perlmeditation japhy <b>Perl Idioms Explained - <c>my $count = () = /.../g</c></b> <c> my $str = "here are some words"; my $count = () = $str =~ /\S+/g; </c> The "empty parentheses" idiom is one that is used to force list context on an expression, and yet return a scalar: the count of items in that list. We are often told that there is no way to get the size of a list -- it must either be iterated over, or stored in an array, or some other means -- because when you <i>try</i> treating a list as a scalar, you're no longer dealing with a list: you're enforcing scalar context on some expression. Here are some examples: <c> my $last_operand = ("a", "b", "c"); # "c" my $true_or_false = /\w+/g; # 1 or "" my $dunno = some_function(); # who knows?! </c> The last example is particularly nasty: the return value of the function will be evaluated in scalar context. If that value is <c>@array</c>, then you'll get the size of the array; if it's <c>@array[0,1]</c>, you'll get <c>$array[1]</c>. <p> So how does the empty parentheses idiom work? It uses a little-exercised rule: <b>a list assignment in scalar context returns the number of elements on the right-hand side of the list assignment</b>. That's probably also rarely-spoken, since it's a bit dense. What it means is that an expression like <c>(LIST1) = (LIST2)</c>, in scalar context, returns the number of elements in <c>LIST2</c> (not <c>LIST1</c>). This means <c>(LIST1)</c> could be replaced with <c>()</c> and the scalar value returned would still be the size of <c>LIST2</c>. <p> This trick is most frequently used in conjunction with a regex with the "global" switch on it. In scalar context, such a regex would only match once, but in list context, it matches exhaustively. To get all such matches, one would do <c>@matches = /.../g</c>. But if we aren't interested in keeping the matches around, and instead just want to know how many there are, we could replace <c>@matches</c> with <c>()</c>, and evaluate it all in scalar context: <c>my $count = (() = /.../g);</c>. Of course, those outer parentheses are unnecessary, so the idiom becomes <c>my $count = () = /.../g;</c>. <p> <b>Summary</b> <p> It forces an expression to be evaluated in list context, and returns the number of elements returned by that expression. <!-- Node text goes above. Div tags should contain sig only --> <div class="pmsig"><div class="pmsig-1936"> <hr/> Jeff <tt><font color="#0000ff">[japhy]</font></tt> Pinyan, [id://371157|P.L., P.M., P.O.D, X.S.]: <a href="http://japhy.perlmonk.org/modules/">Perl</a>, <a href="http://japhy.perlmonk.org/modules/Regexp-Parser/">regex</a>, and <a href="http://lists.perl.org/showlist.cgi?name=perl5-porters"><tt>perl</tt></a> <a href="http://foldoc.doc.ic.ac.uk/foldoc/foldoc.cgi?query=hacker">hacker</a> <br> <i>How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ <b>Meister Eckhart</b></i> </div></div>