regex - How to replace string and preserve its uppercase/lowercase

Question

I want to replace one string with the other in Perl; both are of the same length. I want to replace all occurrences of the string (case insensitive), but I want that the case of the letter will be preserved. So if the first letter was upper case, the first letter after the replacement will be upper case also.

For example, if I want to replace "foo" with "bar", so I want that

foo ==> bar
Foo ==> Bar
FOO ==> BAR

Is there a simple way to do this in Perl?

Answer 1

This might be what you are after:

How do I substitute case insensitively on the LHS while preserving case on the RHS?

This is copied almost directly from the above link:

sub preserve_case($$) {
    my ($old, $new) = @_;
    my $mask = uc $old ^ $old;
    uc $new | $mask .
    substr($mask, -1) x (length($new) - length($old))
}

my $string;

$string = "this is a Foo case";
$string =~ s/(Foo)/preserve_case($1, "bar")/egi;
print "$string
";

# this is a Bar case

$string = "this is a foo case";
$string =~ s/(Foo)/preserve_case($1, "bar")/egi;
print "$string
";

# this is a bar case

$string = "this is a FOO case";
$string =~ s/(Foo)/preserve_case($1, "bar")/egi;
print "$string
";

# this is a BAR case