TIL about Named Capture Groups
OK, a couple of days ago, but today is the first that I made the time to play with them.
We’ll start with something to run regular expressions against. I choose telephone numbers. In the US, they’re split up into three-digit area codes, three-digit exchange codes, and four-digit subscriber codes. This gives us a quick and easy formulation to make regular expressions.
If we’re going to regex phone numbers, we need one to work with, right? I could use the number that Tommy Tutone used, but it doesn’t have an area code and I don’t want to bother Jenny anymore. To lean hackish, I’ll go with Sneakers: at the end of the movie, our heroes have the NSA over a barrel, and they social-engineer themselves into a wishlist of high-ticket items, and Carl, played by River Phoenix, says…
Martin Bishop: Carl?
Carl: The, young lady with the… Uzi. Is she single?
Martin Bishop: Uh, Carl? Excuse us.
[pulls Carl aside]
Martin Bishop: This is the brass ring. Haven’t you got any bigger thoughts?
Carl: I just want her telephone number. Please?
NSA Agent Mary: Wait a minute… you can have anything you want, and you’re asking for my phone number?
Carl: Yes.
NSA Agent Mary: 273-9164. Area code 415.
Carl: I’m Carl.
NSA Agent Mary: I’m Mary.
Bernard Abbott: I’m going to be sick!
The lore is that (415) 273-9164 was the main phone number for the NSA offices in San Francisco, but MovieChat says that it had a recording promoting the movie. It certainly isn’t one of the safe 555 numbers that movies generally use to keep people from calling.
Let’s not actually call it, then, OK?
my $number = ' 415 273 9164 ';
$number =~ /(\d{3})\D+(\d{3})\D+(\d{4})/;
say join '.', $1, $2, $3;
The bits with the parentheses are called Capture Groups, and that the first “There’s a name for that?” bit of Perl I’ve learned in a while.
There are … problems with this. If I was really matching phone numbers, I would probably use a limited number of non-digit characters to separate the numbers, or maybe use a more specific format, like (415)273-9164 or 415.273.9164.
The other problems are that we don’t know what things are by reading $1 and $2, and the next time the code hits a regular expression, those variables are overwritten.
my $number = ' 415 273 9164 ';
my ( $area, $exchange, $subscriber ) = $number =~ m{
(\d{3}) # area code
\D*
(\d{3}) # exchange
\D*
(\d{4}) # subscriber
}mx;
say join '-', $area, $exchange, $subscriber;
The wonder of /mx to allow us to comment our regular expressions, so our code doesn’t seem quite as unreadable, and we put the output into real variables. This is how I’d probably write it still. I mean, if it was just for me, I might not comment my regular expressions, because I recognize how I write regular expressions, but still.
And then I learned about naming your capture groups.
$number =~ m{
(?<area>\d{3})
\D*
(?<exchange>\d{3})
\D*
(?<subscriber>\d{4})
}mx;
say qq{ ($+{area}) $+{exchange}-$+{subscriber} };
So, Perl creates a hash named %+ and puts the results into it. We have the same problem as with $1 and all: next regex wipes your results. But that’s certainly a way to keep track of what your variables mean, without (and likely before) you comment your regex and use the regex flags suggested by Perl Best Practices.
I feel like I should go deeper, because regular expressions are deep, man, but there’s a lot to regular expressions, and I might have to write a series. If you’re new or inexperienced with regexes, maybe start with perlre and go from there, knowing that there are lots of things you shouldn’t use.
I’ll say that, if you ever see more than 1 e flag (especially more than 3), you’re within deep magic, and you shouldn’t add to or change it unless you’re attuned to it.
I’m not sure you should use Named Capture Groups either.