How to write a validator for field validation 7.x-2.x

In field validation 7.x-2.x, validator is a plugin of Ctools, it is very easy to write your own validator, here is an example :

<?php
/**
 * @file
 * Field validation email validator.
 *
 */
$plugin = array(
  'label' => t('Email'),
  'description' => t("Verifies that user-entered data is a valid email address."),
  'handler' => array(
    'class' => 'field_validation_email_validator',
  ),
);

class field_validation_email_validator extends field_validation_validator {

  /**
   * Validate field. 
   */
  public function validate() {
    if ($this->value != '' && (!valid_email_address($this->value))) {
      $this->set_error();
    }
  }

}

It is very simple, this is the email validator shipped with field-validation 7.x-2.0-beta2.

You could copy a validator from field validation, and edit it to meet your requirement.

If you want to set some argument for your validator , here is another example:

$plugin = array(
  'label' => t('Numeric values'),
  'description' => t('Verifies that user-entered values are numeric, with the option to specify min and / or max values.'),
  'handler' => array(
    'class' => 'field_validation_numeric2_validator',
  ),
);

class field_validation_numeric2_validator extends field_validation_validator {

  /**
   * Validate field. 
   */
  public function validate() {
    $settings = $this->rule->settings;
    if ($this->value != '') {
      $flag = TRUE;
      if (!is_numeric($this->value)) {
        $flag = FALSE;
      }
      else{
        if (isset($settings['min']) && $settings['min'] != '' && $this->value < $settings['min']) {
          $flag = FALSE;
        }
        if (isset($settings['max']) && $settings['max'] != '' && $this->value > $settings['max']) {
          $flag = FALSE;
        }       
      }
      if (!$flag) {
        $token = array(
          '[min]' => isset($settings['min']) ? $settings['min'] : '',
          '[max]' => isset($settings['max']) ? $settings['max'] : '',
        );
        $this->set_error($token);
      }
    }
  }
  
  /**
   * Provide settings option
   */
  function settings_form(&$form, &$form_state) {
    $default_settings = $this->get_default_settings($form, $form_state);
    //print debug($default_settings);
    $form['settings']['min'] = array(
      '#title' => t('Minimum value'),
      '#description' => t("Optionally specify the minimum value to validate the user-entered numeric value against."),
      '#type' => 'textfield',
      '#default_value' => isset($default_settings['min']) ? $default_settings['min'] : '',
    );
    $form['settings']['max'] = array(
      '#title' => t('Maximum value'),
      '#description' => t("Optionally specify the maximum value to validate the user-entered numeric value against."),
      '#type' => 'textfield',
      '#default_value' => isset($default_settings['max']) ? $default_settings['max'] : '',
    );
    parent::settings_form($form, $form_state);
  }
  
  /**
   * Provide token help info for error message.
   */
  public function token_help() {
    $token_help = parent::token_help();
    $token_help += array(
      '[min]' => t('Minimum value'), 
      '[max]' => t('Maximum value'),
    );
    return $token_help;
  }
}

This is the numeric validator shipped with field validation.

For a custom validator, create a custom module and add these lines to the .module file:

<?php
function mymodule_ctools_plugin_directory($module, $plugin) {
  if ($module == 'field_validation' && $plugin == 'validator') {
    return 'plugins/' . $plugin;
  }
}

Then add the following folder inside the module:
plugins
and
plugins/validator

Place the validator file insite the validator folder and clear your cache.

Now you should be able to use it via the UI / validators.