cowboy_req:read_and_match_urlencoded_body - Read, parse and match a urlencoded request body against constraints


read_and_match_urlencoded_body(Fields, Req)
    -> read_and_match_urlencoded_body(Fields, Req, #{})

read_and_match_urlencoded_body(Fields, Req, Opts)
    -> {ok, Body, Req}

Fields :: cowboy:fields()
Req    :: cowboy_req:req()
Opts   :: cowboy_req:read_body_opts()
Body   :: #{atom() => any()}

Read, parse and match a urlencoded request body against constraints.

This function reads the request body and parses it as application/x-www-form-urlencoded. It then applies the given field constraints to the urlencoded data and returns the result as a map.

The urlencoded media type is used by Web browsers when submitting HTML forms using the POST method.

Cowboy will only return the values specified in the fields list, and ignore all others. Fields can be either the key requested; the key along with a list of constraints; or the key, a list of constraints and a default value in case the key is missing.

This function will crash if the key is missing and no default value is provided. This function will also crash if a constraint fails.

The key must be provided as an atom. The key of the returned map will be that atom. The value may be converted through the use of constraints, making this function able to extract, validate and convert values all in one step.

Cowboy needs to read the full body before parsing. By default it will read bodies of size up to 64KB. It is possible to provide options to read larger bodies if required.

Cowboy will automatically handle protocol details including the expect header, chunked transfer-encoding and others.

Once the body has been read, Cowboy sets the content-length header if it was not previously provided.

This function can only be called once. Calling it again will result in undefined behavior.



Fields to retrieve from the urlencoded body.

See cowboy(3) for a complete description.


The Req object.


A map of body reading options. Please refer to cowboy_req:read_body(3) for details about each option.

This function defaults the length to 64KB and the period to 5 seconds.

Return value

An ok tuple is returned.

Desired values are returned as a map. The key is the atom that was given in the list of fields, and the value is the optionally converted value after applying constraints.

The map contains the same keys that were given in the fields.

An exception is triggered when the match fails.

The Req object returned in the tuple must be used from that point onward. It contains a more up to date representation of the request. For example it may have an added content-length header once the body has been read.


  • 2.5: Function introduced.


Match fields
%% ID and Lang are binaries.
#{id := ID, lang := Lang}
    = cowboy_req:read_and_match_urlencoded_body(
        [id, lang], Req).
Match fields and apply constraints
%% ID is an integer and Lang a non-empty binary.
#{id := ID, lang := Lang}
    = cowboy_req:read_and_match_urlencoded_body(
        [{id, int}, {lang, nonempty}], Req).
Match fields with default values
#{lang := Lang}
    = cowboy_req:read_and_match_urlencoded_body(
        [{lang, [], <<"en-US">>}], Req).
Allow large urlencoded bodies
{ok, Body, Req} = cowboy_req:read_and_match_urlencoded_body(
    Fields, Req0, #{length => 1000000}).

See also

cowboy_req(3), cowboy_req:has_body(3), cowboy_req:body_length(3), cowboy_req:read_body(3), cowboy_req:read_urlencoded_body(3), cowboy_req:read_part(3), cowboy_req:read_part_body(3)

Cowboy 2.7 Function Reference


Version select

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