Apply your own custom validation to a field

gistfile1.phtml

/*

In this code sample, you can learn how to apply custom
validations on any fields you want.

This can be done using these steps:

1. Edit the field in backend (field modal) 
2. Where it asks for field validation choose "Custom"
3. Enter a unique validation action name e.g. my_8numbers

Now place the following function e.g. in theme functions.php to place 
your new "my_8numbers" which will apply to that field.

in this example, the function registers your new validation and
return an error if the input was not a 8-numbers value. this can
be useful to test specific formula/validation e.g. a serial number

Note how "my_8numbers" is suffix in the function for other validations
just change that name, and also change the rules for validation as you like

This line: $ultimatemember->form->add_error($key, __('Enter a valid 8-number serial please') );
make sure that form does not submit and return error on screen so user can correct their input.

*/

add_action('um_custom_field_validation_my_8numbers','um_custom_field_validation_my_8numbers', 10, 3);
function um_custom_field_validation_my_8numbers( $key, $array, $args ) {
	global $ultimatemember;
	if ( !ctype_digit( $args[ $key ] ) || strlen( $args[ $key ] ) != 8 ) {
		$ultimatemember->form->add_error($key, __('Enter a valid 8-number serial please') );
	}
}

Hi,
I want to collect phone number in format: + < country-code> -<10 digit phone>.

Please support.