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

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6.5: 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))

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