What file extensions does ffmpeg support?

I'm trying to extract a subtitle from a Blu-ray disc and it is an hdmv_pgs_subtitle. So I try

ffmpeg -i file.m2ts -map 0:3 -scodec copy rip3/test.sub 

and get

Could not write header for output file #0 (incorrect codec parameters ?): 
Operation not permitted

Then I do

ffmpeg -i file.m2ts -map 0:3 -scodec copy rip3/test.sup 
ffmpeg -i file.m2ts -map 0:3 -scodec copy rip3/test.srt

and a bunch of other different extensions, until I give .mkv a shot, which works.

How can I find what extensions ffmpeg allows? I haven't found one place that e.g. names .mkv as an extension

ffmpeg -formats mentions matroska, but doesn't say that .mkv is the extension. So is there a way to find all supported file extensions for ffmpeg?

EDIT: After reading the answer from slhck I found the extensions by doing this command in the sourcefolder.

find . -type f -iname "*.c" -exec egrep -H ".extensions" {} \;

The filename will give some information about what kind of format it is.

First off, you can see all formats (containers) by calling ffmpeg -formats. That they don't mention extensions is probably for the reason that an extension on its own is often meaningless, and you can override the output format by using the -f option.

You can see what the common extensions are for a particular (de)muxer. For example:

$ ffmpeg -v 1 -h demuxer=matroska
  Demuxer matroska,webm [Matroska / WebM]:
  Common extensions: mkv,mk3d,mka,mks

Output formats and their extensions are registered in the encoders. So, you'd find this for Matroska, when looking at the ffmpeg source code:

AVOutputFormat ff_matroska_muxer = {
    .name              = "matroska",
    .long_name         = NULL_IF_CONFIG_SMALL("Matroska"),
    .mime_type         = "video/x-matroska",
    .extensions        = "mkv",
    ....

Input formats are usually not parsed by their extension but the actual contained data.

Also see:

• FFmpeg Formats Documentation
• FFmpeg Documentation: Supported File Formats, Codecs, or Features.

The problem is that only two of all the formats ffmpeg can handle support HDMV PGS subtitles:

• MPEG-TS
• MKV (Matroska)

You cannot use an SRT container because it's a different concept. SRT subtitles are based on text, while HDMV PGS subtitles are actual bitmaps.

So, this should work if all you need to do is store the subtitles somewhere:

ffmpeg -i input.mkv -c:s:0 copy -map 0:s:0 output.mks