_l_l_o_".into());
+        let e = Expression::new("'hello hello'.replace('h', '')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "ello ello".into());
+
+        let e = Expression::new("'hello hello hello'.split(' ')").expect("This must be valid CEL");
+        assert_eq!(e.eval(), vec!["hello", "hello", "hello"].into());
+        let e =
+            Expression::new("'hello hello hello'.split(' ', 0)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), Value::List(vec![].into()));
+        let e =
+            Expression::new("'hello hello hello'.split(' ', 1)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), vec!["hello hello hello"].into());
+        let e =
+            Expression::new("'hello hello hello'.split(' ', 2)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), vec!["hello", "hello hello"].into());
+        let e =
+            Expression::new("'hello hello hello'.split(' ', -1)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), vec!["hello", "hello", "hello"].into());
+
+        let e = Expression::new("'tacocat'.substring(4)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "cat".into());
+        let e = Expression::new("'tacocat'.substring(0, 4)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "taco".into());
+        let e = Expression::new("'ta©o©αT'.substring(2, 6)").expect("This must be valid CEL");
+        assert_eq!(e.eval(), "©o©α".into());
+    }
+}