Why does this PHP regex give me error?

Question

Need Some Help With Regex:

I want to replace

[url=http://youtube.com]YouTube.com[/url]

with

<a href="http://youtube.com"&gt;YouTube.com&lt;/a&gt;

the regex

preg_replace("/[url=(.*?)](.*?)[/url]/is", '<a href="$1">$2</a>', $text);

why does this give me:

Warning: preg_replace() [function.preg-replace]: Unknown modifier 'r' in C:\Programa\wamp\www\func.php on line 18

Answer 1

PHP is interpreting the '/' in /url as being the end of the regex and the start of the regex options. Insert a '\' before it to make it a literal '/'.

You need to escape the '['s in the same way (otherwise they will be interpreted as introducing a character class).

preg_replace("/\[url=(.*?)](.*?)\[\/url]/is", '<a href="$1">$2</a>', $text);

Answer 2

You should escape special characters in your regular expression:

preg_replace('/\[url=(.*?)](.*?)\[\/url]/is', '<a href="$1">$2</a>', $text);

I have escaped the [ characters (they specify the start of a character class) and the / character (it specifies the boundaries of the regular expression.)

Alternatively (for the / character) you can use another boundary character:

preg_replace('#\[url=(.*?)](.*?)\[/url]#is', '<a href="$1">$2</a>', $text);

You still have to escape the [ characters, though.

Answer 3

Both the slashes and square brackets are special characters in regex, you will need to escape them:

\/ \[ \]

Answer 4

The 2nd '/' in a regex string ends the regex. You need to escape it. Also, preg_replace will interpret the '[url=(.*?)]' as a character class, so you need to escape those as well.

preg_replace('/\[url=(.*?)\](.*?)\[\/url\]/is', '<a href="$1">$2</a>', $text);

Answer 5

You seem to be just starting out with regular expressions. If that is the case - or maybe even if it isn't - you will find the Regex Coach to be a very helpful tool. It provides a sandbox for us to test our pattern matches and our replace strings too. If you had been using that it would have highlighted the need to escape the special characters.