"expr" won't match empty strings

[Luke Kendall]

I'm hesitant to report this, but I think it's an actual bug in expr 
that's been there from day one.

I believe that expr, when used to match regular expressions, should use 
the success/failure of the pattern match to determine the exit code.

But instead, I believe "expr" uses the length of the matched string to 
determine its exit code.  So when the regexp correctly matches an empty 
string, expr returns failure, despite the match.  Here's a simple example:

$ expr " " : "^ *$" && echo Matched.
1
Matched.
$ expr "" : "^ *$" && echo Matched.
0

And compare that to what sed and grep do:


$ echo "" | sed -n 's/^,*$/& - yep/p'
 - yep

$ printf "a\n\n" | grep '^$' && echo "A match."

A match.

I'd like to suggest that expr be changed to use the success/fail of the 
pattern match to determine the exit status, as all the other unix tools do.

I don't think this alteration of semantics would break many existing 
scripts, for two reasons:
1) It must be unusual to use regexps that can match an empty string, 
because expr does not report a match for that corner case, so to 
correctly handle it, the user must have had to add an explicit test for 
the input string being empty: and this will still work (it's just that 
with the suggested change, that extra code becomes redundant).
2) Based on my own experience, it's unusual to use expr ":" with 
patterns that can match the empty string - it's taken me over 30 years 
to notice this oddity!

If you think this would be a good change, but don't have time to do 
anything, let me know and I'll have a go and submit a patch.

Regards,

luke