Replace last occurrence in string

JavaScript, String, Regexp · Apr 22, 2021

Replaces the last occurrence of a pattern in a string.

Use typeof to determine if pattern is a string or a regular expression.
If the pattern is a string, use it as the match.
Otherwise, use the RegExp constructor to create a new regular expression using the RegExp.prototype.source of the pattern and adding the 'g' flag to it. Use String.prototype.match() and Array.prototype.slice() to get the last match, if any.
Use String.prototype.lastIndexOf() to find the last occurrence of the match in the string.
If a match is found, use String.prototype.slice() and a template literal to replace the matching substring with the given replacement.
If no match is found, return the original string.

const replaceLast = (str, pattern, replacement) => {
  const match =
    typeof pattern === 'string'
      ? pattern
      : (str.match(new RegExp(pattern.source, 'g')) || []).slice(-1)[0];
  if (!match) return str;
  const last = str.lastIndexOf(match);
  return last !== -1
    ? `${str.slice(0, last)}${replacement}${str.slice(last + match.length)}`
    : str;
};

replaceLast('abcabdef', 'ab', 'gg'); // 'abcggdef'
replaceLast('abcabdef', /ab/, 'gg'); // 'abcggdef'
replaceLast('abcabdef', 'ad', 'gg'); // 'abcabdef'
replaceLast('abcabdef', /ad/, 'gg'); // 'abcabdef'

Written by Angelos Chalaris

I'm Angelos Chalaris, a JavaScript software engineer, based in Athens, Greece. The best snippets from my coding adventures are published here to help others learn to code.

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