why not using ^ and $ instead ?

So \A and \z match head and end collectly than ^ and $.
Is it bothering you?

\A and \z can be useful for multiline input where ^ and $ will match begining and ending of every line, \A and \z will match only begining and ending of whole input text.

I think that @stof is right. Because you always will have one line in input for version number.

but here, we are not in a multiline regex (we don't have the m modifier), so they are equivalent. I would rather use ^ and $, to be consistent with the Symfony codebase

OK. I fixed it for consistency.

Just for your information.

but here, we are not in a multiline regex (we don't have the m modifier), so they are equivalent.

They are not equivalent.

$ matches "\n" at the end, but \z does not match "\n" at the end.
So string like "2.2.5\n" passes the validation.

Of course nobody types the version number like that.

@kenjis a human won't type the version as 2.2.5\n, but beware that this tool could be used inside an automated command and then this trailing \n character could be indeed wrongly passed inside the version number.

@stof A regex has already been fixed. Anything else?

@javiereguiluz Thank you for your comment.

I think 2.2.5\n is not valid version number. So we should use \z instead of $ ?

@kenjis Indeed, I also think \z is valid than $ whether or not multiline because it doesn't match \n.
However, @stof mentioned the consistency, so I think this PR is enough.

(英語かけてるかわからないので日本語で書いておきますが、\n にマッチしたところでたいした問題ではないので、気持ちもわかるんですが、一貫性を優先してこれでよいかと思っています。)

Thanks for fixing this bug @fivestar.