extract string from binary file - regex issue

Question

I have a binary file that contains a readable filename* bounded by 'namexx:' and 'xx:piece', where x is any digit from 0-9 in both cases.

I am working on a Mac in bash 5.

I have tried using sed:

cat filename.xxx | sed -E 's/^.*name[0-9]{2}:(.*)[0-9]{2}:piece.*$/\1/'

The problem is that the regex does not consume the whole file, so I get a lot of random stuff returned in addition to the captured filename.

I've tried prefixing sed with LC_ALL=C as I read in another answer that this will treat all binary data as 'consumable' with wildcards, but it makes no difference (and I may have misunderstood).

I have also tried removing the beginning and end anchors, but that makes no difference either.

*The file is a torrent file from which I just want to extract the filename. I have looked at bencoding and trying to extract the filename, but it seemed too complex for a trivial task.

Maybe all you need is sed -n -E 's/^.*name[0-9]{2}:(.*)[0-9]{2}:piece.*$/\1/p;'? — Wiktor Stribiżew
– Wiktor Stribiżew, Commented May 23, 2019 at 12:16
Try grep -m 1 -o 'name[0-9]\{2\}:$.*$[0-9]\{2\}:piece' filename.xxx | sed 's/^name[0-9]\{2\}://' | sed 's/[0-9]\{2\}:piece$//' — Wiktor Stribiżew
– Wiktor Stribiżew, Commented May 23, 2019 at 12:30

Wiktor Stribiżew · Accepted Answer · 2019-05-23 13:25:47Z

2

You may use

sed -n -E 's/^.*name[0-9]{2}:(.*)[0-9]{2}:piece.*$/\1/p;' filename.xxx

Here, -n prevents line from being printed and p prints the matches (what remains after replacement).

As an alternative, you may use something like

grep -m 1 -o 'name[0-9]\{2\}:\(.*\)[0-9]\{2\}:piece' filename.xxx | \
   sed -E 's/^name[0-9]{2}:(.*)[0-9]{2}:piece$/\1/'

The first grep will only extract the first (-m 1) match and then sed will only keep the capturing group value inside the result.

edited May 23, 2019 at 13:25

answered May 23, 2019 at 12:54

Wiktor Stribiżew

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