Andreas has a good point. See RFC 3986 on URI schemes:
http://tools.ietf.org/html/rfc3986#page-12

The colon is a reserved general delimiter (similar in use to the / in a
typical URL, but applies to URNs etc). As suggested, we get req:something
being changed to one of the unreserved characters that do not have to be
URL encoded. Again, from the RFC these are

* Option A: req_something (underscore)
* Option B: req-something (hyphen)
* Option C: req~something (tilde)
* Option D: req.something (period)

Personally, my eye likes Option B, the hyphen.

On 31 January 2012 22:14, Andreas Schildbach <andreas@schildbach.de> wrote:

> On 01/31/2012 07:22 PM, Matt Corallo wrote:
>
> > that "It is recommended that additional variables prefixed with
> > mustimplement: not be used in a mission-critical way until a grace
>
> Is the ':' sign actually allowed in URL parameter names
> (unescaped/unencoded)? If not, I'd propose an unrestricted char instead,
> maybe '_'.
>
>
>
>
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