Local Regex Match Variable Not Updated

Question

I am looping through an array of objects and mapping them to my own custom objects. I am extracting data via regular expressions. My first run through the loop works fine, but in subsequent iterations, although they match, the match variables do not get set.

Here is one of the regex's:

var gameRegex   = /(\^?)([A-z]+)\s?(\d+)?\s+(at\s)?(\^?)([A-z]+)\s?(\d+)?\s\((.*)\)/g;

Here is the initial part of my loop:

for(var i = 1; i <= data.count; i++) {
    var gameMatch = gameRegex.exec(data["left" + i]);
    var idMatch = idRegex.exec(data["url" + i]);

First time through, gameMatch and idMatch have values. The following iterations do not work even though I have tested that they do work.

Is there something about regular expressions, maybe especially in Node.js, that I need to do if I use them more than once?

This might help http://stackoverflow.com/questions/889067/javascript-regular-expressions-exec-infinite-loop — elclanrs, Jan 06 '13 at 08:13
Thanks for the link, but I'm not looking at extracting multiple matches from the same string. I'm extracting one match (with groups) per string. — Benny, Jan 06 '13 at 08:17
If you want to match the regex only once per string, remove the `g`. — Tomalak, Jan 06 '13 at 08:24
@Tomalak, that was it! Thank you! Can you please post that as an answer so I can accept it? — Benny, Jan 06 '13 at 08:31

Tomalak · Accepted Answer · 2013-01-06T09:19:29.533

When you have a regular expression with a global flag /.../g and use exec() with it, JavaScript sets a property named lastIndex on that regex.

s = "abab";
r = /a/g;
r.exec(s);    // ["a"]
r.lastIndex   // 1
r.exec(s);    // ["a"]
r.lastIndex   // 3
r.exec(s);    // null
r.lastIndex   // 0

This is meant to be used for multiple matches in the same string. You could call exec() again and again and with every call lastIndex is increased - defining automagically where the next execution will start:

while (match = r.exec(s)) {
  console.log(match);
}

Now lastIndex will be off after the first invocation of exec(). But since you pass in a different string every time, the expression will not match anymore.

There are two ways to solve this:

manually set the r.lastIndex = 0 every time or
remove the g global flag

In your case the latter option would be the right one.

Local Regex Match Variable Not Updated

1 Answers1