In code in my Re^2: creating qr from existing regex reply to bart, there is the possibility of creating an empty  (?) embedded pattern-match modifier construct. However, all the docs I've seen specify:

"(?adlupimsx-imsx)"
"(?^alupimsx)"
        One or more embedded pattern-match modifiers ...

(emphasis added), not zero. Note, though, that (all) the docs go on to say:

This is particularly useful for dynamic patterns, such as those read in from a configuration file, taken from an argument, or specified in a table somewhere.

To me, dynamic generation implies the possibility of zero modifiers: one should seek to support the most general case.

The code here and below works with an empty  (?) construct for Perl standard releases 5.8 - 5.14, all I can test with ATM. (I haven't tested with other modifier groupings than i or not-i.)

c:\@Work\Perl\monks>perl -wMstrict -le
"my $string = '/.*u\/ba$/';
 ;;
 my $flag = qr{ [msix] }xms;
 ;;
 my $convertable =
 my ($pattern, $modifiers) = $string =~ m{
   \A \s* / (.*?) / ($flag*) \z
   }xms;
 die qq{bad rx: '$string'} unless $convertable;
 ;;
 my $rx = qr{(?$modifiers)$pattern};
 print $rx;
 print 'A: match' if 'u/ba' =~ $rx;
 ;;
 my $ry = qr{ \A foo $rx }xms;
 print $ry
 ;;
 print 'B: match' if 'foolubatu/ba' =~ $ry;
 print 'C: match' if 'Foolubatu/ba' =~ $ry;
"
(?-xism:(?).*u\/ba$)
A: match
(?msx-i: \A foo (?-xism:(?).*u\/ba$) )
B: match
[download]

My Questions:

• Does the code behave identically with other, later Perl releases?
• Does anyone have experience with or insight concerning the "empty" variation of this construct?
• Is use of  (?) a good idea, or should it be considered "experimental and subject to change"?

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