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

Here's a lovely Perlish solution by Larry Rosler. It exploits properties
of bitwise xor on ASCII strings.

$_= "this is a TEsT case";

$old = 'test';
$new = 'success';

{ uc $new | (uc $1 ^ $1) .
(uc(substr $1, -1) ^ substr $1, -1) x
(length($new) - length $1)


And here it is as a subroutine, modeled after the above:

sub preserve_case($$) {
my ($old, $new) = @_;
my $mask = uc $old ^ $old;

uc $new | $mask .
substr($mask, -1) x (length($new) - length($old))

$string = "this is a TEsT case";
$string =~ s/(test)/preserve_case($1, "success")/egi;
print "$string\n";

This prints:

this is a SUcCESS case

As an alternative, to keep the case of the replacement word if it is
longer than the original, you can use this code, by Jeff Pinyan:

sub preserve_case {
my ($from, $to) = @_;
my ($lf, $lt) = map length, @_;

if ($lt < $lf) { $from = substr $from, 0, $lt }
else { $from .= substr $to, $lf }

return uc $to | ($from ^ uc $from);

This changes the sentence to "this is a SUcCess case."

Just to show that C programmers can write C in any programming language,
if you prefer a more C-like solution, the following script makes the
substitution have the same case, letter by letter, as the original. (It
also happens to run about 240% slower than the Perlish solution runs.)
If the substitution has more characters than the string being
substituted, the case of the last character is used for the rest of the

# Original by Nathan Torkington, massaged by Jeffrey Friedl
sub preserve_case($$)
my ($old, $new) = @_;
my ($state) = 0; # 0 = no change; 1 = lc; 2 = uc
my ($i, $oldlen, $newlen, $c) = (0, length($old), length($new));
my ($len) = $oldlen < $newlen ? $oldlen : $newlen;

for ($i = 0; $i < $len; $i++) {
if ($c = substr($old, $i, 1), $c =~ /[\W\d_]/) {
$state = 0;
} elsif (lc $c eq $c) {
substr($new, $i, 1) = lc(substr($new, $i, 1));
$state = 1;
} else {
substr($new, $i, 1) = uc(substr($new, $i, 1));
$state = 2;
# finish up with any remaining new (for when new is longer than old)
if ($newlen > $oldlen) {
if ($state == 1) {
substr($new, $oldlen) = lc(substr($new, $oldlen));
} elsif ($state == 2) {
substr($new, $oldlen) = uc(substr($new, $oldlen));
return $new;


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