I have a string which contains characters that should be replaced once (at first appearance).

These characters are:

• L => will be replaced with the lecture name
• N => will be replaced with a name
• D => will be replaced with a date

Example input:

L_N_L_D

Desired result (note that only the first L is replaced):

Math_Ex01_L_2018-10-05

My current code (simplified for the example):

let res = file_string.replace(/L|N|D/, x => {
  switch (x) {
    case 'L': return lecture;
    case 'N': return name;
    case 'D': return date;
    default: return x;
  }
});

What I get is this:

Math_L_N_D

If I change the regex to /L|N|D/g the second L will also be replaced which is not what I want.

How could this be implemented?

I have a string which contains characters that should be replaced once (at first appearance).

These characters are:

• L => will be replaced with the lecture name
• N => will be replaced with a name
• D => will be replaced with a date

Example input:

L_N_L_D

Desired result (note that only the first L is replaced):

Math_Ex01_L_2018-10-05

My current code (simplified for the example):

let res = file_string.replace(/L|N|D/, x => {
  switch (x) {
    case 'L': return lecture;
    case 'N': return name;
    case 'D': return date;
    default: return x;
  }
});

What I get is this:

Math_L_N_D

If I change the regex to /L|N|D/g the second L will also be replaced which is not what I want.

How could this be implemented?

• javascript
• replace

• 2 let res = file_string.replace('D', date).replace('N', name).replace('L', lecture) - note the order – Jaromanda X Commented Oct 8, 2018 at 11:52
• I also tried this, but if the lecture contains the character 'N' for example the 'N' of the lecture will be replaced instead of the one in the file_string. – Vkay Commented Oct 8, 2018 at 11:56
• not if you do it in the order I did it ... note the order – Jaromanda X Commented Oct 8, 2018 at 11:57
• The order is not fixed in my case, it can be dynamically changed by the user with a textfield. So if the name contains L I have the same problem as in the second ment. – Vkay Commented Oct 8, 2018 at 12:01
• oh ... OK ... thought it was fixed order :p – Jaromanda X Commented Oct 8, 2018 at 12:01

2 Answers 2

Put your replacements in a map, once a replacement is made, set map[x] to x:

let lecture = "Math";
let name = "Ex01";
let date = "2018-10-05";

let repl = {
  'L': lecture,
  'N': name,
  'D': date
};

let file_string = "L_N_L_D"

let result = file_string.replace(/[LND]/g, x => {
  let r = repl[x];
  repl[x] = x;
  return r;
});

console.log(result)

Apart from solving the problem at hand, this also greatly simplifies your replacement function (think adding new placeholders, for example).

That being said, a real solution to your problem would be to follow the @marsze's advice and use unambiguous placeholders, like {...}, in which case the whole enterprise bees simply

 repl = {...as before...}
 result = subject.replace(/{(.+?)}/g, (_, x) => repl[x])

Answering your general question "replace only the first occurrence of a character", you could do it like this:

var lecture = "Math";
var name = "Ex01";
var date = "2018-10-05";
var found = {};
var file_string = "L_N_L_D";
var filename_result = file_string.split("").map(function (character) {
  if (!found[character]) {
    found[character] = true;
    switch (character) {
      case "L": return lecture;
      case "N": return name;
      case "D": return date;
    }
  }
  return character;
}).join("");
console.log(filename_result);

You should probably explain where that odd format of the file_string es from. Are there other similar use cases? Or is this just about this specific example? Understanding the original requirements would help a lot.

If the file_string is supposed to be a configurable format string, then it should be improved. A sequence which serves as a placeholder to be replaced should (or must) be different from a literal, e.g.:

var filename_format = "{L}_{N}_L_{D}";