Julius Plenz – Blog

Perl: use feature;

With it's 5.10 version, Perl learned some pretty interesting new features. Some of them modify the syntactic interpretation of Perl code, so to stay backwards-compatible, you have to manually enable them. To do this, add a line like

use feature qw(:5.10);

to your other use declarations. (For one-liners, add the -E command line switch.) The documentation is available via perldoc feature or online.

There's one pretty trivial change that nonetheless has bothered me for a long time already. Why is there no function in Perl to print out some strings, and then finish with a newline? I mean, almost every sane programming language has this, and it helps writing clean code a lot (no fiddling with \n and so on). – Now, they backported the say function from Perl 6, which does just that: add an omplicit newline.

The new concept of state variables is nothing I need for now; however, the feature they strangely call switch (although the actual keywords are given/when) is pretty nice: It's pretty much like what you'd expect with a typical C construct like this:

switch(var) {
    case 1:
        /* do something */

        /* do default thing */

In Perl that's now:

given($var) {
    when(condition1) {
        # do something

    do_other_thing when condition2; # as one-liner

    default {
        # fallthrough case

Update: The statement when condition; one-liner doesn't work. Don't know where I got that from.

You can use simple strings (maps to $_ eq "string"), regular expressions (maps to $_ =~ /regex/) or function references like \&myfunc (maps to myfunc($_)).

The best improvement, however, are so-called named captures. (You don't have to enable those specifically.) It allows regexes to contain pairs of parentheses that you can assign a name to later extract the match. That means: no stupid and mind-boggling re-numbering of $2 to $3 etc., just because you added a set of brackets around something you want to extract from a string. Consider this example regex:

my $re = qr/

You can now access the user-part of the e-mail address via $+{user}, not necessarily via $1. The immediate gain is obvious: if you add parentheses around the whole expression (to capture the whole e-mail address) the user part is still available via $+{user}. The numbered variable would be $2 now, though.

Another side-effect is that you can test several regexes in a row now, and later extract matching parts from all of these – of course only if you use distinct capture names. Hash entries in %+ will only be updated upon a match (and won't be cleared AFAIK, at least in the lexical scope).

posted 2011-07-02 tagged perl