How to modify a JavaScript RegExp pattern

Question

I want to create a new RegExp object by modifying the pattern from another RegExp object.

The output of RegExp.prototype.toString() includes leading and trailing slashes.

    // This is nice and clean, but produces incorrect output.
    function addBarWrong(re) {
            return new RegExp(re+"bar");
    }

    addBarWrong(new RegExp("foo")); // -> /\/foo\/bar/

My naive approach is pretty inelegant.

    // This is unpleasant to look at, but produces correct output.
    function addBarUgly(re) {
            var reString = re.toString();

            reString = reString.substr(1, reString.length - 2);

            return new RegExp(reString+"bar");
    }

    addBarUgly(new RegExp("foo")); // -> /foobar/

Surely there's a better way. What is the clean solution here?

Barmar · Accepted Answer · 2021-03-26 15:32:00Z

16

Use the source (Luke) property.

function addBarElegant(re) {
  var reString = re.source;
  return new RegExp(reString + "bar", re.flags);
}
console.log(addBarElegant(/foo/));

edited Mar 26, 2021 at 15:32

answered Apr 5, 2018 at 0:13

Barmar

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1 Comment

Mmmh mmh Over a year ago

Would we be complete with new RegExp(re.source, re.flags)?

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