Drupal 7 Module Development: Validate

espn_news.module

<?php

  /**
   * Implements hook_validate().
   *
   * Validation for configuration form. Since we need a number in the
   * limit-textfield, we check that the input is correct.
   *
   * @url: https://api.drupal.org/api/drupal/modules!node!node.api.php/function/hook_validate/7
   */
  function espn_news_form_validate($form, &$form_state) {
    // Get current value from input.
    $espn_news_limit = $form_state['values']['espn_news_limit'];

    // Check that input is a number.
    if (!is_numeric($espn_news_limit)) {
      form_set_error('espn_news_limit', t('Limit must be a number.'));
    }
    // Here you can check for negative numbers.
    
  }