Global regexes & .test()

The Problem

The more I've used regular expressions in JavaScript, the more I've tended to use the global flag, since this is typically the behavior I want (for example, when doing str.replace(regex) or regex.exec(str)).

Recently, I created a regex to determine if a SQL statement manipulates the DB (insert/update/delete) or if it's a simple query (select):

let is_change_regex = /^INSERT|^UPDATE|^DELETE/gi;

Notice that I included a g flag at the end to make it global out of habit.

However, when I ran is_change_regex.test(sql) against several DELETE statements, every other one returned true. They are all DELETE statements, so I had expected all of them to return true.

Understanding Why

As usual, MDN gives a clear explanation of what's going on in the RegExp.test() docs:

When a regex has the global flag set, test() will advance the lastIndex of the regex. Further calls to test(str) will resume searching str starting from lastIndex. The lastIndex property will continue to increase each time test() returns true.

Then it highlights the gotcha with a Note:

Note: As long as test() returns true, lastIndex will not reset — even when testing a different string!

This mostly makes sense when thinking about it from an implementation standpoint. The .lastIndex property is how a global regex can maintain state when replacing something multiple times throughout a string, or when .exec() is called multiple times against the same string. It makes sense that using a global regex with .test() would allow the caller to test() the string multiple times, walking through the string to see how many regex matches there are in it.

But it's counter-intuitive that it doesn't reset the .lastIndex when testing a different string. Perhaps the ECMAScript Committee felt like that was too much magic, or may not be performant.

TL/DR

• Unless you're intentionally calling .test() multiple times with the same string to see how many matches are in the string, you probably don't want to use .test() with a global regex. It's best to use a regular regex, to avoid unintuitive behavior.
• It's probably better to only use global regexes when you know you need them (typically for str.replace(regex) and regex.exec(str)), and in other cases to default to regular, non-global regexes.