PHP Regex validate letters and Spanish accent

As found in the answer to this question, you could match accented characters using the full Letter Unicode property \p{L}. That includes regular a-z characters along with accented ones, so replace a-z in your expression with this like so:

$reg = "#[^\p{L}\s-]#u";

Note that to use this you need the UTF-8 modifier u after your closing delimiter, as the docs say such Unicode "character types are available when UTF-8 mode is selected".