CODEWITHSUNDEEP
javascriptregex
ruslan.savenok
ruslan.savenok

Reputation: 105

How can I replace a match only at a certain position inside the string?

So, I have a function which has two params: string and match index to replace and i need to replace only match with that index. How can i do that?

Example:

replace('a_a_a_a_a', 1)

Result:

a__a_a_a

Upvotes: 9

Views: 12521

Answers (5)

Tigregalis
Tigregalis

Reputation: 666

You need to use the function parameter of the replace function, and also maintain an index (i) of matches.

function replaceAt(string, replaceThis, replaceWith, at) {
  let i = 0;
  if (Array.isArray(at)) {
    return string.replace(replaceThis, (match, offset) => at.includes(i++) ? replaceWith : match)
  } else if (Number.isInteger(at)) {
    return string.replace(replaceThis, (match, offset) => i++ === at ? replaceWith : match)
  } else {
    return string;
  }
}
// usage:
console.log(replaceAt("aaaaabcde", /a/g, ".", [2, 0])); // ".a.aaabcde"
console.log(replaceAt("aaaaabcde", /a/g, ".", 2)); // "aa.aabcde"

String.prototype.replaceAt = function(replaceThis, replaceWith, at) {
  let string = this, i = 0;
  if (Array.isArray(at)) { // e.g. console.log("aaaaabcde".replaceAt(/a/g, ".", 2)); => "aa.bcde"
    return string.replace(replaceThis, (match, offset) => at.includes(offset) ? replaceWith : match)
  } else if (Number.isInteger(at)) { //e.g. console.log("aaaaabcde".replaceAt(/a/g, ".", [2, 0])); => ".a.bcde"
    return string.replace(replaceThis, (match, offset) => offset === at ? replaceWith : match)
  } else {
    return string;
  }
}
// usage:
console.log("aaaaabcde".replaceAt(/a/g, ".", [2, 0])); // ".a.aaabcde"
console.log("aaaaabcde".replaceAt(/a/g, ".", 2)); // "aa.aabcde"

function replaceExceptAt(string, replaceThis, replaceWith, at) {
  let i = 0;
  if (Array.isArray(at)) { // e.g. console.log(replaceExceptAt("aaaaabcde", /a/g, ".", 2); => "..a..bcde"
    return string.replace(replaceThis, (match, offset) => !at.includes(i++) ? replaceWith : match)
  } else if (Number.isInteger(at)) { //e.g. console.log(replaceExceptAt("aaaaabcde", /a/g, ".", [2, 0])); => "a.a..bcde"
    return string.replace(replaceThis, (match, offset) => i++ !== at ? replaceWith : match)
  } else {
    return string;
  }
}
// usage:
console.log(replaceExceptAt("aaaaabcde", /a/g, ".", [2, 0])); // "a.a..bcde"
console.log(replaceExceptAt("aaaaabcde", /a/g, ".", 2)); // "..a..bcde"

String.prototype.replaceExceptAt = function(replaceThis, replaceWith, at) {
  let string = this, i = 0;
  if (Array.isArray(at)) { // e.g. console.log("aaaaabcde".replaceExceptAt(/a/g, ".", 2); => "..a..bcde"
    //return string.replace(replaceThis, (match, offset) => !at.includes(offset) ? replaceWith : match)
    return string.replace(replaceThis, (match, offset) => !at.includes(i++) ? replaceWith : match)
  } else if (Number.isInteger(at)) { //e.g. console.log(replaceAt("aaaaabcde", /a/g, ".", [2, 0])); => "a.a..bcde"
    return string.replace(replaceThis, (match, offset) => i++ !== at ? replaceWith : match)
  } else {
    return string;
  }
}
// usage:
console.log("aaaaabcde".replaceExceptAt(/a/g, ".", [2, 0])); // "a.a..bcde"
console.log("aaaaabcde".replaceExceptAt(/a/g, ".", 2)); // "..a..bcde"

Upvotes: 0

Shadow Wizard
Shadow Wizard

Reputation: 66389

For those like me who find regex to be cryptic, here is "pure JavaScript" way as well:

function CustomReplace(strData, strTextToReplace, strReplaceWith, replaceAt) {
    var index = strData.indexOf(strTextToReplace);
    for (var i = 1; i < replaceAt; i++)
        index = strData.indexOf(strTextToReplace, index + 1);
    if (index >= 0)
        return strData.substr(0, index) + strReplaceWith + strData.substr(index + strTextToReplace.length, strData.length);
    return strData;
}

Usage:

var mystr = 'a_a_a_a_a';
var newstr = CustomReplace(mystr, "_", "__", 2); //replace the second appearance

Live test case: http://jsfiddle.net/tXx5n/2/

Upvotes: 9

jAndy
jAndy

Reputation: 235982

Could look like:

var mystr = 'a_a_a_a_a';

function replaceIndex(string, at, repl) {
   return string.replace(/\S/g, function(match, i) {
        if( i === at ) return repl;

        return match;
    });
}

replaceIndex(mystr, 2, '_');

The above code makes usage of the fact, that .replace() can take a funarg (functional argument) as second parameter. That callback is passed in the current match of the matched regexp pattern and the index from that match (along with some others which we are not interested in here). Now that is all information we need to accomplish your desired result. We write a little function wish takes three arguments:

  • the string to modify
  • the index we wish to change
  • the replacement character for that position

Upvotes: 12

rajani
rajani

Reputation: 66

Use regex match with g flag var str="test string".match(/t/g) output would be array of matched strings in this case "t" [t,t,t]

Upvotes: 0

sergio
sergio

Reputation: 69027

Javascript match returns an array, in case of multiple matches, so you could so something like this:

var string = "....";
var patt1 = /..../gi;
var results = string.match(patt1);
var newString = results.splice(i, i).join();

Instead of using a match, you could use split instead in your specific case:

var results = string.split("_");
var newString = results.splice(i, i).join("_");

It depends on how your input data can vary and where you need to do the split/match (that is why I did not specify any regex above, the split example is complete)...

Upvotes: 1

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