Adds valid_json which is using json_last_error() === JSON_ERROR_NONE

system/Validation/FormatRules.php

@@ -241,6 +241,21 @@ public function valid_base64(string $str = null): bool
 
 	//--------------------------------------------------------------------
 
+	/**
+	 * Valid JSON
+	 *
+	 * @param	string
+	 *
+	 * @return	bool
+	 */
+	public function valid_json(string $str = null): bool
+	{
+		json_decode($str);
+		return json_last_error() === JSON_ERROR_NONE;
+	}
+
+	//--------------------------------------------------------------------
+
 	/**
 	 * Checks for a correctly formatted email address
 	 *

tests/system/Validation/FormatRulesTest.php

@@ -596,6 +596,44 @@ public function base64Provider()
 
 	//-------------------------------------------------------------------
 
+	/**
+	 * @dataProvider jsonProvider
+	 *
+	 * @param $str
+	 * @param $expected
+	 */
+	public function testJson($first, $expected)
+	{
+		$data = [
+			'foo' => $first,
+		];
+
+		$this->validation->setRules([
+			'foo' => "valid_json",
+		]);
+
+		$this->assertEquals($expected, $this->validation->run($data));
+	}
+
+	//--------------------------------------------------------------------
+
+	public function jsonProvider()
+	{
+		return [
+			['null', true],
+			['"null"', true],
+			['600100825', true],
+			['{"A":"Yay.", "B":[0,5]}', true],
+			['[0,"2",2.2,"3.3"]', true],
+			[null, false],
+			['600-Nope. Should not pass.', false],
+			['{"A":SHOULD_NOT_PASS}', false],
+			['[0,"2",2.2 "3.3"]', false]
+		];
+	}
+
+	//-------------------------------------------------------------------
+
 	/**
 	 * @dataProvider timezoneProvider
 	 *

user_guide_src/source/libraries/validation.rst

@@ -670,6 +670,7 @@ is_unique               Yes         Checks if this field value exists in the dat
                                     column and value to ignore, useful when updating records to ignore itself.
 timezone                No          Fails if field does match a timezone per ``timezone_identifiers_list``
 valid_base64            No          Fails if field contains anything other than valid Base64 characters.
+valid_json              No          Fails if field does not contain a valid JSON string.
 valid_email             No          Fails if field does not contain a valid email address.
 valid_emails            No          Fails if any value provided in a comma separated list is not a valid email.
 valid_ip                No          Fails if the supplied IP is not valid. Accepts an optional parameter of ‘ipv4’ or               valid_ip[ipv6]