diff --git a/relog-web/package-lock.json b/relog-web/package-lock.json
index 8e075b9..118b5f4 100644
--- a/relog-web/package-lock.json
+++ b/relog-web/package-lock.json
@@ -16,6 +16,7 @@
         "d3-array": "^2.12.1",
         "dagre": "^0.8.5",
         "idb": "^6.1.5",
+        "jsep": "^1.3.8",
         "leaflet": "^1.8.0",
         "react": "^17.0.2",
         "react-dom": "^17.0.2",
@@ -11201,6 +11202,14 @@
         }
       }
     },
+    "node_modules/jsep": {
+      "version": "1.3.8",
+      "resolved": "https://registry.npmjs.org/jsep/-/jsep-1.3.8.tgz",
+      "integrity": "sha512-qofGylTGgYj9gZFsHuyWAN4jr35eJ66qJCK4eKDnldohuUoQFbU3iZn2zjvEbd9wOAhP9Wx5DsAAduTyE1PSWQ==",
+      "engines": {
+        "node": ">= 10.16.0"
+      }
+    },
     "node_modules/jsesc": {
       "version": "2.5.2",
       "resolved": "https://registry.npmjs.org/jsesc/-/jsesc-2.5.2.tgz",
@@ -25021,6 +25030,11 @@
         "xml-name-validator": "^3.0.0"
       }
     },
+    "jsep": {
+      "version": "1.3.8",
+      "resolved": "https://registry.npmjs.org/jsep/-/jsep-1.3.8.tgz",
+      "integrity": "sha512-qofGylTGgYj9gZFsHuyWAN4jr35eJ66qJCK4eKDnldohuUoQFbU3iZn2zjvEbd9wOAhP9Wx5DsAAduTyE1PSWQ=="
+    },
     "jsesc": {
       "version": "2.5.2",
       "resolved": "https://registry.npmjs.org/jsesc/-/jsesc-2.5.2.tgz",
diff --git a/relog-web/package.json b/relog-web/package.json
index 67e313c..ce0553a 100644
--- a/relog-web/package.json
+++ b/relog-web/package.json
@@ -17,6 +17,7 @@
     "d3-array": "^2.12.1",
     "dagre": "^0.8.5",
     "idb": "^6.1.5",
+    "jsep": "^1.3.8",
     "leaflet": "^1.8.0",
     "react": "^17.0.2",
     "react-dom": "^17.0.2",
diff --git a/relog-web/src/casebuilder/expr.js b/relog-web/src/casebuilder/expr.js
new file mode 100644
index 0000000..d871a34
--- /dev/null
+++ b/relog-web/src/casebuilder/expr.js
@@ -0,0 +1,50 @@
+import { Jsep } from "jsep";
+import { exportValue } from "./export";
+
+export const evaluateExpr = (expr, data) => {
+  const node = Jsep.parse(expr);
+  return evaluateNode(node, data);
+};
+
+const evaluateNode = (node, data) => {
+  if (node.type == "BinaryExpression") {
+    return evaluateBinaryExprNode(node, data);
+  } else if (node.type == "UnaryExpression") {
+    return evaluateUnaryExprNode(node, data);
+  } else if (node.type == "Literal") {
+    return node.value;
+  } else if (node.type == "Identifier") {
+    return data[node.name];
+  } else {
+    throw `Unknown type: ${node.type}`;
+  }
+};
+
+const evaluateBinaryExprNode = (node, data) => {
+  const leftVal = evaluateNode(node.left, data);
+  const rightVal = evaluateNode(node.right, data);
+  if (node.operator == "+") {
+    return leftVal + rightVal;
+  } else if (node.operator == "*") {
+    return leftVal * rightVal;
+  } else if (node.operator == "/") {
+    return leftVal / rightVal;
+  } else if (node.operator == "-") {
+    return leftVal - rightVal;
+  } else if (node.operator == "^") {
+    return Math.pow(leftVal, rightVal);
+  } else {
+    throw `Unknown operator: ${node.operator}`;
+  }
+};
+
+const evaluateUnaryExprNode = (node, data) => {
+  const arg = evaluateNode(node.argument, data);
+  if (node.operator == "+") {
+    return arg;
+  } else if (node.operator == "-") {
+    return -arg;
+  } else {
+    throw `Unknown operator: ${node.operator}`;
+  }
+};
diff --git a/relog-web/src/casebuilder/expr.test.js b/relog-web/src/casebuilder/expr.test.js
new file mode 100644
index 0000000..ddd293a
--- /dev/null
+++ b/relog-web/src/casebuilder/expr.test.js
@@ -0,0 +1,19 @@
+import { evaluateExpr } from "./expr";
+
+test("parse expression", () => {
+  // Basic expressions
+  expect(evaluateExpr("1 + 1")).toEqual(2);
+  expect(evaluateExpr("2 * 5")).toEqual(10);
+  expect(evaluateExpr("2 * (3 + 5)")).toEqual(16);
+  expect(evaluateExpr("14 / 2")).toEqual(7);
+  expect(evaluateExpr("10 - 3")).toEqual(7);
+  expect(evaluateExpr("-10")).toEqual(-10);
+  expect(evaluateExpr("+10")).toEqual(10);
+  expect(evaluateExpr("2^3")).toEqual(8);
+  expect(evaluateExpr("2^(3 + 1)")).toEqual(16);
+
+  // With data
+  expect(evaluateExpr("x + 1", { x: 10 })).toEqual(11);
+  expect(evaluateExpr("2 ^ (3 + x)", { x: 1 })).toEqual(16);
+  expect(evaluateExpr("x + y", { x: 1, y: 2 })).toEqual(3);
+});