Merge pull request #1849 from tordans/patch-5

Readme: Add example for string validation for an optional field to chapter "Unions"

README.md

@@ -1284,6 +1284,25 @@ For convenience, you can also use the [`.or` method](#or):
 const stringOrNumber = z.string().or(z.number());
 ```
 
+**Optional string validation:**
+
+To validate an optional form input, you can union the desired string validation with an empty string [literal](#literals).
+
+This example validates an input that is optional but needs to contain a [valid URL](#strings):
+
+```ts
+const optionalUrl = z.union( [
+    z.string().url().nullish(),
+    z.literal( '' ),
+] )
+
+console.log( optionalUrl.safeParse( undefined ).success ) // true
+console.log( optionalUrl.safeParse( null ).success ) // true
+console.log( optionalUrl.safeParse( '' ).success ) // true
+console.log( optionalUrl.safeParse( 'https://zod.dev' ).success ) // true
+console.log( optionalUrl.safeParse( 'not a valid url' ).success ) // false
+```
+
 ## Discriminated unions
 
 A discriminated union is a union of object schemas that all share a particular key.