animesavour

I think string_copy(filename,string_length(filename)-9,9)
Try that in the debugger or something to make sure this is what you want... you might need to change it to string_copy(filename,string_length(filename)-8,9).  Like I said before, GameMaker has a weird way of numbering the character positions and it confuses me...

So anyway, your whole code would be

filename = get_open_filename("DG files|*.dg|All files|*.*","");
if(string_replace(string_copy(filename,string_length(filename) - 9, 9), "/", "\") == "\blank.dg"){
    // DO SOMETHING
}

Edit - for future reference, the reason string_copy() was not working is because it does not actually change the string. All it does is returns a copy of the string and it's up to you to set the string as the copy afterwards. Also, the way you had it won't work anyway because that would get the first 9 characters, not the last 9.

To make a file,

var f;
f = file_bin_open("blank.dg",2);
file_bin_close(f);

To let the user search for it, I use

filename = get_open_filename("DG files|*.dg|All files|*.*","");

(I'm not sure what that last argument should be... I just use a blank string and it seems to work).
Anyway, so after that then the filename variable will be a string containing the path to the file that the player has selected... so to find out if the player has selected "blank.db" then use string_copy() to get the last 9 characters.  If it is "\blank.db" then do whatever.

if filename_pos("\blank.dg",filename) 
{

// DO SOMETHING

}

if filename = "\blank.dg"
{

// DO SOMETHING

}

filename = get_open_filename("DG files|*.dg|All files|*.*","");
string_copy(filename,1,9)

To make a file,

var f;
f = file_bin_open("blank.dg",2);
file_bin_close(f);

To let the user search for it, I use

filename = get_open_filename("DG files|*.dg|All files|*.*","");

(I'm not sure what that last argument should be... I just use a blank string and it seems to work).
Anyway, so after that then the filename variable will be a string containing the path to the file that the player has selected... so to find out if the player has selected "blank.db" then use string_copy() to get the last 9 characters.  If it is "\blank.db" then do whatever.

Are you using game makerstudio or an earlier gamemaker version?
The file functions have changed in studio.