You should check pack and unpack on how to handle binary strings. open or sysopen should show you how to open a file.

as soon as possible

binmode $filehandle; is also important.

Don't forget index to find the byte sequence.

McA

Don't forget binmode and possibly a proper I/O layer .

See:

• open
• perlopentut
• PerlIO

• perlpacktut
• the open pragma

It's better if you can work out how to do this on your own by reading the sources mentioned above. Here is some example code, although much of it may still be confusing without doing some additional reading:

# Set the input and output filenames
my $ifilename = 'file.in';
my $ofilename = $ifilename . '.bin';

# Open the files and ensure we handle them as binary data
open my $ifile, '<', $ifilename or die "could not open input file $!";
open my $ofile, '>', $ofilename or die "could not open output file $!"
+;
binmode $ifile;
binmode $ofile;

# Read the binary data
$_ = do { local $/; <$ifile> };

# Replace the initial bytes with zero
s/^(.*?)(\x4F\xFF)/"\x0" x length($1) . $2/se;

# Store the result in the output file
print $ofile $_;

# Tidy up
close $ifile;
close $ofile;
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This makes the assumption that the file fits in RAM. This is usually, but not always, a safe assumption. Things may get complicated if that's not the case. Since we're not talking about line-oriented text files read (and possibly sysread and syswrite) would be good additional reading.

If there are fixed-length records there are ways to deal with that. If there are record terminators consistent throughout the file, there's a way to deal with that, too. See $/ for more information.

Good point about it all having to fit into RAM. Should be relatively straightforward to read byte by byte, although looking ahead for the two byte separator would need some finagling. hmmm, how about:

my $ifilename = 'file.in';
my $ofilename = $ifilename . '.bin';

open my $ifile, '<', $ifilename or die "could not open input file $!";
open my $ofile, '>', $ofilename or die "could not open output file $!"
+;

binmode $ifile;
binmode $ofile;

my $n = eof($ifile) ? "" : getc($ifile);
while ((my $c = $n) ne "") {
        $n = eof($ifile) ? "" : getc($ifile);
        print $ofile "$c$n" eq "\x4F\xFF" .. 1 ? $c : "\x0";
}

close $ifile;
close $ofile;
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