Question

When I want to replace some parts of a string, should I call replace directly like this?

var r1 = /"\+((:?[\w\.]+)(:?(:?\()(:?.*?)(:?\))|$){0,1})\+"/g;
arg = arg.replace(r1, function(outer, inner){
    return eval(inner);
});

Or test for a match first, and then replace if it’s a hit, like this?

var r1 = /"\+((:?[\w\.]+)(:?(:?\()(:?.*?)(:?\))|$){0,1})\+"/g;
if (r1.test(arg)) {
    arg = arg.replace(r1, function(outer, inner){
        return eval(inner);
    });
}

I guess this boils down to how the string.replace(regex, string) function works. Will it go into my callback even if there is no match, or will it then simply return arg? In that case I assume the calling replace directly is the right way to go to avoid having the regex engine match the string twice?

Answer 1

You don have to use test. The function in replace only executed when a match occurs.

• No matches: No function (eval) call
• 1 match: 1 call
• 2 matches: 2 calls
• etc.

Also, why are you using eval? eval executes the parameter, as if it’s a JavaScript expression. Since you know the input format, it’s likely that you’re able to achieve the same behaviour without eval.