How to use backreferences in PHP

I want to add a character to the end of each file extension found in a body of text usi开发者_如何学编程ng preg_replace().

Here is some sample text:

$string='http://www.mysite.com/expert/images/imageone.jpghttp://www.mysite.com/expert/images/imagetwo.jpg';

This search & replace works fine in TextWrangler, appending a semi colon to file extensions:

(\.(jpg|gif|html?|php|tiff?|pdf|png))


\1;

Translated to PHP, however does not work, having no effect; no errors.

preg_replace("/(\.(jpg|gif|html|php|tif|tiff|pdf|htm|png))/","\\1;",$string);

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It works perfectly for me, but you should try using $1:

$string = preg_replace("/.../","$1;",$string);

or put the replacement in single quotes:

$string = preg_replace("/.../",'\\1;',$string);