A regex is a double-quote-ish expression, so variables are interpolated, including dereferencing references. Any block of code can be stuck in place of a reference (it should return a reference, in order to dereference properly). So you can execute any code you want within a regex or a string. The result of the block will be dereferenced and interpolated into the string (here I return a ref to an empty string, so the pattern isn't changed):

It will be executed each time the regex is looked at (compiled), even if the match doesn't get that far. For example: 

for (qw{dog pig cat horse}) {
    /.o${print "$_ gets printed\n";\''}/ and print "$_ matched\n";
}
__END__
dog gets printed
dog matched
pig gets printed
cat gets printed
horse gets printed
horse matched

[download]
Contrast that with (?{}), which executes arbitrary code if the expression matched up to that point (and does not insert anything into your pattern): 
for (qw{dog pig cat horse}) {
    /.o(?{print "$_ gets printed\n"})/ and print "$_ matched\n";
}
__END__
dog gets printed
dog matched
horse gets printed
horse matched

[download]
And finally (??{}), which acts the same, but does insert into your pattern: 
for (qw{dog pig cat horse}) {
    /.o(??{print "$_ gets printed\n"; 'g'})/ and print "$_ matched\n";
}
__END__
dog gets printed
dog matched
horse gets printed

[download]
The latter two are "experimental" features, but ref interpolation is not.
Caution: Contents may have been coded under pressure.

In reply to Re^3: ??{ } oddity by Roy Johnson
in thread ??{ } oddity by insaniac

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