added pin.go to contain all PIN related functionality

common/pin.go

@@ -0,0 +1,14 @@
+package common
+
+import "regexp"
+
+// Define the regex pattern as a constant
+const (
+	pinPattern = `^\d{4}$`
+)
+
+// checks whether the given input is a 4 digit number
+func IsValidPIN(pin string) bool {
+	match, _ := regexp.MatchString(pinPattern, pin)
+	return match
+}