All you need to do is a little buffering like: 

my $last4 = '';
my $buffer = '';
my $find = qr /1234/;
my $chunk = 7;  # 2**20 or some such is more logical!
while ( read ( DATA, $buffer, $chunk ) ) {
    print "got last-$last4\tbuffer-$buffer\n";
    print "simple match\n" while $buffer =~ s/$find//;
    $buffer = $last4 . $buffer;
    $last4 = substr $buffer, -4, 4, '';
    print "buffer match\n" if $buffer =~ m/$find/;
}
__DATA__
1234567890123456789012345678901234567890123456789012345678901234567890

__END__
got last-    buffer-1234567
simple match
got last-567    buffer-8901234
simple match
got last-7890    buffer-5678901
got last-8901    buffer-2345678
buffer match
got last-5678    buffer-9012345
simple match
got last-8905    buffer-6789012
got last-9012    buffer-3456789
buffer match
got last-6789    buffer-0123456
simple match
got last-9056    buffer-7890123
got last-0123    buffer-4567890
buffer match
got last-7890    buffer-

[download]

In this example I use a chunk size of 7 so that we need the buffer often but 2**20 (megabyte) chunks work effectively with big files (I wrote a node about processing big files fast at Re: Performance Question). You should be able to get around a 4-8MB/s throughput depending on your hardware.

I'll leave it as an exercise for you as to what to do with the data between matches. Logging the positions and making a second pass through the file to get the data may well be most efficient.

Note if we match out of the initial buffer we s/// it out before we add the previously buffered chunk and retest (avoids double match) - replace with a filler string to maintain length. There are 7 matches available and 7 found. A dozen lines with debugging - you gotta love Perl.

Update

Here is a version that records the positions of matches for you 

my $last4 = '';
my $buffer = '';
my $find = '1234';
my $find_re = qr /$find/;
my $length = length $find;
my $chunk = 7;  # 2**20 or some such is more logical!
my $pos = 0;
while ( my $read_length = read ( DATA, $buffer, $chunk ) ) {
    $buffer = $last4 . $buffer;
    print "got last-$last4\tbuffer-$buffer\n";
    while ($buffer =~ m/$find_re/g) {
         print "match at ", $pos - (length $last4) -$length  + pos $bu
+ffer, "\n";
    }
    $last4 = substr $buffer, -$length, $length, '';
    $last4 = '' if $last4 =~ m/$find_re/;  # stop double match bug
    $pos += $read_length;
}
__DATA__
1234567890123456789012345678901234567890123456789012345678901234567890

[download]

cheers

tachyon

s&&rsenoyhcatreve&&&s&n.+t&"$'$`$\"$\&"&ee&&y&srve&&d&&print

In reply to Re: How do I search this binary file? by tachyon
in thread How do I search this binary file? by John M. Dlugosz

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