Drupal 7 custom module form validate - drupal

Question

I'm new to Drupal 7 enviroment. I want to create a custom module form and validate it. I'm facing problems in custom validating the form. Please help. I am providing the code below.
<?php
//implementing hook permissions
function userform_2_permission(){
return array(
'submit userform_2' => array(
'title' => t('Submit Userform_2'),
'description' => t('Submit username in the field'),
),
);
}
// implementing hook menu
function userform_2_menu(){
$items = array();
$items['userform_2'] = array(
'title' => 'Userform 2',
'description' => 'Input the username',
'type' => MENU_NORMAL_ITEM,
'access arguments' => array('access userform_2'),
'page callback' => 'drupal_get_form',
'page arguments' => array('userform_2_form'),
);
return $items;
}
// implementing form
function userform_2_form($form,&$form_state){
$form['username'] = array(
'#type' => 'textfield',
'#title' => t('Username'),
'#description' => t('Please provide your username'),
'#size' => 60,
'#maxlength' => 128,
);
$form['password'] = array(
'#title' => t('Password'),
'#type' => 'password', // it provdes the password + password_confirm field
'#size' => 60,
'#description' => 'Please provide a password',
'#maxlength' => 128,
);
$form['submit'] = array(
'#type' => 'submit',
'#value' => t('Login'),
);
return $form;
}
I want to add form validation here. Please provide the solution for this.
function useform_2_form_validate($form,&$form_state){
// please provide solution
}
// implementing submit handler
function userform_2_form_submit($form, &$form_state){
$u_id = db_insert('userform_2') -> fields(array(
'username' => $form_state['values']['username'],
'password' => $form_state['values']['password'],
)) ->execute();
drupal_set_message(t('the username has been added'));
}
?>

Answer 1

Attach custom validation :
$form['#validate'][] = 'useform_2_form_validate';
But I think if you search a little more you can do it alone ;)
https://api.drupal.org/api/drupal/developer!topics!forms_api_reference.html/7.x/#validation