feat: add string rule to the core rules

README.md

@@ -948,6 +948,12 @@ Field under this rule may be empty.
 
 </details>
 
+<details><summary><strong>string</strong></summary>
+
+Field under this rule must be a string.
+
+</details>
+
 ## Register/Override Rule
 
 Another way to use custom validation rule is to create a class extending `Rakit\Validation\Rule`.

src/Rules/String.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace Rakit\Validation\Rules;
+
+use Rakit\Validation\Rule;
+
+class String extends Rule
+{
+
+    /** @var string */
+    protected $message = "The :attribute only allows a string";
+
+    /**
+     * Check the $value is valid
+     *
+     * @param mixed $value
+     * @return bool
+     */
+    public function check($value): bool
+    {
+        return is_string($value);
+    }
+}

src/Validator.php

@@ -162,6 +162,7 @@ protected function registerBaseValidators()
             'defaults'                  => new Rules\Defaults,
             'default'                   => new Rules\Defaults, // alias of defaults
             'nullable'                  => new Rules\Nullable,
+            'string'                    => new Rule\String,
         ];
 
         foreach ($baseValidator as $key => $validator) {

tests/Rules/StringTest.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Rakit\Validation\Tests;
+
+use Rakit\Validation\Rules\String;
+use PHPUnit\Framework\TestCase;
+use stdClass;
+
+class StringTest extends TestCase
+{
+
+    public function setUp()
+    {
+        $this->rule = new String;
+    }
+
+    public function testValids()
+    {
+        $this->assertTrue($this->rule->check('foo'));
+        $this->assertTrue($this->rule->check('123asd'));
+        $this->assertTrue($this->rule->check('asd123'));
+        $this->assertTrue($this->rule->check('foo123bar'));
+        $this->assertTrue($this->rule->check('foo bar'));
+        $this->assertTrue($this->rule->check('<p><a href="#">Lorem ipsum dolor sit amet</a> cum omnis voluptatum! </p>'));
+    }
+
+    public function testInvalids()
+    {
+        $this->assertFalse($this->rule->check(2));
+        $this->assertFalse($this->rule->check([]));
+        $this->assertFalse($this->rule->check(new stdClass));
+    }
+}