Parse Accept Header

Question

Does anyone have any suggestions (or a regular expression) for parsing the HTTP Accept header?

I am trying to do some content-type negotiation in ASP.NET MVC. There doesn't seem to be a built in way (which is fine, because there are a lot of schools of thought here), but the parsing is not entirely trivial and I would rather not re-invent the wheel if someone has already done it well and is willing to share.

Good question - I'm looking for this in another project, as well! — Jarrod Dixon, Nov 01 '08 at 04:00

score 9 · Accepted Answer · answered Nov 01 '08 at 16:50

9

Have you seen this article? It gives a pretty comprehensive implementation for parsing the Accept header and subsequently doing something useful with it.

answered Nov 01 '08 at 16:50

Hank Gay

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Mark Bell · Answer 2 · 2018-02-27T15:08:18.640

As of .NET 4.5 (I think—Microsoft have made info on framework versions < 4.5 rather obscure these days), you can use one of the the built in parsers from System.Net.Http.Headers:

public IOrderedEnumerable<MediaTypeWithQualityHeaderValue> GetMediaTypes(string headerValue) =>
    headerValue?.Split(',')
        .Select(MediaTypeWithQualityHeaderValue.Parse)
        .OrderByDescending(mt => mt.Quality.GetValueOrDefault(1));

Then you can do something like this:

var headerValue = "application/json, text/javascript, */*; q=0.01";
var mediaTypes = GetMediaTypes(headerValue);

Giving you a nice list of all the media types, where the preferred option is the first item. Here's a LINQPad Dump of the mediaTypes result from the example:

Hat tip to this answer, for getting me on the right track.

score 3 · Answer 3 · answered Apr 24 '09 at 08:16

3

I've written a parser in PHP. It's not complex, but it will give you an array of mime types in order of preference.

answered Apr 24 '09 at 08:16

Jrgns

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the linked site is "disabled" – MageWind Feb 04 '15 at 18:30

score 1 · Answer 4 · answered Jan 30 '13 at 13:15

After reading the xml.com article I decided to not write a function for the Accept header myself ;)

Fortunately the article points to a good library: https://code.google.com/p/mimeparse/ - in my case I need it as a Node.js module: https://github.com/kriskowal/mimeparse

score 1 · Answer 5 · answered Feb 15 '11 at 10:04

1

Found another implementation in php here

answered Feb 15 '11 at 10:04

Christophe Furmaniak

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score 1 · Answer 6 · answered Oct 15 '18 at 09:13

Building on https://stackoverflow.com/a/49011308/275501 from https://stackoverflow.com/users/43140/mark-bell above:

public class MyController : Controller
{

    [HttpGet]
    [Route("/test")]
    public ActionResult Index() {

        // does this request accept HTML?
        var acceptsHTML = IsAcceptable("text/html");
        var model = FetchViewModel();
        return acceptsHTML ? (ActionResult) View(model) : Ok(model);

    }

    private bool IsAcceptable(string mediaType) =>
        Request.Headers["Accept"].Any(headerValue =>
            !string.IsNullOrWhiteSpace(headerValue) &&
            headerValue.Split(",").Any(segment => MediaTypeHeaderValue.Parse(segment).MediaType == mediaType));

    private object FetchViewModel() {

        return new { Description = "To be completed" };

    }

}

score 0 · Answer 7 · answered Nov 01 '08 at 04:48

The RFC is quite complex. If the regex where to follow these rules to the letter, it would become several lines long.

If you already have the Accept-header, and ignore the quotes and the parameters, you could do something like this to match each pair:

/([^()<>@,;:\\"\/[\]?={} \t]+)\/([^()<>@,;:\\"\/[\]?={} \t]+)/

* is included in the character class, so it does not need any special case in the regex.

Parse Accept Header

7 Answers7

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