如何使preg_replace处理文件名忽略，文件扩展名？

Question

I have a file name as Dona&ld #Duck .jpg. I want to replace all the special characters before .jpg or any file extension with _ (underscore). Output should be Don_a_ld__Duck__.jpg

How to achieve this using preg_replace? I have done as following.

$fileName ='Don.a&ld #Duck .jpg';
$fileName = trim($fileName);
$fileExtension = substr($fileName,-4);
$fileName = substr($fileName,0,-4); // file name without extension
echo $filename = preg_replace('/[^a-z0-9]/i', '_', $fileName).$fileExtension; 

I do not want append file extension separately.

Answer 1

Use the following solution with a negative lookahead that protects the last dot:

$fileName ='Don.a&ld #Duck .jpg';
echo $filename = preg_replace('/[^a-z0-9](?![^.]*$)/i', '_', $fileName);
                                          ^^^^^^^^^ 

See the PHP demo

The (?![^.]*$) just fails the match if a special char is followed by 0+ chars other than a dot up to the end of the string.

Note that /[^a-z0-9]/i matches _ and replaces with _, which is a redundant operation. You may avoid it with a \W subpattern:

preg_replace('/\W(?![^.]*$)/', '_', $fileName)