Question

I was surprised to see that

/a/ === /a/

evaluates to false in JavaScript. Reading through the specs:

Two regular expression literals in a program evaluate to regular
expression objects that never compare as === to each other even if the
two literals’ contents are identical.

Since === cannot be used to test for equality, how can equality of regular expressions be tested in JavaScript?

Answer 1

Here’s a function that fully tests all the relevant regex properties and makes sure it’s the right type of object:

function regexSame(r1, r2) {
    if (r1 instanceof RegExp && r2 instanceof RegExp) {
        var props = ["global", "multiline", "ignoreCase", "source", "dotAll", "sticky", "unicode"];
        for (var i = 0; i < props.length; i++) {
            var prop = props[i];
            if (r1[prop] !== r2[prop]) {
                return false;
            }
        }
        return true;
    }
    return false;
}

---

And, since flags sometimes get added to the regex object with new features (as has happened since this original answer in 2012 – though the above code has been updated as of 2019), here’s a version that is a bit more future proof on future flags being added since it compares whatever flags are there rather than looking for a specific set of flags. It sorts the flags before comparing to allow for minor differences in how the regex was specified that wouldn’t not actually change functionality.

function regexSame(r1, r2) {
    return r1 instanceof RegExp && 
           r2 instanceof RegExp &&
           r1.source === r2.source &&
           r1.flags.split("").sort().join("") === r2.flags.split("").sort().join("");
}