Why Does Javascripts Regex Exec Not Always Return The Same Value Duplicate

If you’ve been using JavaScript and tinkering with regular expressions, you might have stumbled upon a peculiar behavior of the `exec` method. Sometimes, it doesn’t quite return what you were expecting, especially when dealing with duplicate values. Let’s dive into why this happens and how you can handle it.

In JavaScript, the `exec` method is commonly used with regular expressions to search for matches within a string. When a match is found, `exec` returns an array containing the matched values. However, when you have a regex pattern with the global flag (`/g`), things can get a bit tricky, especially with duplicate matches.

The global flag in JavaScript regular expressions allows the regex engine to find all matches within the given string rather than stopping at the first match. This is where the behavior of `exec` can be a bit surprising. Each time you call `exec` on a regex with the global flag, it will return the next match until no more matches are found, and then it will return `null`.

So, why does `exec` not always return the same value for duplicates? Well, it’s important to understand that `exec` keeps track of the last index where a match occurred in the string. When you call `exec` again on the same string, it starts searching for a match from this last index onwards, which means you might not always get the same duplicate value in subsequent calls to `exec`.

To illustrate this behavior, consider the following code snippet:

const regex = /hello/g;
const str = 'hello world, hello universe';
let match;

while ((match = regex.exec(str)) !== null) {
    console.log(`Found '${match[0]}' at index ${match.index}`);
}

In this code, we have a simple regex pattern `/hello/g` to match the word "hello" globally within the string `'hello world, hello universe'`. The `exec` method is used in a loop to find and log each match along with its index in the string.

When you run this code, you might notice that the output is:

Found 'hello' at index 0
Found 'hello' at index 12

As you can see, the first call to `exec` finds the first occurrence of "hello" at index 0, and the second call finds the next occurrence at index 12.

To ensure you capture all duplicate values, you can reset the regex’s last index property manually before each call to `exec`. This can be done by setting the `lastIndex` property of the regex to 0 before calling `exec`. This ensures that the search starts from the beginning of the string on each iteration.

Here’s how you can modify the previous code snippet to reset the last index:

const regex = /hello/g;
const str = 'hello world, hello universe';
let match;

regex.lastIndex = 0; // Reset the lastIndex before each search

while ((match = regex.exec(str)) !== null) {
    console.log(`Found '${match[0]}' at index ${match.index}`);
}

By resetting the `lastIndex` property, you should now see both occurrences of "hello" logged correctly:

Found 'hello' at index 0
Found 'hello' at index 12

In conclusion, understanding the behavior of `exec` when dealing with global regex patterns is crucial to avoid unexpected results, especially with duplicate matches. By resetting the `lastIndex` property before each call to `exec`, you can ensure consistent behavior and capture all duplicate values successfully.