Allow qr/(?[ [a] ])/ interpolation in (?[...])

Interpolation fails if the interpolated extended character class con-
tains any bracketed character classes itself.

The sizing pass looks for [ and passes control to the regular charac-
ter class parser.  When the charclass is finished, it begins scanning
for [ again.  If it finds ], it assumes it is the end.

That fails with (?[  (?a:(?[  [a]  ]))  ]).  The sizing pass hands
[  [a]  ]... off to the charclass parser, which parses [  [a] and
hands control back to the sizing pass.  It then sees ‘  ]))  ])’,
assumes that the first ]) is the end of the entire construct, so the
main regexp parser sees the parenthesis following and dies.

If we change the sizing pass to look for ?[ we can simply record the
depth (depth++) and then when we see ] decrement the depth or exist
the loop if it is zero.

1 files changed, 3 insertions, 0 deletions

diff --git a/t/re/regex_sets.t b/t/re/regex_sets.t
index b70e7ec0c2..9ddceae577 100644
--- a/t/re/regex_sets.t
+++ b/t/re/regex_sets.t
@@ -85,6 +85,9 @@ my $still_fold = qr/(?[ $kelvin_fold ])/;
 like("K", $still_fold, "/i on interpolated (?[ ]) is retained in outer without /i");
 like("k", $still_fold, "/i on interpolated (?[ ]) is retained in outer without /i");
 
+eval 'my $x = qr/(?[ [a] ])/; qr/(?[ $x ])/';
+is($@, "", 'qr/(?[ [a] ])/ can be interpolated');
+
 done_testing();
 
 1;