Question

I notice a lot of code where people do something like:

myClass.someMethod(something here, $1);

The $1 is picking up a value from “something here”?

What is this known as? I can’t seem to find it anywhere? But this step, process is used in cases with regex quite a bit..

Answer 1

You’ll often see this in regular expressions where $1 represents a capture group that you’re carrying over into the new value.

For example, suppose we’re building a tweet-parser for our website. We want to find @ references in the tweet, and convert them into links to those particular accounts:

// Our Tweet
$t = "I am @jonathansampson, and I work with @appendTo.";

// Find every occurence of @something, capture 'something'
echo preg_replace("/@([a-z]+)/i", "<a href='http://twitter.com/$1'>$0</a>", $t);

Note here that we’re matching every occurrence of @something, but we’re wrapping the username portion in ( and ) so that we can handle it individually in our replacement text. The entire pattern is represented by $0, which will hold the value in its entirety from @ to the last char in the username.

The same is true for JavaScript:

var tweet = "I am @jonathansampson, and I work with @appendTo.",
    patrn = /@([a-z]+)/gi,
    links = tweet.replace(patrn, "<a href='http://twitter.com/$1'>$&</a>");

The variable links now contains the value:

console.log(links);
/*
    "I am <a href='http://twitter.com/jonathansampson'>@jonathansampson</a>, and 
     I work with <a href='http://twitter.com/appendTo'>@appendTo</a>."
*/

You might note that I used $& in JavaScript to grab the entire matched pattern while using $0 in PHP – we have to deal with these differences in life. My apologies 😉