Parsing and unparsing of strings according to a format

[Kai.Grossjohann@CS.Uni-Dortmund.DE (Kai Großjohann)]

In Tramp, I'm operating a lot on filenames.  A typical filename might
look like this:

    /method:user@host:/path/to/file

For generating this, I use the format-spec function from Gnus which
groks percent-escapes.  So I have a format string like so:

    /%m:%u@%h:%p

Of course, %m stands for the method and so on.  I also have a
home-grown regular expression with parentheses to parse the structure.

Is there a library that will help me with this?  So I specify the
filename syntax and the library gives me two functions, one function
that takes method, user, host, path and gives me a string, and
another function that takes a string and gives me method, user, host,
path.

Another filename format is

    /[method/user@host]/path/to/file

where the square brackets are part of the filename syntax.  The
library should allow me to easily specify which of the two syntaxes I
want (and allow other syntaxes, as well), and then produce a parsing
and an unparsing function.

As a final complication, some of the components of the filename are
optional.  For example, the "user@" part is optional.  Also, the
"method:" part is optional.  And the path can be the empty string.
(In the alternate syntax, it's "method/" instead of "method:".)

So, I specify what the filename looks like and then I have a parsing
function such that

    (filename-parse "/grossjoh@schulz:/tmp")

returns a structure where method=nil, user="grossjoh", host="schulz",
path="/tmp".  And I have an unparsing function such that

    (filename-unparse nil "grossjoh" "schulz" "/tmp")

returns "/grossjoh@schulz:/tmp".

At least for the unparsing part, it seems that something like
mode-line-format ought to do the trick.  But is there a function with
the necessary functionality?

And what about parsing?

tia,
kai
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