$_against this pattern: the beginning of the string (
^) followed by zero or more whitespace characters (
\s*), then a semicolon (
;), then possibly some more whitespace (
\s*). If it matches, the matching part of the string is stripped out (ie, substituted with nothing) and the whole expression has a true value. If not, it just has a false value.
s///) are true if the substitute pattern applied and was performed and false if they weren't.