diff --git a/CLAUDE.md b/CLAUDE.md
index 15f587d..302c813 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -8,6 +8,7 @@ Greenfield project. Implementation is driven by the Vexa AI-SDLC pipeline (Jira
/
├── index.html # Calculator UI (semantic HTML5)
├── styles.css # Responsive CSS layout and styling
+├── validator.js # Numeric input validator (validateNumericInput)
├── README.md # Project overview
└── CLAUDE.md # This file
```
diff --git a/index.html b/index.html
index ccb86de..eeaf14a 100644
--- a/index.html
+++ b/index.html
@@ -5,6 +5,7 @@
Simple Calculator
+
diff --git a/validator.js b/validator.js
new file mode 100644
index 0000000..162126b
--- /dev/null
+++ b/validator.js
@@ -0,0 +1,47 @@
+/**
+ * Validates a numeric input string.
+ *
+ * Allowed characters: digits 0-9, a single decimal point, a leading minus sign.
+ * Returns an object with:
+ * isValid {boolean} - true when the string represents a well-formed number
+ * value {number|null} - parsed numeric value, or null when invalid
+ * sanitized {string} - trimmed input string
+ */
+function validateNumericInput(input) {
+ var str = String(input).trim();
+ var result = { isValid: false, value: null, sanitized: str };
+
+ if (str === '') {
+ return result;
+ }
+
+ // Reject characters other than digits, decimal point, and minus sign
+ if (/[^0-9.\-]/.test(str)) {
+ return result;
+ }
+
+ // Minus sign is only valid as the very first character
+ var minusCount = (str.match(/-/g) || []).length;
+ if (minusCount > 1 || (minusCount === 1 && str.charAt(0) !== '-')) {
+ return result;
+ }
+
+ // Only one decimal point is allowed
+ if ((str.match(/\./g) || []).length > 1) {
+ return result;
+ }
+
+ // A bare '-' or '.' is not a valid number
+ if (str === '-' || str === '.') {
+ return result;
+ }
+
+ var parsed = parseFloat(str);
+ if (isNaN(parsed)) {
+ return result;
+ }
+
+ result.isValid = true;
+ result.value = parsed;
+ return result;
+}