PHP - preg_replace backreferencing
backreference, php, preg-replace
Solution
This will do the job for you. Please see below for an explanation on how it all works.
$string = 'some text www.example.com more text http://example.com more text https://www.example.com more text';
$string = preg_replace('#\b(?:http(s?)://)?((?:[a-z\d-]+\.)+[a-z]+)\b#', "<a href='http$1://$2'>http$1://$2</a>", $string);
echo $string; // some text <a href='http://www.example.com'>http://www.example.com</a> more text <a href='http://example.com'>http://example.com</a> more text <a href='https://www.example.com'>https://www.example.com</a> more text
`\b` match word boundary `(?:http(s?)://)?` optionally match string if it contains `'http://'` or `'https://'`, if https grab the `'s'` so we can build correct URL
`(?:[a-z\d-]+\.)+` match one or more occurrence of of series of letter/numbers followed by a period
`[a-z]+` match one ore more occurrences of a series of letters, TLD, note TLDs are now open for purchase so can't limit length anymore. see http://tinyurl.com/cle6jqb
We then capture both of the last two sections in addition to the `'s'` in a backreference by enclosing them in parentheses.
We then build the URL:
<a href='http$1://$2'>http$1://$2</a>
`http$1://` create HTTP if HTTPS the backreference `$1` will contain an `'s'`
`$2` will contain the domain name. We make the link where the URL is made the link text.
Problem
I have big problems understanding how to use `preg_replace` with backreferencing. I have a plain-text string and want to replace every link with the HTML syntax for a link. So "www.mydomain.tld" or "http://www.mydomain.tld" or "http://mydomain.tld" should be wrapped in an HTML a-tag. I have found a working function that does this online, but I want to understand how to do it myself. In the function I found, this is the replacement: ``` "\\1<a href=\"http://\\2\" target=\"_blank\" rel=\"nofollow\">\\2</a>" ``` I see some escaped quotation marks in there and these bits: `\\1` `\\2`. According to the PHP documentation these are backreferences. But how do I use them, what do they do? I found nothing about that in the spec, so any help would be greatly appreciated!