RegEx - How To Insert String Before File Extension

How would I insert "_thumb" into files that are being dyanmically generated.

For example, I have a site that allows users to upload an image. The script takes the image, optimizes it and saves to file. How would I make it insert the string "_thumb" for the optimized image?

I'm currently saving 1 version of the otpimized file. ch-1268312613-photo.jpg

I want to save the original as the above string, but want to append, "_thumb" like the following string ch-1268312613-photo_thumb.jpg

No need to use regex. This will do it:

$extension_pos = strrpos($filename, '.'); // find position of the last dot, so where the extension starts
$thumb = substr($filename, 0, $extension_pos) . '_thumb' . substr($filename, $extension_pos);

Rather than take the regular expression route (or other crude string manipulation) perhaps some filename-related functions might be easier to live with: namely pathinfo (to get the file extension) and basename (to get the filename minus extension).

$ext   = pathinfo($filename, PATHINFO_EXTENSION);
$thumb = basename($filename, ".$ext") . '_thumb.' . $ext;

Edit: @tanascius sorry for the very similar answer, guess I should've checked the little pop-up whilst writing this (it took a while, I got distracted).

Why do you want to use a RegEx? Is the extension always .jpg and is there only one extension? Maybe you can replace it by _thumb.jpg?

If it is not that simple, you can use a function like pathinfo to extract the basename + extension and do a replace there. That is working without a regex, too, which might be overkill here:

$info = pathinfo( $file );
$no_extension =  basename( $file, '.'.$info['extension'] );
echo $no_extension.'_thumb.'.$info['extension']

Assuming you only have the occurrence of ".jpg" once in the string, you can do a str_replace instead of regex

$filename = str_replace(".jpg", "_thumb.jpg", $filename);

or even better, substr_replace to just insert the string somewhere in the middle:

$filename = substr_replace($filename, '_thumb', -4, 0);

 preg_replace("/(\w+)\.(\w+)/","'\\1_thumb.\\2'","test.jpg"); 

I think that using a regexp is far more speed effective AND really better for flexibility.

But I admit Chad's solution is elegant and effective

$str = "ch-1268312613-photo.jpg";
print preg_replace("/\.jpg$/i","_thumb.jpg",$str);

Assuming you are wanting to inject your substring just before the first occurring dot, preg_replace() is a direct technique. Using a start of string anchor and a negated character class in the pattern, there can only be one replacement at most. Because \K restarts the fullstring match, the replacement string only replaces the literal dot that is matched.

Code: (Demo)

$strings = [
    "ch-1268312613-photo.jpg",
    "foo.inc.php",
    ".htaccess",
    "no_ext",
];

var_export(
    preg_replace(
        '~^[^.]*\K\.~',
        '_thumb.',
        $strings
    )
);

Output:

array (
  0 => 'ch-1268312613-photo_thumb.jpg',
  1 => 'foo_thumb.inc.php',
  2 => '_thumb.htaccess',
  3 => 'no_ext',
)

Just for fun, these 2-call techniques also work: (Demo)

echo implode('_thumb.', explode('.', $string, 2));

and

echo implode('_thumb', sscanf($string, '%[^.]%s'));