perlmeditation
tachyon
<p>Consider that you want a regex to find lots of things in a string and store them into an array. Perl has a very convenient idiom for this:
<code>
@ary = $str =~ m/(stuff)/g;
</code>
<p>If you are after a single match you use a scalar in list context as the L-VALUE ie
<code>
($scalar) = $str =~ m/(this)/;
</code>
<p>Note if you forget the ( ) around $scalar and you get a match $scalar will contain the integer value 1 so don't forget the ( ). The ( ) gets you list context which you need.
<p>Anyway, although this might not make a lot of sense at first glance it is really very simple. If we had just this:
<code>
$str =~ m/(stuff)/;
print $1
</code>
<p>then we would expect our code to print 'stuff' if the string contained the literal 'stuff' as this gets captured into $1. The addition of /g means the $1 will sequentailly contain 'stuff' EVERY time <code>$str =~m/(stuff)g</code> is true ie 0..n times. Now if we know that a regex is a valid R value in an expression, we know we can write <code>L-VALUE = R-VALUE</code> so we can understand that:
<code>
@all_the_matches = $str =~ m/(stuff)/g;
</code>
<p>In array context we get all the matches into our array. So for example you can do:
<code>
@links = $html =~ m/<a[^>]+href\s*=\s*["']?([^"'> ]+)/ig;
</code>
<p>This is a reasonably reliable and quick way to extract all the <a...href=...> links from HTML. Although you can certainly use [cpan://HTML::LinkExtor] or any of the other [cpan://HTML::Parser] based widgets there are times when you want to say extract all the links that look like:
<code>
<A CLASS="blah" HREF="foo.com">
</code>
<p>A carefully chosen regex can extract exactly what you want, without any excess, as you can make it match a specific link subset with ease. Using this idiom you can grok the matches into an array in one elegant line of Perl.....
<p>The uses are of course only limited by your imagination. Using ^ and /m you can do things like extract a specific field from a space separated data set:
<code>
$data = '
f1 f2 f3
f4 f5 f6
f7 f8 f9
';
@second = $data =~ m/^\S+\s+(\S+)/mg;
print "@second";
</code>
<p>As always YMMV and you should pick the best hammer to drive the nail at hand.
<h3>Update</h3>
<p>Technical inaccurary removed. For the details on what happens if you put a scalar on the LHS of this idiom see [id://292089|this] where [bart] gets to poke fun at me for making an untested assumption and my pitiful excuses [id://292107|here] and a round about hack....