perl -le '$re = "(hello)"; $_ = "Hello world";  print $1 if /$re/i;'

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perl -le '$re = qr/(hello)/; $_ = "Hello world";  print $1 if /$re/i;'

##</code><code>##

perl -le '$re = qr/(hello)/i; $_ = "Hello world";  print $1 if /$re/;'