R devel - Jul 2024 - Question about regexp edge case

On StackOverflow (here: 
https://stackoverflow.com/questions/78803652/why-does-gsub-in-r-match-one-character-too-many)
there was a question about this result:

 > gsub("^([0-9]{,5}).*","\\1","123456789")
[1] "123456"

The OP expected "12345" as the result.  Several points were raised:

  - The R docs don't mention the case of {,5} for the default perl = 
FALSE which uses TRE.
  - perl = TRUE gives the OP's expected result of "12345".
  - perl = TRUE does *not* give the documented result on at least one 
system (which is "123456789", because "{,5}" is documented
to not be a
quantifier, so it should only match the literal string "{,5}").
  - Some regexp engines (including Perl and Awk) document that "12345"
is correct.

Is any of this worth fixing?

Duncan Murdoch

? Sun, 28 Jul 2024 20:02:21 -0400
Duncan Murdoch <murdoch.duncan at gmail.com> ?????:
> gsub("^([0-9]{,5}).*","\\1","123456789")  
> [1] "123456"
This is in TRE itself: for "^([0-9]{,1})" tre_regexecb returns {.rm_so
= 0, .rm_eo = 1}, matching "1", but for "^([0-9]{,2})" and
above it
returns an off-by-one result, {.rm_so = 0, .rm_eo = 3}.

Compiling with TRE_DEBUG, I see it parsed correctly:

catenation, sub 0, 0 tags
  assertions: bol
  iteration {-1, 2}, sub -1, 0 tags, greedy
    literal (0, 9) (48, 57), pos 0, sub -1, 0 tags

...but after tre_expand_ast I see

catenation, sub 0, 1 tags
  assertions: bol
  catenation, sub -1, 1 tags
    tag 0
    union, sub -1, 0 tags
      literal empty
      catenation, sub -1, 0 tags
        literal (0, 9) (48, 57), pos 2, sub -1, 0 tags
        union, sub -1, 0 tags
          literal empty
          catenation, sub -1, 0 tags
            literal (0, 9) (48, 57), pos 1, sub -1, 0 tags
            union, sub -1, 0 tags
              literal empty
              literal (0, 9) (48, 57), pos 0, sub -1, 0 tags

...which has one too many copies of "literal (0,9)". I think it's
due
to the expansion loop on line 942 of src/extra/tre/tre-compile.c being

for (j = iter->min; j < iter->max; j++)

...where 'min' is -1 to denote no minimum. This is further confirmed by
"{0,3}", "{1,3}", "{2,3}", "{3,3}" all
working correctly.

Neither TRE documentation [1] nor POSIX [2] specify the {,n} syntax:
from my reading, it looks like if the upper boundary is specified, the
lower boundary must be specified too. But if we do want to fix this, it
will have to be a special case for iter->min == -1.

-- 
Best regards,
Ivan

[1]
https://laurikari.net/tre/documentation/regex-syntax/

[2]
https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap09.html#tag_09_03_06