input

<a href="http://mysite.com">My Site</a>
<a href="http://mysite.com/image.jpg"><img src="http://mysite.com/image.jpg"/></a>
    <a href="http://mysite.com/image.gif"><img src="http://mysite.com/image.gif"/></a>
<a href="http://yoursite.com">Your Site</a>

output

<a href="http://mysite.com/image.jpg"><img src="http://mysite.com/image.jpg"/></a>
<a href="http://mysite.com/image.gif"><img src="http://mysite.com/image.gif"/></a>

Thank's for help

I am not a PHP developer,but I can give you a javascript demo,hope it can give you some help :)

var reg=/<a\b[^>]*?href=\"((?!jpg|gif|png).)*?"[^>]*?>.*?<\/a>/gi;
yourstr=yourstr.replace(reg,'');

Description

This will skip over all the other attributes in the anchor tag, even if they have a value which would look like an attribute nested in the value.

<a(?=\s|>)                  # validate this is an anchor tag
(?!                         # start look ahead ! must not contain, = must contain
  (?:[^>=]|='[^']*'|="[^"]*"|=[^'"][^\s>]*)*?   # move through tag, skipping over quoted or non quoted values
  \shref="[^"]*(?:jpg|png|gif)"                 # find href, capture value including quotes if they exist
  )                             # end look ahead
[^>]*>.*?<\/a>                  # capture the entire to the close tag

PHP Code Example:

Sample Text

note the second line

<a href="http://mysite.com">My Site</a>
<a wrongtag=" href='http://mysite.com/image.jpg' " href="http://mysite.com">My Site</a>
<a href="http://mysite.com/image.jpg"><img src="http://mysite.com/image.jpg"/></a>
    <a href="http://mysite.com/image.gif"><img src="http://mysite.com/image.gif"/></a>
<a href="http://yoursite.com">Your Site</a>

Code

<?php
$sourcestring="your source string";
echo preg_replace('/<a(?=\s|>)
(?!  # start look ahead ! must not contain, = must contain
(?:[^>=]|=\'[^\']*\'|="[^"]*"|=[^\'"][^\s>]*)*?  # move through tag, skipping over quoted or non quoted values
\shref="[^"]*(?:jpg|png|gif)"    # find href, capture value including quotes if they exist
)    # end look ahead
[^>]*>.*?<\/a>   # actually capture the string
/imsx','',$sourcestring);
?>

Matches

[0] => <a href="http://mysite.com/image.jpg"><img src="http://mysite.com/image.jpg"/></a>
[1] => <a href="http://mysite.com/image.gif"><img src="http://mysite.com/image.gif"/></a>