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Adding support for `fn:replace`

See original GitHub issue

I just looked into this. I would like to be able to use the functx library. I have gone through it and commented out all the functions that fontoxpath cannot currently compile. There are quite a lot, but they depend on a relatively few missing XQuery functions. The most interesting, for me, is replace.

I had a look at CONTRIBUTING.md and started to dig in. I see that the matches implementation uses xspatterns, which in turn relies on whynot. Currently, xspatterns only returns a boolean, which is enough for matches but not for replace. whynot in turn will return the traces that matched, which is enough for replace, but will still need some work to feed through xspatterns, which would also need extending to implement captures.

Meanwhile, I see that the fontoxpath implementation of tokenize does not use xspatterns at all, but JavaScript’s RegExp! (Again, not surprising, as xspatterns would not be sufficient in this case.)

I am unsure how to proceed.

I see that the implementation of matches does not support the flags argument; in the first instance, this is fine for me, as I don’t need flags for my use of replace.

I would be happy to implement replace using JavaScript regexes directly, which would be simple, but presumably not strictly XQuery-compliant. However, fontoxpath already does this for tokenize.

It looks fairly simple to extend xspatterns to return the match; but replace (and tokenize) need all the non-overlapping matching (effectively a g flag).

If you can let me know what would work for the fontoxpath maintainers, I can consider how I might be able to help; in the mean time, I suspect I will simply implement the desired functx functions directly in JavaScript. (In the first instance, I simply want trim: as you can see, I’ve already gone down quite a deep rabbit hole in an effort to see how to do this “properly”!)