In the first case, there is nothing stored in \1 so there shouldn't be any match (and it is an error.) In the latter case, \1 probably refers to the outer parentheses, but that hasn't taken effect yet, so there is no match and no error (perhaps it really doesn't make sense but perl tries to interpret it that way...)
If you instead do 
print "Match: $1\n" if $test_string =~ /((?:f))\1/;

[download]
the result is: Match: f
(Doing that doesn't seem especially useful, though...)
chas

In reply to Re^3: Regular Expressions by chas
in thread Regular Expressions by y8

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