matching regex for custom params in URI

I am building a web framework and REGEX is really hostile today. I do not like the django way of formatting custom params with angle brackets

url/<param>/... or
<str:token>/

I would prefer the way js and other programs handle this

name/:token/:another_param

After trying for 45 minutes I am giving up. I would like to allow only characters like a-zA-Z0-9_:/. The main issue here is that I do not want to allow recursive colons like this

:::id

These are strings I would like to match

empty string (although I could check prior matching)
/
:id
/:id
/:id/
person/:name/:id/:token_person2/image
person////
////

Could someone help me?

Solution

You don't want to match :: and apparently also not :/

What you can do is use a single negative lookahead to assert that those 2 strings do not occur.

^(?![/\w:]*:[:/])[/\w:]*$

Explanation

^ Start of string
(?![/\w:]*:[:/]) Negative lookahead, assert not :: or :/ to the right
[/\w:]* Optionally repeat matching one of /, \w (word character) or :
$ End of string

const regex = /^(?![/\w:]*:[:/])[/\w:]*$/;
[
  "",
  "/",
  ":id",
  "/:id",
  "/:id/",
  "person/:name/:id/:token_person2/image",
  "person////",
  "////",
  ":::id",
  "person/:/name",
].forEach(s =>
  console.log((regex.test(s) ? "" : "No ") + `Match --> '${s}'`)
)

See a regex demo