So the anwser is simply "no"?

Because I already found a way to add special characters like "é". I was wondering if there is a similar solution for " ' ".

I didn't say no. I provided you a link because the solution is there. Did you try adding apostrophe to the regular expression as the instructions indicate?

The value for sync_chars_regex must be a regular expression which your attempt is not.
In the default regex within the [] character class the a-z means all small ASCII letter, A-Z all capital ASCII letters, 0-9 all ASCII digits plus the three characters hyphen, underscore and dot.

Now this '[A-z\s\d][\\'\"\\^\&\+\\-\°\#\!\?\@]' probably wouldn't match any of your file names even if it was a valid expression because already A-z does not what you think it would do. It is not a range from capital letter A to small letter z because the codepoint order is different, small letters are before capital letters. It's not like in your phone book, hence the original expression uses separate sets within the class. Furthermore you defined an expression with two character classes [A-z..] and [\\...] that both would have to match two consecutive characters of your file name. But overall the \\ is one escaped backslash and the following ' ends the PHP string already and the rest remains as garbage. Because you said "I already found a way to add special characters like "é"" I thought you would know, but apparently you didn't. So, keep the original expression intact but just add the desired characters to the one character class, but you will have to backslash-escape an apostrophe that otherwise would end the string. Probably something like this:

$conf['sync_chars_regex'] = '/^[a-zA-Z0-9-_.\']+$/'

Now if the list should include the characters you gave in your try it's probably

$conf['sync_chars_regex'] = '/^[a-zA-Z0-9-_.\'"^&+°#!?@]+$/'

You can test expressions (the part between the leading '/ and the trailing /') against sample file names at https://regex101.com/ like this specific example one saved as https://regex101.com/r/TmugD3/1/

Hi,
Like said, including apos ' in your file names and allowing them would break Piwigo SQL queries.
More information on code side: [synchronization] Sync fail with files containing ' (apostrophe) in filename
[Github] Piwigo issue #250

On filename side, you may find a nice solution replacing it ' with ’ Unicode prefered one (U+2019)

' replaced by ’ (which is not `)

Wikipedia wrote:

This is the preferred character to use for apostrophe according to the Unicode standard.[110][111]
https://en.wikipedia.org/wiki/Apostrophe#Unicode