Maybe this is what you mean — i.e. replace (for example) 'cat' with 'dog', but only if 'Cat' is capitalized, the replacement word should also be capitalized, otherwise not (?)

#!/usr/bin/perl -wl
use strict;

my %repl = (
   cat => 'dog',
   foo => 'bar',
);

sub repl {
    my $word = shift;
    my $lookup = lc $word;
    return $word unless exists $repl{$lookup}; # nothing to replace
    my $is_up = $word =~ /^[[:upper:]]/;       # determine caps style
    my $repl = $repl{$lookup};
    $repl = ucfirst $repl if $is_up;           # apply caps style
    return $repl;
}

my $s = 'cat Cat foo Foo Blah blub';

$s =~ s/\b(\w+)\b/repl($1)/eg;

print $s;   # dog Dog bar Bar Blah blub
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This could also be extended to handle all-uppercase words, too, or in theory any cOMbinatION, in which case the source and replacement word would have to have the same length, though.

Bingo!! This is exactly what I wanted!!

This script beautifully handles the first letter of the word. You'll still have to define the (complex) rules for this in case you need more than just first letter. For example if you want "animal" to be replaced by "pet" then what should "ANiMal" be replaced with?

-- Regards - Samar

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

s/cat Cat/dog Dog/
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Sorry, I should be more specific, I need a generic reg exp taht can handle all words. The 'cat' word is just an example, it could be anything else.

You're in luck, /\w+/ is a regular expression that can handle all words!

...roboticus

Mr. Tautology

</snark-mode>

so use \w+

$_ = 'blah Blah';
s/blah Blah/dog Dog/;
print $_;
__END__
dog Dog
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