Regex - Has to start with certain characters but can't contain this string

Question

I have this regex that I been using, but now I need to add something to it.

^[|a-z-&+\/](?!\|)

I tried a few things, but I just can't come up with the solution.

I want it to add to it, to not match anything that contains a "##" in any position.

These match:

|test
test#test
apple

These don't match:

||test
test##test
apple##

Answer 1

One option is to add a negative lookahead at the beginning after the ^ anchor: (?!.*##) .

Example Here

^(?!.*##)[|a-z-&+\/](?!\|)

---

As Casimir et Hippolyte points out , since # can't be the first character, you could alternatively add this check as an alternation in the existing lookahead:

Example Here

^[|a-z-&+\/](?!\||.*##)

Explanation:

• ^[|az-&+\\/] - Match a single character in the character set at the start of the string (or line if you're using the multi-line flag).
• (?! - Start of a negative lookahead
• \\| - Match the | character literally
• | - Alternation; or..
• .*## - Check for the ## characters
• ) - Close the negative lookahead