Re: [openpgp] Possible ambiguity in description of regular expressions:

On 05/01/2021 15:33, Daniel Kahn Gillmor wrote:
> But beyond the wordsmithing, if anyone thinks that Neal's interpretation
> (or my proposed clarification) is actually wrong or problematic, please
> speak up!
The original proposed change: "A range is a non-empty sequence of 
characters enclosed in []" is clear and (mostly) effective, so IMO 
should be adopted despite the fact that it is insufficient in itself.
Now, consider the remaining corner cases:

[]]     : matches a closing square bracket
[^]]    : matches anything other than a closing square bracket
[[]     : matches an opening square bracket
[^[]    : matches anything other than an opening square bracket
[][]    : matches either square bracket
[^][]   : matches anything other than either square bracket
[^]     : is an incorrectly nested sequence

To tackle the insufficiency, I propose an additional change:

-If the sequence begins with '^', it matches any single character not 
from the rest of the sequence.
+If the sequence begins with '^', it matches any single character not 
from the rest of the sequence, which must then contain at least one 
further character following the '^'.
When read alongside "To include a literal ']' in the sequence, make it 
the first character (following a possible '^')", this should be 
sufficient to cover all corner cases.
(I considered "... the rest of the sequence, which must be non-empty", 
but it is unclear whether "which" refers to "the sequence" or "the rest 
of the sequence")
However...

We should probably also explicitly note how to negate the special 
meaning of '^':
+To include a literal '^', locate it somewhere other than the first 
character of the sequence.
Now, this doesn't cover the (contrived) case where we may want to use a 
literal '^' as the beginning of an ASCII range:
[^-~]

But if absolutely necessary, one could refactor:

[_-~^]

While we're at it, we should also clarify range inclusivity:

-this is shorthand for the full list of ASCII characters between them
+this is shorthand for them and the full list of ASCII characters 
between them
Also, many regex engines support backslash-escaping within a character 
class. Does RFC4880 support this or not? My reading is that it doesn't, 
but it may be worth explicitly clarifying this also (even though 
backslash escaping would be a more elegant solution to [^-~]).
Is there anything to be said for referring out to an external regex 
definition instead of reinventing the wheel? :-)
--
Andrew Gallagher

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