I've looked for a solution and played around a bit with some examples but couldn't find what I'm looking for. (I'm using javascript)
Basically it should only match a string of 18 digits if the first character isn't @, it should only include the 18 digits.
So something like /[^@]\d{18}/g but if I do this it includes the first character that is not an @
Examples: test 123456789012345678 abc match 123456789012345678
test @123456789012345678 abc don't match.
One option is to match an @ and a digit that you don't want, and then capture 18 digits between word boundaries \b in capture group 1 that you want to keep.
The word boundaries are to prevent a partial match for the digits.
@\d|\b(\d{18})\b
If supported, you can also use a negative lookbehind:
\b(?<!@)\d{18}\b