javascript replace function to replace substring

Question

I have this string:

global_filter[default_fields[0]]

I want to convert it to this string:

global_filter[dashboard_fields[0]]

I would like to use the replace function to detect the matched substring 'default_fields' and then replace it with 'dashboard_fields'.

First I try this:

var str = "global_filter[default_fields[0]]";
var regex = /\w+\[(.+)\[\d+\]\]/;
str.replace(regex, function(match, index){
  console.log(match); console.log(index);
});
=> global_filter[default_fields[0]]
=> default_fields

For some strange reason, the matched substring comes as the index argument, not the match argument. The index argument is supposed to be a position where the match is found. So obviously, I am doing something wrong. So I try to use positive lookahead instead:

str.replace(/(?=\w+\[).+(?=\[\d+\]\])/, function(match, index){
  console.log(match); console.log(index);
})
=> global_filter[default_fields
=> 0

That clearly does not work either. How am I supposed to make match equal the substring I want?

Um var str = global_filter[default_fields[0]]; I do not see the string. Are you missing the quotes? — epascarello
– epascarello, Commented Oct 7, 2015 at 21:44
Given what you just said, can you literally replace('default_fields', 'dashboard_fields')? — Brian
– Brian, Commented Oct 7, 2015 at 21:45

anubhava · Accepted Answer · 2015-10-07 21:50:32Z

1

You don't need to replace callback function, just use captured groups as:

var str = "global_filter[default_fields[0]]";
str = str.replace(/(\w+\[).+?(\[\d+\]\])/g, '$1dashboard_fields$2');
//=> global_filter[dashboard_fields[0]]

Reason why you're seeing a captured group when you print index is because index is always supposed to be last argument of callback function after all the matched groups. Do note that you have (.+) as a captured group.

So if you use:

str.replace(/\w+\[(.+)\[\d+\]\]/, function(m0, m1, index){
  console.log(m0); console.log(index);
});

Then it will correctly print:

global_filter[default_fields[0]]
0

edited Oct 7, 2015 at 21:50

answered Oct 7, 2015 at 21:46

anubhava

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1 Comment

Daniel Viglione Over a year ago

Thanks for answer. Can you also explain why the replace function is passing a string for the index argument, when the index is supposed to be the position where the string was found.

user2182349 · Accepted Answer · 2015-10-07 21:44:35Z

0

You don't have to use a regular expression, you can just use a string.

var str = 'global_filter[default_fields[0]]';

console.log(str.replace('default','dashboard'));

answered Oct 7, 2015 at 21:44

user2182349

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