Google Apps Script Regex exec() returning null

Question

When I run the code below in debug mode I get the expected value on the first iteration of the for loop but null on the second as seen on the images:

First iteration:

Second iteration:

What am I dong wrong?

The code I am using is:

var newer_than = ' newer_than:2d'; //added for faster debugging
var subjectIdentifier = '"Ingress Portal Submitted: "';
var searchString = 'subject:'+subjectIdentifier+newer_than;

function getPortalName(string) {
  var myRegexp = /: (.+)/g;
  var match = myRegexp.exec(string);
  var portalName = match[1];
  return portalName;
}

function getPortalsSubmitted() {
  var threads = GmailApp.search(searchString);
  for (i=0; i<threads.length; i++) {
    var subject = threads[i].getFirstMessageSubject();

    var portalName = getPortalName(subject);
    var subDate = threads[i].getMessages()[0].getDate();

    Logger.log([portalName,subDate]);
  }
}

function updatePortals() {
  var threads = GmailApp.search('subject:"Ingress Portal"');
  for (i=0; i<threads.length; i++) {
    Logger.log(threads[i].getFirstMessageSubject());
  }
}

Answer 1

Although this question was already answered on the comments, I'll make a proper answer.

One important issue to understand this problem is on the exec behavior when the regex have the g flag. Which, when called sequentially will try to look for the "next" match, even if you pass on a different string. Here is the documentation link on MDN.

And although MDN states that you should take care not to re-create the RegExp object (even a literal), because it might reset the lastIndex property. At least in Apps Script that's not true. If a regex literal is used in the exact same spot in the code over and over, Apps Script caches the regex and re-uses the same object.

This two effects combined meant you were triggering this "next match" behavior on your code unknowingly.

The easiest solution for you is to just drop the g flag, since you don't need it anyway (you're getting only the first result). But you could have also fixed this by replacing the var myRegexp = /: (.+)/g; line with var myRegexp = new RegExp(': (.+)','g');, forcing Apps Script to give you a new object.

I think a good lesson we can learn from this is: don't use a flag if you don't need it. Sometimes we're lazy and set flags without thinking, "just in case".