Perl正则表达式替换

Question

The code is here. My question is since $1 is Fred and this is a replacement, which means replacing fred or barney with Fred , why cannot the answer be like I saw FRED with FRED. i have no idea for what's going on here. Thank you for your answer!

$_ = "I saw Barney with Fred.";
s/(fred|barney)/\U$1/gi; # $_ is now "I saw BARNEY with FRED."

Answer 1

Because the capturing group not only captures the string Fred but also Barney . So if the regex engine see Fred , it replaces immediately with Uppercase FRED and if the engine sees Barney, it replaces it with Uppercase Barney.

Answer 2

$1 refers to "whatever the first capturing group captured". So when (fred|barney) matches fred , that's what $1 contains, but when it matches barney , that is then what $1 contains.