How do I swap substrings within a string?

Question

I am trying to swap all occurrences of a pair of substrings within a given string.

For example, I may want to replace all occurrences of "coffee" with "tea" and all occurrences of "tea" with "coffee".

This is the first thing I thought of:

var newString = oldString.replace(/coffee/g, "__").replace(/tea/g, "coffee").replace(/__/g, "tea");

It works most of the time, but if my input string contains the substring "__", it will not work properly.

I am looking for something that works regardless of what input I give it, so I thought some more and came up with this:

var pieces = oldString.split("coffee");
for (var i = 0; i < pieces.length; i++)
  pieces[i] = pieces[i].replace(/tea/g, "coffee");
var newString = pieces.join("tea");

It works fine but it is kind of ugly and verbose. I tried to come up with something more concise and I used the map function built into jQuery to come up with this:

var newString = $.map(oldString.split("coffee"), function(piece) {
  return piece.replace(/tea/g, "coffee");
}).join("tea");

That is better but I still have a feeling that there is some brilliantly simple method that is failing to come to my mind. Does anyone here know a simpler way?

score 8 · Accepted Answer · edited Aug 22 '11 at 19:43

8

What about

theString.replace(/(coffee|tea)/g, function($1) {
     return $1 === 'coffee' ? 'tea' : 'coffee';
});

(Personally I think it's a crime to swap coffee and tea, but that's your business)

edited Aug 22 '11 at 19:43

Robert Harvey

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answered Aug 22 '11 at 17:58

Alexander Gessler

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be careful if one of the strings is a substring of the other. put the longer one first. (and obviously escape any special characters) – Patrick Parker Mar 07 '19 at 06:31

score 2 · Answer 2 · edited Aug 22 '11 at 19:42

2

You can use a function:

var str = "I like coffee more than I like tea";
var newStr = str.replace(/(coffee|tea)/g, function(x) {
   return x === "coffee" ? "tea" : "coffee";
});
alert(newStr);

Running example

edited Aug 22 '11 at 19:42

Elias Zamaria

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answered Aug 22 '11 at 18:00

epascarello

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score 0 · Answer 3 · answered Mar 08 '20 at 20:08

Based on @alexander-gessler's answer, but with support for dynamic inputs.

(While potentially opening the door to code injection.)

function swapSubstrings(string, a, b) {
  return string.replace(
    new RegExp(`(${a}|${b})`, "g"),
    match => match === a ? b : a
  )
}

score 0 · Answer 4 · answered Aug 22 '11 at 17:57

0

Close. Consider this:

var newString = oldString.replace("tea|coffee", function(match) {
    // now return the right one, just a simple conditional will do :)
});

Happy coding.

answered Aug 22 '11 at 17:57

score 0 · Answer 5 · answered Aug 22 '11 at 18:03

Use a map (an object) to define what replaces what and then a function to tap into the map:

D:\MiLu\Dev\JavaScript :: type replace.js
function echo(s) { WScript.Echo(s) }
var
map = { coffee: "tea", tea: "coffee" },
str = "black tea, green tea, ice tea, teasing peas and coffee beans";
echo( str.replace( /\b(tea|coffee)\b/g, function (s) { return map[s] } ) );

D:\MiLu\Dev\JavaScript :: cscript replace.js
black coffee, green coffee, ice coffee, teasing peas and tea beans

How do I swap substrings within a string?

5 Answers5

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