Well, that's certainly an improvement -- I had forgotten about all of the behaviors of the ? modifier. However, I must still be missing something. My code now reads:

use strict;
use warnings;

for (
    'oh {hello} there',
    'oh {hello there',
    'oh hello there',
    'oh hello} there',
) {
    print '',
     ( $_ =~ /
              (\{)?       # optional opening brace
              hello       # .. followed by 'hello'
              (?(1)\})    # a closing brace iif the open brace was the
+re
             /x ? 'YEP ' : 'NOPE' ),
     " - $1\n";
}
[download]

But my output is:

YEP  - {
YEP  - 
YEP  - 
YEP  - 

However, now warnings are thrown for using an uninitialized '$1' in the last three cases, as it should be.

What I'm struggling with is how to say that '{hello}' is OK, and 'hello' is OK, but '{hello' and 'hello}' are NOT OK. Or, put another way, I want to require a closing brace if an opening brace is found; however, if there is no opening brace then there must be no closing brace either.

Thanks for the help, though, it's an improvement.