Replacing multiple patterns in a block of data

Question

I need to find the most efficient way of matching multiple regular expressions on a single block of text. To give an example of what I need, consider a block of text:

<

Answer 1

An improved answer:

var index = {
    'Hello': 'Bye',
    'World': 'Universe'
};

var pattern = '';
for (var i in index) {
    if (pattern != '') pattern += '|';
    pattern += i;
}

var text = "Hello World what a beautiful day";
text.replace(new RegExp(pattern, 'g'), function($0) {
    return index[$0] != undefined ? index[$0] : $0;
});