Update API.md with string.regex invalidate config

Author: WesTyler

API.md

@@ -1542,14 +1542,26 @@ const schema = Joi.object({
 });
 ```
 
-#### `string.regex(pattern, [name])`
+#### `string.regex(pattern, [name | config])`
 
 Defines a regular expression rule where:
 - `pattern` - a regular expression object the string value must match against.
 - `name` - optional name for patterns (useful with multiple patterns). Defaults to 'required'.
+- `config` - an optional configuration object with the following supported properties:
+  - `name` - optional pattern name. Defaults to `required`.
+  - `invalidate` - optional boolean flag. Defaults to `false` behavior. If specified as `true`, the provided pattern will be disallowed instead of required.
 
 ```js
 const schema = Joi.string().regex(/^[abc]+$/);
+
+const namedSchema = Joi.string().regex(/[0-9]/, { name: 'numbers'});
+namedSchema.validate('alpha'); // ValidationError: "value" with value "alpha" fails to match the numbers pattern
+
+const invalidatedSchema = Joi.string().regex(/[a-z]/, { invalidate: true });
+invalidatedSchema.validate('lowercase'); // ValidationError: "value" with value "lowercase" matches the invalidated pattern: [a-z]
+
+const invalidatedNamedSchema = Joi.string().regex(/[a-z]/, { name: 'alpha', invalidate: true });
+invalidatedNamedSchema.validate('lowercase'); // ValidationError: "value" with value "lowercase" matches the invalidated alpha pattern
 ```
 
 #### `string.replace(pattern, replacement)`