Parsing URL Query Parameters using regex

Question

I would like to parse url query params intelligently using regex.

Things I've had to consider: 1) params can be out of order 2) only certain params must match

Given a query string: "?param1=test1&param2=test2&parm3=test3" I would like to run javascript regex to parse param1's value and param3's value.

The regex I've come up with so far is:

/(?:[?&](?:param1=([^&]*)|param3=([^&]*)|[^&]*))+$/g

This regex seems to work fine for me in sites like https://regex101.com/.

However, when I run the JS method below, I always get undefined for 2,ドル which is what param1's value should be parsing to. Any help or suggestion?

"?param1=test1&param2=test2&param3=test3".replace(
/(?:[?&](?:param1=([^&]*)|param3=([^&]*)|[^&]*))+$/g,
 function (0,ドル 1,ドル 2,ドル 3ドル) { return 0ドル + ' ' + 1ドル + ' ' + 2ドル + ' ' + 3ドル; });

This returns 2ドル as undefined and 3ドル as test3. However, if I exclude both param2 and param3 from the url query string, I am successfully able to parse param1 as 2ドル. A bit confused about that.

thanks!

• prasanth: Add with javascript tag
• Damon: What is your expected output of the replace?
• programmer33: I would like it 0ドル and 1ドル to output 'test1' and 'test3' it might actually be a javascript limitation: stackoverflow.com/questions/3537878/…
• Damon: You are finding those capture groups, but this regex only MATCHES one time. I still don't understand what your expected output of this operation is. Do you want to extract the values from param 1 and 3 into an array like ['test1', 'test3'] or do you want to replace them? Do you want to replace the whole string?
• Damon: Javascript replace will replace only the matched text with the return value of the callback. That capture group is undefined because you are only accessing the final repetition of that alternation group. tl;dr - I can't see this regex solving any real world problem the way it is written. You must give us the expected output for any meaningful assistance :).

Answer 1

.*parameterName=([^&|\n|\t\s]+)

Using this pattern will give you in a group the parameter value. For example for this URL: https://www.youtube.com/watch?v=aiYpDDHKy18&list=RDaiYpDDHKy18&start_radio=1

.*list=([^&|\n|\t\s]+)

will get you: the list id: "RDaiYpDDHKy18"

.*start_radio=([^&|\n|\t\s]+)

will get you the number 1.

Answer 2

If they're in an arbitrary order, you can use lookaheads to find them and capture the param values. The pattern you want (for your test string) is this:

^(?=.*param1=([^&]+)|)(?=.*param2=([^&]+)|)(?=.*param3=([^&]+)|).+$

Demo on Regex101

It'll also accept query strings that are missing parameters, because each lookahead alternates on either the parameter/value pair or the empty string, using the |s at the end.

You'll need an additional lookahead for each parameter you hope to grab, and there's no way around this. Because every regex under the sun is a state machine under the hood, the only thing you're gonna be hanging on to using your original pattern is the most recent match for a given capture group.