diff --git a/src/sort/i64.rs b/src/sort/i64.rs
index fef30925..977d3bdf 100644
--- a/src/sort/i64.rs
+++ b/src/sort/i64.rs
@@ -59,6 +59,13 @@ impl Sort for I64Sort {
 
         add_primitives!(typeinfo, "to-string" = |a: i64| -> Symbol { a.to_string().into() });
 
+        // Must be in the i64 sort register function because the string sort is registered before the i64 sort.
+        typeinfo.add_primitive(CountMatches {
+            name: "count-matches".into(),
+            string: typeinfo.get_sort(),
+            int: self.clone(),
+        });
+
     }
 
     fn make_expr(&self, _egraph: &EGraph, value: Value) -> (Cost, Expr) {
@@ -83,3 +90,32 @@ impl FromSort for i64 {
         value.bits as Self
     }
 }
+
+struct CountMatches {
+    name: Symbol,
+    string: Arc<StringSort>,
+    int: Arc<I64Sort>,
+}
+
+impl PrimitiveLike for CountMatches {
+    fn name(&self) -> Symbol {
+        self.name
+    }
+
+    fn accept(&self, types: &[ArcSort]) -> Option<ArcSort> {
+        if types.len() == 2
+            && types[0].name() == self.string.name
+            && types[1].name() == self.string.name
+        {
+            Some(self.int.clone())
+        } else {
+            None
+        }
+    }
+
+    fn apply(&self, values: &[Value]) -> Option<Value> {
+        let string1 = Symbol::load(&self.string, &values[0]).to_string();
+        let string2 = Symbol::load(&self.string, &values[1]).to_string();
+        Some(Value::from(string1.matches(&string2).count() as i64))
+    }
+}
diff --git a/src/sort/string.rs b/src/sort/string.rs
index a8ffb03c..34bbac78 100644
--- a/src/sort/string.rs
+++ b/src/sort/string.rs
@@ -6,7 +6,7 @@ use super::*;
 
 #[derive(Debug)]
 pub struct StringSort {
-    name: Symbol,
+    pub name: Symbol,
 }
 
 impl StringSort {
@@ -33,6 +33,10 @@ impl Sort for StringSort {
     fn register_primitives(self: Arc<Self>, typeinfo: &mut TypeInfo) {
         typeinfo.add_primitive(Add {
             name: "+".into(),
+            string: self.clone(),
+        });
+        typeinfo.add_primitive(Replace {
+            name: "replace".into(),
             string: self,
         });
     }
@@ -85,3 +89,34 @@ impl PrimitiveLike for Add {
         Some(Value::from(res_symbol))
     }
 }
+
+struct Replace {
+    name: Symbol,
+    string: Arc<StringSort>,
+}
+
+impl PrimitiveLike for Replace {
+    fn name(&self) -> Symbol {
+        self.name
+    }
+
+    fn accept(&self, types: &[ArcSort]) -> Option<ArcSort> {
+        if types.len() == 3
+            && types[0].name() == self.string.name
+            && types[1].name() == self.string.name
+            && types[2].name() == self.string.name
+        {
+            Some(self.string.clone())
+        } else {
+            None
+        }
+    }
+
+    fn apply(&self, values: &[Value]) -> Option<Value> {
+        let string1 = Symbol::load(&self.string, &values[0]).to_string();
+        let string2 = Symbol::load(&self.string, &values[1]).to_string();
+        let string3 = Symbol::load(&self.string, &values[2]).to_string();
+        let res: Symbol = string1.replace(&string2, &string3).into();
+        Some(Value::from(res))
+    }
+}
diff --git a/tests/string.egg b/tests/string.egg
index 34e84b3a..74a879ef 100644
--- a/tests/string.egg
+++ b/tests/string.egg
@@ -1,2 +1,8 @@
 ; Tests for the string sort
+
+; Concatenation
 (check (= (+ "a" "bc" "de") "abcde"))
+; Counting the number of substring occurances
+(check (= (count-matches "ab ab" "ab") 2))
+; replacing a substring
+(check (= (replace "ab ab" "ab" "cd") "cd cd"))