trigger onAccount on Account (before insert, before update) {
    for(Account a: Trigger.new){
        if(!Pattern.matches('[ABCDEFGHJKLMNPQRSUVW][0-9][0-9][0-9][0-9][0-9][0-9][0-9][0-9A-J]', a.cif__c)){
            a.cif__c.addError('CIF wrong format');
        }else{
            if(!validateCif(a)){
            	a.cif__c.addError('CIF invalid');
        	}
        }
    }

    public boolean validateCif(Account a){
        String[] letters = new String[]{'J','A','B','C','D','E','F','G','H','I'};
        String digits = a.cif__c.substring(1, a.cif__c.length()-1);
        String letter = a.cif__c.substring(0,1);
        String control = a.cif__c.substring(a.cif__c.length() -1);
        Integer sum = 0;
        Integer i;
        Integer digit;

        for(i=0;i<digits.length();i++){
            digit = Integer.valueOf(digits.substring(i,i+1));
            system.debug('digit: '+digit);

            if(Math.mod(i, 2) == 0){
                digit *= 2;
                if(digit > 9){
                    digit = (digit / 10) + (Math.mod(digit, 10));
                }
                sum += digit;
            }else{
                sum += digit;
            }
        }

        sum = Math.mod(sum, 10);
        if(sum != 0){
            digit = 10 - sum;
        }else{
            digit = sum;
        }

        if(Pattern.matches('[ABEH]', letter)){
            return String.valueOf(digit) == control;
        }
        if(Pattern.matches('[NPQRSW]', letter)){
            return letters[digit] == control;
        }
        return String.valueOf(digit) == control || letters[digit] == control;
    }
}