Someone help me construct this regular expression

[pkipper]

Hi guys,

I am poor at regular expression, and I need something that I am sure can be achieved with regular expression but just that I cannot figure out how.

I need to pick out file all filesnames that ends with .jpg or .gif, but the left side of the filename (left side of the . ) cannot end with tn.

For example, it needs to pick out filenames such as

hello123.jpg
whatisthis.gif
034984.jpg

but not

hello123tn.jpg
toirkrtn.gif
019a3tn4tn.gif

I have constructed the regular expression that picks out all the jpg/gif files, but that is it. I do have a work around by using explode then substr which works just fine, but i rather keep it simple with just regular expression.

This is what I have now, the dodgy method using explode and substr:

while($file_name = readdir($dir_handle)) {
	if($file_name != "." && $file_name != ".." && filetype($file_name)!="dir") {
    	if(preg_match("/(JPG|JPEG|GIF|JPE)$/i", $file_name)) {
    		$parts = explode(".", $file_name);
    		if(substr($parts[0], -2)!="tn") {
    			$dirArray[] = $file_name; 
    		}
    	}
    }
}

I hope someone can help me out 🙁

[tsinka]

Hi,

try:

preg_match("/(?<!tn).(JPG|JPEG|GIF|JPE)$/i", $file_name)

This is called "lookbehind assertion".

Thomas

[pkipper]

Hi Thomas,

It works perfectly, what I need to do now is to understand how you got that to work 🙂

Thanks again.