Well, my gut reaction would be to open the file and read in the first few lines and see if any of them match a regex. But that assumes that what you are looking for would always be near the start of the file.

On a more general note, anytime I hear a question that begins with "Is it possible to do ..." my immediate response is always "Yes! Just give me enough time and money." If I can't do it myself, I can hire the people who can. Do you want faster than light travel? Do you want anti-gravity? Just give me enough time and money.

Update: Here is a very crude approach that might work. (Please note that this is basic logic, NOT working code!)

open file for reading;

$found_dtd = 0;
while not end of file {
    next unless string has pattern known to be after DTD or schema
    if line matches DTD or schema regex {
        $found_dtd = 1;
        last;
    }
}

return $found_dtd;
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