Using $1, $2, etc without knowing if the regexp matched is dangerous. For example, 
foreach ('foo', 'bar') {
   /(oo)/;
   print("$1\n");
}

[download]
outputs 
oo
oo

[download]

In reply to Re^2: Regex Optimization Question by ikegami
in thread Regex Optimization Question by gawatkins

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