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JavaScript replace

In the following code, the alert(a) inside the JavaScript replace function will alert the matched strings, which, in this case, will be {name} and {place}.

This works as described by the documentation javascript docs , namely, the first argument to the function in the replace method will be the matched string. In the code below, the alert(b) will alert 'name' and 'place' but without the curly braces around them.

Why is that? How does it strip the curly braces for 'b'? Here's a fiddle http://jsfiddle.net/mjmitche/KeHdU/

Furthermore, looking at this example from the docs, 

function replacer(match, p1, p2, p3, offset, string){
 // p1 is nondigits, p2 digits, and p3 non-alphanumerics
 return [p1, p2, p3].join(' - ');
};

Which of the parameters in this example would the 'b' in function(a,b) of the replace function below represent?

Part of my failure to understand might be due to the fact that I'm not sure what javascript does, for example, with the second parameter if the maximum number of arguments aren't used.

code

var subObject = {
 name: "world",
 place: "google"
}; 
var text = 'Hello, {name} welcome to {place}';
var replace = function (s, o) {
 return s.replace(/\{([^{}]*)\}/g,
 function (a, b) {
 alert(a);
 alert(b);
 var r = o[b];
 return typeof r === 'string' || typeof r === 'number' ? r : a;
 }
 );
}; 
var replacedText = replace(text, subObject); 
alert(replacedText); ​

Answer*

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  • so the curly braces aren't part of the capturing groups (and therefore not included in argument b) because they're not in the parentheses? Commented Dec 8, 2012 at 23:43
  • @BrainLikeADullPencil: Yep. The parentheses include everything that you want to capture. You can have multiple sets of parentheses. Commented Dec 8, 2012 at 23:43
  • @BrainLikeADullPencil: Here's a quick demo. It should be helpful: jsfiddle.net/KeHdU/5 Commented Dec 8, 2012 at 23:47

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