How to use preg_match function to test if a string has the allowed characters?

Hi Experts

Could you point a what must be changed  at preg_match function  to test if a string in a manner that:



Accept  only
"_"   (underline)
Any alphabetical character
Any numerical digit


So, doesn't accept:
"."    (point)
" "    (blank spaces)


Accordingly to:

  function alpha_only_space($str)
    {

        if (!preg_match("/^([-a-z ])+$/i", $str))
        {
            $this->form_validation->set_message('alpha_only_space', 'O nome deste %s campo deve conter apenas alfanuméricos');
            return FALSE;
        }
        else
        {
            return TRUE;
        }
    }
    

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Thanks in advance!