Why does regex test method return alternating true/false results in JavaScript?

Other Questions JavaScript
1

I’m having trouble understanding why my regex pattern behaves strangely when used in a loop. Here’s what I’m working with:

var pattern = /\$[A-Z]+[0-9]+/g;
for (var counter = 0; counter < 6; counter++) {
    if (pattern.test("$B2")) {
        console.log("Found match");
    }
    else {
        console.log("No match found");
    }
}

When I run this code, instead of getting consistent results, the output switches between “Found match” and “No match found” on each iteration. The string “$B2” should match the pattern every time, but it doesn’t. What’s causing this weird alternating behavior? I expected it to match all six times since the test string fits the regex pattern perfectly.

2

Indeed, this confused me as well initially. The core issue lies in JavaScript’s handling of the global flag in regex patterns. When the /g flag is used, the regex state is maintained between calls, meaning it tracks where the last match occurred. After finding a match, it moves the internal pointer past the matched text. Given that your test string “$B2” is only 3 characters long, subsequent calls to test() start from position 3, where it finds nothing, and subsequently resets to the beginning. You can resolve this by either removing the global flag for consistent results when testing the same string or manually resetting lastIndex to 0 before each test() call if the global flag is necessary.

3

The global flag /g is your problem. JavaScript maintains an internal lastIndex property that records where the last match ended. Each time you invoke test(), it starts searching from that position instead of the beginning. Thus, the first call successfully finds “$B2” at position 0, setting lastIndex to 3. On the next call, it begins at position 3, finds nothing, returns false, and resets lastIndex to 0. This leads to the alternating results you observe. To achieve consistent true results, consider removing the global flag, changing the pattern to /\$[A-Z]+[0-9]+/.

4

exactly! that g flag messes with things by keeping track of where it last matched. so when it finds a match, it starts looking from there on the next test, which can lead to those weird results. just remove it or reset lastIndex.