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"Y" =~ /\u(?x: y)/
    or  die "\\u and (?x: ) don't cooperate";

/$foo bar[5]/x; # Not the same as /$foobar[5]/
/\ u2/x;        # Not the same as /\u2/, obviously
/\01 2/x;       # Not the same as /\012/
/$hash{a Key}/x;# Not the same as /$hash{aKey}/

#!/usr/bin/perl -w
my $x= " b c ";
print for "abcd" =~ /$x/xg;
# prints "bc"

print "($1)($2)\n"
    while  "fFFggGGGG" =~ /([a-z]+)\U(\1*)/g;
...
(gg)()
---
(gg)(G)