javascript - Regex expression to allow # only in between chars in a string

Question

How can I write a JavaScript regex to handle

case 1. allow all chars, but '#' in between characters only

case 2. allow only alphanumeric and '#', but '#' in between chars

For example :
case1 : var formattedText = inputText.replace(/^(.+#+.)$/g, ''); //not working

Answer 1

To match all chars except # at the start and end, you might use a negated character class [^# ] excluding # (or newlines if desired)

^[^#
]+(?:#+[^#
]+)*$

Regex demo

To match only alphanumeric and # not at the start or end of the string, you might use:

^[a-zA-Z0-9]+(?:#+[a-zA-Z0-9]+)*$

Regex demo

const regex = /^[a-zA-Z0-9]+(?:#+[a-zA-Z0-9]+)*$/;
[
  "#test",
  "test#",
  "test",
  "test#test",
  "test#test#test"
].forEach(s => console.log(`${s} --> ${regex.test(s)}`));