String.replace method in javascript using regex

Question

I came across this example in MDN documentation on using the replace method on strings.

This is the example cited there

var re = /(\w+)\s(\w+)/;
var str = "John Smith";
var newstr = str.replace(re, "$2, $1");
print(newstr);//Smith,John

I changed the regex to the following and tested it.

var re = /(\w?)(\w+)/;
var str = "John Smith";
var newstr = str.replace(re, "$1, $1");
newstr;//J, ohn Smith
var newstr1=str.replace(re,"$2, $1");
newstr1;//ohn, J Smith.

$1 must be J and $2 must be ohn Smith in this example. when I reversed the order of $n for newstr1,it should be 'ohn Smith, J`. but it is not.

is my understanding of $1 and $2 (substring matches correct) and why newstr1 is different?

Thanks for the comments

Paul S. · Accepted Answer · 2013-06-21 23:55:35Z

2

Actually, $1 is "J", $2 is "ohn" and the " Smith" is unmatched.

var re = /(\w?)(\w+)/,
    str = "John Smith";

str.replace(re, function (match, $1, $2) {
    console.log('match', match);
    console.log('$1', $1);
    console.log('$2', $2);
    return ''; // leave only unmatched
});
/* match John
   $1 J
   $2 ohn
   " Smith"
*/

Therefore, your swap is switching around the J with the ohn, giving you newstr1.

Why is this happening? because \w matches a word, but ? makes it optional, so just like (.*?)(.) captures one letter in $1, (\w?) is doing the same. The second capture, (\w+) can then only extend to the end of the word, dispite the +, because \w doesn't match whitespace \s.

edited Jun 21, 2013 at 23:55

answered Jun 21, 2013 at 23:50

Paul S.

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