bug#20657: Traditional range expression not accepted in regex/dfa

[Paul Eggert]

address@hidden wrote:

> The bugaboo here is the "---"; it's
> a range expression consisting of minus through minus, and apparently long
> ago was how one got a minus into a bracket expression.

Actually, long ago expressions like '[^0-9-]' worked just as they do now, and it 
wasn't ever necessary to use trailing "---".  That being said, it is true that 
in 7th Edition Unix '[^0-9---]' meant the same thing as '[^0-9-]', so in that 
sense we have an incompatibility with 7th Edition Unix here.

>         $ ./src/grep '[^0-9---]' /dev/null
>         ./src/grep: Invalid range end
>
> The underlying regex and, I believe, dfa routines don't accept this.

Yes, that's correct.  It's not a bug, though, as the regexp is ambiguous and 
does not conform to POSIX, which says the following about RE bracket 
expressions: "To use a <hyphen> as the starting range point, it shall either 
come first in the bracket expression or be specified as a collating symbol; for 
example, "[][.-.]-0]", which matches either a <right-square-bracket> or any 
character or collating element that collates between <hyphen> and 0, inclusive." 
<http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_05> 
In your correspondent's example, the hyphen is a starting range point but is 
neither first in the bracket expression nor is specified as a collating symbol, 
so the regexp doesn't conform to POSIX.

Even though it's not a bug I suppose it wouldn't hurt to make the GNU matchers 
compatible with 7th Edition Unix here, if someone really wants to take that task 
on; it's not urgent, though.