WooCommerce: Enforce minimum length phone number field

I have installed WooCommerce in my WordPress based website. Now my problem is when a customer checks out or creates an ID, then there is a field where the user can insert his phone number. That field accepts 9 numbers, so I want to apply a minimum length function on that field so the user will be prompted with an error message.

I have tried by adding these lines in function.php:

add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );
function custom_override_checkout_fields( $fields )
{        
     $fields['billing']['billing_phone']['minlength'] = 10;      
     return $fields;    
}

but this is not working and the strange thing is that when I use

['maxlength'] = 10; 

it actually works.

Answer

By default WooCommerce checkout fields support the following attributes for the fields

$defaults = array(
    'type'              => 'text',
    'label'             => '',
    'description'       => '',
    'placeholder'       => '',
    'maxlength'         => false,
    'required'          => false,
    'id'                => $key,
    'class'             => array(),
    'label_class'       => array(),
    'input_class'       => array(),
    'return'            => false,
    'options'           => array(),
    'custom_attributes' => array(),
    'validate'          => array(),
    'default'           => '',
);

You can add custom attributes by passing an array to custom_attributes

add_filter( 'woocommerce_checkout_fields' , 'custom_override_checkout_fields' );
function custom_override_checkout_fields( $fields )
{        
     $fields['billing']['billing_phone']['custom_attributes'] = array( "minlength" => "12" );      
     return $fields;    
}

Which would produce the following HTML

<input type="text" minlength="12" value="" placeholder="" id="billing_phone" name="billing_phone" class="input-text ">

If minlength doesn’t work ( and I have a suspicion that it probably won’t ) , try using the pattern attribute

$fields['billing']['billing_phone']['custom_attributes'] = array( "pattern" => ".{12,}" ); //min 12 characters