Re: help with pattern matching needed

[Chet Ramey]

On 1/7/22 5:15 PM, Christoph Anton Mitterer wrote:
> On Fri, 2022-01-07 at 12:05 -0500, Chet Ramey wrote:
> > > It looks like \] is being treated as a literal ] in both cases.
> > > The difference seems to be in the parsing: dash gives up on the
> > > bracket expression, while bash consumes the rest of the script
> > > trying to close it.
> > >
> > >       % cat ex1.sh
> > >       case $1 in
> > >           [.*^$[\]) printf '%s matched\n' "$1" ;;
> > >           *) printf "%s didn't match\\n" "$1" ;;
> > >       esac
> >
> > The parser sees the (deprecated) `$[' and tries to read an entire
> > `$[]'
> > expansion as part of a word. It fails to close that expansion and
> > reports
> > an EOF error.
>
> So the failure is not because of the missing ] for the pattern, but the
> missing ] for the $[], right?

Correct.

> Is that deprecated form actually scheduled to go away?

Not scheduled, no.

> > > That is it, though.  The empty $[] substitutes "0" into the
> > > pattern.
> > >
> > >       % cat ex2.sh
> > >       case $1 in
> > >           [.*^$[]) printf '%s matched\n' "$1" ;;
> > >           *) printf "%s didn't match\\n" "$1" ;;
> > >       esac
> > >       % bash ex2.sh '[.xxxxxxx^0'
> > >       [.xxxxxxx^0 matched
> > >
> > > However, as you noted, bash doesn't parse quite so greedily this
> > > time.  I don't know why not.
> >
> > Because the closing `]' isn't escaped and the $[] expansion is
> > complete.
>
> Shouldn't it then be made failing because of the invalid pattern, that
> misses an ] ?

There is no invalid pattern. There is only a pattern that doesn't contain
a bracket expression. It might not be what you intend, but it's valid.


> And is it possible to get the whole thing better documented? I mean I
> probably won't be the last one to stumble over it.
>
> And from reading either POSIX or Bash manpage I had already the
> impression that there would be a 2 stage unescaping... first the normal
> quote removal, then making special characters in patterns (*?[\) loose
> their special meaning?

There was a pretty spirited argument about that. (The messages in the URL
below don't capture the entire discussion.)

https://www.austingroupbugs.net/view.php?id=1234

You can see in the proposed interpretation how difficult it is to capture
the intended behavior.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/