How can I validate exposed filter from View? I have build View which work for search exactly content by number filed "ICO".
How can I validate exposed filter form for only number and regular expression which enable only eight digit number?

Can you any help me with some example code for build it?
You should use hook_form_alter() to add a custom submit handler to the form like so:

$form['#submit'][] = 'my_custom_validation';

function my_custom_validation() {
  // Add the logic that you want in here
}


Then use form_set_error() to indicate if the value is wrong.