[NTG-context] Re: XML: Correct usage of hash symbol in url / new upload

[Hans Hagen via ntg-context <ntg-context@ntg.nl>]

Hi Denis,

> I need to process URLs in XML documents differently depending on the target. Thanks to Hans and Thomas I can now deal with the attributes.
> Now I need to prefix certain kind of targets with a certain URL (a web viewer for IIIF images in this particular case). The prefix contains a #, which, as a special character seems to cause an error. Adding a double hash seems to work, but those ## will end up in the link URL. How can I properly escape a single #? I've already tried using \Ux{23}, but that won't work either.
The duplication of hashes is kind of special. Consider this:

\def\foo#1{#1 test ##}

\def\foo#1{#1 test\def\more##1{(##1)}}

the internal representation of #1 is a reference to parameter 1 while 
the ## becomes one # (cc parameter) in the second exmaple followed by a 
character 1 (cc other)

then, when tex serializes e.g. in tracing it duplicates the hash (with 
cc parameter) so that the user is not confused (an alternative could 
have been to indicate a parameter reference differently but changing 
that now is no real option - what symbol to use anyway?)

with lua(meta)tex opening up matters this duplication becomes a bit 
annoying so i decided to see if it could be avoided in some cases

first i decided to provide an escape, possible because we already have 
such mechanism using # (not advertised because it is still experimental)

then i decided to just not duplicate unless we trace

in the lowlevel macro manual there is a section added that goes in more 
detail

to come back to the escaping: here is a list:

#I  loop iteraor
#P  parent loop iterator
#G  grandparent loop iterator

#H  hash
#S  space (ascii 32)
#T  tab \t
#L  newline \n
#R  return \r
#X  backslash

considering

#N nbsp

and some more. Keep in mind that #1-#9 #A-#E are in use as parameter 
references

but in the case of your url's escaping should not be needed (hopefully)

Hans

ps. all kind of expereimental because it's a bit hairy and we don't want 
to loose compatibility (hashes get interpreted in the macro preamble, 
macro body, running text, serialization etc)

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                                           Hans Hagen | PRAGMA ADE
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