I should allow 2 different input strings formats, with each their own validation. So eg:
AA2222222222222222
and
2222222222222222
This means that if the first character is a letter, I should validate for ^[a-zA-Z]{2}\d{16}$. If the first character is numeric, I should validate for d{16}.
I tried to write it in an conditional regex:
^(([a-zA-Z])(?([a-zA-Z])^[a-zA-Z]{2}\d{16}$|d{16})
but I get a pattern error and can't figure out what exactly is wrong.
Any insight would be apreciated
I tried to write it in an conditional regex
JavaScript doesn't support regular expression conditional syntax, so (?ifthen|else) doesn't work in JavaScript.
This means that if the first character is a letter, I should validate for ^[a-zA-Z]{2}\d{16}$. If the first character is numeric, I should validate for d{16}.
Since the \d{16} part is the same, you can just make the [a-zA-Z]{2} part optional:
/^(?:[a-zA-Z]{2})?\d{16}$/
That uses a non-capturing group around the [a-zA-Z]{2} and makes the entire group optional via the ? after it.
If the validation were different (say, maybe the version with the letters at the start only does \d{14}), you could use an alternation:
/^(?:[a-zA-Z]{2}\d{14}|\d{16})$/
(Beware the gotcha: Without the non-capturing around around the alternation, the ^ would be part of the first alternative but not the second, and the $ would be part of the second alternative, but not the first.)
But in your specific case, you don't need that.