RegExp.prototype.exec() can loop forever

Question

Steps to reproduce:

Open the MDN docs page: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/RegExp/exec.
Open 'Developer Tools' and switch to the Console tab.
Replace foo* with .* on the docs page and click 'Run' button.

This will result in an infinite loop. How do I prevent this from happening? In my Node application, the user can specify .* as the pattern and it hangs. I do not think doing a string-match and preventing this specific regex pattern would help as there might be many other patterns that will result in an infinite loop.

Source code (in case the content of the MDN page changes):

const regex1 = RegExp('.*', 'g');
const str1 = 'text';
let array1;

while ((array1 = regex1.exec(str1)) !== null) {
  console.log(array1.index);
}

Thanks, I found this on the page: **If the regex may match zero-length characters (e.g. /^/gm), increase its lastIndex manually each time to avoid being stuck in the same place.** I hope this works for me. — Jatin Sanghvi, Sep 25 '22 at 01:52
All patterns that can match an empty string exhibit this behaviour — Bergi, Sep 25 '22 at 02:10

score 1 · Answer 1 · answered Sep 25 '22 at 01:56

You could add a check that the lastIndex property of the regex has changed, and if it hasn't (indicating a zero-length match, which would then loop infinitely), break out of the loop:

const regex1 = RegExp('.*', 'g');
const str1 = 'text';
let array1;
let lastIndex = 0;

while ((array1 = regex1.exec(str1)) !== null) {
  console.log(array1);
  if (regex1.lastIndex == lastIndex) break;
  lastIndex = regex1.lastIndex;
}

Note you can put the test before doing anything with the result of the exec, depending on whether you want to capture the zero-length match or not.

Bergi · Accepted Answer · 2022-09-25T02:21:30.047

All patterns that can match an empty string exhibit this behaviour - see Regex that can match empty string is breaking the javascript regex engine or Zero-Length regexes and infinite matches?.

The simple solution would be to just use matchAll instead testing for some complicated loop condition yourself:

const regex = /.*/g;
const str = 'text';
for (const match of str.matchAll(regex)) {
  console.log(match.index, match[0]);
}

Alternatively, using str.match(regex) instead of regex.exec(str) would also help.

RegExp.prototype.exec() can loop forever

2 Answers2