Replace part of string that doesn't match regex

Question

I am attempting to replace parts of a string that don't match a regular expression pattern using JavaScript. This is functionally equivalent to using the -v flag in a GNU grep to invert results. Here is an example:

// I want to replace all characters that don't match "fore" 
// in "aforementioned" with "*"

'aforementioned'.replace(new RegExp(pattern, 'i'), function(match){
    //generates a string of '*' that is the length of match
    return new Array(match.length).join('*');
});

I am looking for a regex for pattern . This would be something like the opposite of (fore) . I've searched around but haven't been able to implement any related question's answers to fit my needs. Here is a list anyway, maybe it will point us in the right direction:

• Regular Expressions and negating a whole character group
• Match everything except for specified strings

Answer 1

If I understand you correctly, this should be one possible solution:

'aforementioned'.replace(new RegExp(pattern + '|.', 'gi'), function(c) {
    return c === pattern ? c : '*';
});

>> "*fore*********"