Thank you guys
I followed the method proposed by jwkrahn. Thus, if I well understand, by putting the quantifier inside the parentheses the backreference is set to 0 or 1, whereas by using the ? character outside the parentheses means that the pattern optionally includes the preceding expression. If I well understand again, in the fisrt case there's no concern to the possibility that the backreference has an "undefined value" (it will be 0 or 1). Is not it?
On the contrary I don't like the method proposed by davido (thank you anyway); even though it looks functional, I do not believe that to set warnings off is the right way, even within the narrowest scope possible.
I'm almost new to Perl, so maybe I'm in wrong. Thank you again

In reply to Re^2: undefined backreferences by frasco
in thread undefined backreferences by frasco

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