As the ACF GUI only gives the option for a maximum character limit, you can use the acf/validate_value filter along with the PHP: strlen function to count the number of characters. I would also add a note under Field Instructions so the user knows the limit before submitting.
Assuming you want a minimum of ten characters:
add_filter('acf/validate_value/name=validate_this_image', 'my_acf_validate_value', 10, 4);
function my_acf_validate_value( $valid, $value, $field, $input ){
// bail early if value is already invalid
if( !$valid ) {
return $valid;
}
// load data
$my_text_field = get_field('text_field');
if ( strlen($my_text_field) < 10 ) {
$valid = 'Must be a minimum of 10 characters.';
}
// return
return $valid;
}
