RegEx if then else for conditional test depending on first character

Question

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

You can use a single pattern `^(?:[A-Z][A-Z])?\d{16}$` with an optional group. Javscript does not support a conditional syntax. — The fourth bird, Nov 18 '21 at 13:56
@T.J.Crowder I mean this syntax https://www.regular-expressions.info/conditional.html — The fourth bird, Nov 18 '21 at 13:59

T.J. Crowder · Answer 1 · 2021-11-18T14:15:31.400

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.

RegEx if then else for conditional test depending on first character

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