The "alpha" class (correct syntax is [:alpha:]) matches any "letter". The meaning of "letter" depends on the locale, in en-US locale this includes A-Z and accented characters in ISO-8859 encoding.
You can easily find out what it does on your system using a test code like this:
$all = implode(' ', range("!","\\xFF"));
'<font color=red>$0</font>', $all);