Fix tchar pattern in RestRequest

[pgomulka]

in https://community.sonarsource.com/t/blog-post-regular-expressions-present-challenges-even-for-not-so-regular-developers/38304
an author pointed out that Elasticsearch has a bug in its tchar regex.

_

So now it should take you only one second to find a bug in this Elasticsearch code which is commented "defined by RFC7230 section 3.2.6" for this expression:

Pattern.compile("[a-zA-z0-9!#$%&'*+\\-.\\^_`|~]+"); 

Source (RestRequest.java)

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and indeed the A-z is a mistake and includes acciental [, ], / characters

This commits fixes this.

Just to explain why we validate this way:
the correct value of a content-type is a Media type https://www.rfc-editor.org/rfc/rfc7231#section-3.1.1.5
a media-type is defined in https://www.rfc-editor.org/rfc/rfc7231#section-3.1.1.1 and uses token
and a token is defined in https://www.rfc-editor.org/rfc/rfc7230#section-3.2.6
and it uses tchar defined in the same RFC

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[elasticsearchmachine]

Hi @pgomulka, I've created a changelog YAML for you.

[pugnascotia]

The changes look good, but there are other instances of A-z in the codebase. In particular, these look like they should be fixed or refactored:

libs/x-content/src/main/java/org/elasticsearch/xcontent/ParsedMediaType.java x-pack/plugin/sql/sql-proto/src/main/java/org/elasticsearch/xpack/sql/proto/content/ParsedMediaType.java 

I'm not sure about the other instances. I searched using rg -F A-z

[pgomulka]

The changes look good, but there are other instances of A-z in the codebase. In particular, these look like they should be fixed or refactored:
libs/x-content/src/main/java/org/elasticsearch/xcontent/ParsedMediaType.java
x-pack/plugin/sql/sql-proto/src/main/java/org/elasticsearch/xpack/sql/proto/content/ParsedMediaType.java
I'm not sure about the other instances. I searched using rg -F A-z

good point on the ParsedMediaType. I fixed those too. It hurts a little to have this duplicated, but I guess we would have to create a utility class somewhere in x-content?core? lib with all http related rfc patterns.

I think we only want to fix a-zA-z as if someone is using A-z it means they probably want a quick way to mark all letters (plus some accidental [,],_ etc)