I wonder if it is possible to modify previously parsed content using Karma. An example is some text like:

coordinate (1, 4.15, 1.49, 0.81) // some comment 1 coordinate (1, -19, 4, -1) // some comment 2

Now suppose the numbers are all parsed, and I change the 0.81 to 71.44. I want to update the text accordingly:

coordinate (1, 4.15, 1.49, 71.44) // some comment 2 coordinate (1, -19, 4, -1) // some comment 2

Note how the overall formatting and comments are preserved. The text could contain lots of other strings etc. and they would be all left untouched. This part - the preservation - is the hard one, and I ask, is this possible using Karma?

If Spirit.Qi is for turning unstructured input (most often text/character streams) into structured (internal) data (most often called parsing) then Spirit.Karma is for doing the opposite: turning structured (internal) data into unstructured (external) text/character streams (some people call it unparsing, we call it generating). Both Qi and Karma use similar grammars to describe the format of the external representation while using C++ data structures as the internal representation. So the answer to your question is: as long as you have/create a format description for your external textual representation it is possible to use Karma to generate text matching the required format from your internal data. See here for a more elaborate higher level description: http://tinyurl.com/nca9pk. HTH Regards Hartmut