Programming PHP

A simple use of positive lookahead is splitting a Unix mbox mail file into individual
messages. The word "From" starting a line by itself indicates the start of a new message,
so you can split the mailbox into messages by specifying the separator as the
point where the next text is "From" at the start of a line:
$messages = preg_split('/(?=^From )/m', $mailbox);
A simple use of negative lookbehind is to extract quoted strings that contain quoted
delimiters. For instance, here’s how to extract a single-quoted string (note that the
regular expression is commented using the x modifier):
$input =