JavaScript - Regex: URL has a param

Question

I'm trying to create an analog for php's isset ($_GET[param]) but for JavaScript.
So long, I get this

[?&]param[&=]

But if param is at the end of URL (example.com?param) this regex won't work.
You can play with it here: https://regex101.com/r/fFeWPW/1

• Wiktor Stribiżew: Replace [&=] with (?:[&=]|$) or (?![^&=])
• George: This answer may help guide you: stackoverflow.com/a/901144/1612146
• mthomp: Do you not just need to add on ? as in [?&]param[&=]? to make the [=&] optional? Then it matches example.com?param
• Barmar: Why don't you just parse the entire query string into an object, then you can just check if a particular key is in the object.
• vaso123: @Quentin developer.mozilla.org/en-US/docs/Web/API/URL/URL Maybe, because it not compatible with all browsers?

Answer 1

If you want to make sure your match ends with &, = or end of string, you may replace the [&=] character class with a (?:[&=]|$) alternation group that will match &, = or end of string (note that $ cannot be placed inside the character class as it would lose its special meaning there and will be treated as a $ symbol), or you may use a negative lookahead (?![^&=]) that fails the match if there is no & or = immediately after the current location, which might be a bit more efficient than an alternation group.

So, in your case, it will look like

[?&]param(?:[&=]|$)

or

[?&]param(?![^&=])

See a regex demo

JS demo:

var strs = ['http://example.com?param', 'http://example.com?param=123', 'http://example.com?param&another','http://example.com?params'];
var rx = /[?&]param(?![^&=])/;
for (var s of strs) {
 console.log(s, "=>", rx.test(s))
}

• Wiktor Stribiżew: Note that [?&]param(?![^&=]) does not work inside a multiline test at regex101, but it works in real JS code when the strings are tested separately.