Re: AS_DIRNAME and expr

[Akim Demaille]

I tried to summarize the thread, and here is my proposal.  Comments
are welcome, suggestions are most welcome :)



`expr'
     No `expr' keyword starts with `x', so use `expr x"WORD" : 'REGEX''
     to keep `expr' from misinterpreting WORD.

     Don't use `length', `substr', `match' and `index'.

`expr' (`|')
     You can use `|'.  Although POSIX does require that `expr '''
     return the empty string, it does not specify the result when you
     `|' together the empty string (or zero) with the empty string.  For
     example:

          expr '' \| ''

     GNU/Linux and POSIX.2 return the empty string for this case, but
     traditional Unix returns `0'.  In the latest POSIX draft, the
     specification has been changed to match traditional Unix's behavior
     (which is bizarre, but it's too late to fix this).  Please note
     that the same problem does arise when the empty string results
     from a computation, as in:

          expr bar : foo \| foo : bar

     Avoid this portability problem by avoiding the empty string.

`expr' (`:')
     Don't use `\?', `\+' and `\|' in patterns, they are not supported
     on Solaris.

     The POSIX.2-1996 standard is ambiguous as to whether `expr a : b'
     (and `expr 'a' : '\(b\)'') output `0' or the empty string.  In
     practice, it outputs the empty string on most platforms, but
     portable scripts should not assume this.  For instance, the QNX
     4.2.5 native `expr' returns `0'.

     You may believe that one means to get a uniform behavior would be
     to use the empty string as a default value:

          expr a : b \| ''

     unfortunately this behaves exactly as the original expression, see
     the ``expr' (`:')' entry for more information.

     Older `expr' implementations (e.g. SunOS 4 `expr' and Solaris 8
     `/usr/ucb/expr') have a silly length limit that causes `expr' to
     fail if the matched substring is longer than 120 bytes.  In this
     case, you might want to fall back on `echo|sed' if `expr' fails.

     Don't leave, there is some more!

     The QNX 4.2.5 `expr', in addition of preferring `0' to the empty
     string, has a funny behavior wrt exit status: it's always 1 when
     the parenthesis are used!

          $ val=`expr 'a' : 'a'`; echo "$?: $val"
          0: 1
          $ val=`expr 'a' : 'b'`; echo "$?: $val"
          1: 0
          
          $ val=`expr 'a' : '\(a\)'; echo "?: $val"
          1: a
          $ val=`expr 'a' : '\(b\)'; echo "?: $val"
          1: 0