If a letter is found, the subject is matched against the first alternative; otherwise it isIf a letter is found, the subject is matched against the first alternative; otherwise it is matched against the second.
The \A, \Z, and \z assertions differ from the traditional circumflex and dollar (described below) in that they only ever match at the very start and end of the subject string, whatever options are set. They are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options. The difference between \Z and \z is that \Z matches before a newline that is the last character of the string as well as at the end of the string, whereas \z matches only at the end.