Running exec() inside a loop in Javascript

Question

I am not sure what seems to be the issue but running exec() inside for loop in Javascript(Nodejs, Chrome console, Firefox console) is producing incorrect results. As far as I know, exec() is a synchronous method, called inside a synchronous loop which should produce the expected output.

const FILTER_REGEX = /(\w+)\s+(below|under|lesser than|lower than|above|over|more than|higher than|greater than|equals|equal to|same as|>|<|=)\s+(\d+)/gi;
const searchQuery = "Package Quantity > 50000 Date yearly ListPrice above 100";
const filterMatches = searchQuery.match(FILTER_REGEX) || [];
for(const filterMatch of filterMatches) {
    const filterMatchGroups = FILTER_REGEX.exec(filterMatch);
    console.log(`filterMatch: ${filterMatch}, filterMatchGroups: ${filterMatchGroups}`);
}

Currently, I am getting the following as output, sometimes filterMatchGroups for first string becomes null and gives filterMatchGroups for second one.

filterMatch: Quantity > 50000, filterMatchGroups: Quantity > 50000,Quantity,>,50000
filterMatch: ListPrice above 100, filterMatchGroups: null

It almost is, but this question is still open. Problem is that your `FILTER_REGEX` is global and has a *state* (the `lastIndex` property). If you ran a new regex every time, output would be as expected. https://jsfiddle.net/fy0p9jbk/ — CertainPerformance, Mar 04 '19 at 04:51
https://stackoverflow.com/questions/1520800/why-does-a-regexp-with-global-flag-give-wrong-results — Andreas, Mar 04 '19 at 04:52
@CertainPerformance Doesn't global identifier mean that it finds all matches rather than stopping at the first one right? Correct me if i am wrong. — PrivateOmega, Mar 04 '19 at 04:52
Not when you use `.exec`, which will return the *next* match (until it gets to the end and can find no more matches, and then it'll start over) — CertainPerformance, Mar 04 '19 at 04:53
@Andreas Thanks, I wasn't aware that it uses last index for the next check also, So I am hoping resetting lastIndex of regex should make it all good. It's like a really good pitfall I would have never realized if I didn't ask. — PrivateOmega, Mar 04 '19 at 04:56
I generally don't bother with `exec`; it might be a bit of a performance overhead, but I'll generally use `replace` to search for all results instead of `exec` (even as I'm not interested in replacing anything). No pesky state with `replace`. — Amadan, Mar 04 '19 at 05:22
You can use `RegExp.lastMatch` (MDN says it has poor cross-browser compatibility, but I haven't seen it fail for me yet); but the standard way would be via [callback parameters](https://jsbin.com/kahivolulo/edit?js,console). — Amadan, Mar 04 '19 at 06:06

score 0 · Accepted Answer · answered Mar 04 '19 at 05:03

This should do it.

const FILTER_REGEX = /(\w+)\s+(below|under|lesser than|lower than|above|over|more than|higher than|greater than|equals|equal to|same as|>|<|=)\s+(\d+)/gi;
const searchQuery = "Package Quantity > 50000 Date yearly ListPrice above 100";
const filterMatches = searchQuery.match(FILTER_REGEX) || [];
for(const filterMatch of filterMatches) {
    FILTER_REGEX.lastIndex = 0;
    const filterMatchGroups = FILTER_REGEX.exec(filterMatch);
    console.log(`filterMatch: ${filterMatch}, filterMatchGroups: ${filterMatchGroups}`);
}

The only visible difference is FILTER_REGEX.lastIndex = 0, we have to reset lastIndex property of a Regex if we have to use it again else it would start the regex check from the previously stored lastIndex. Thanks @CertainPerfomance @Andreas for the links and explanation.

Ref: Detailed Explanation

Running exec() inside a loop in Javascript

1 Answers1