RFR 8197462 : Inconsistent exception messages for invalid capturing group names

[Xueming Shen]

Hi Ivan,

The code handles group name was added later. So "historically" those cases
trigger "unknow look-behind group" when the first character after "<" is not
"=" or "?". With the addition of the group name support, it's actually 
hard to
say which one is more accurate, incorrect group name or incorrect "looks
behind". Sure with a tailing ">" it might be more desired to lean to group
name.

It's definitely a bug not to check whether or not the first char is 
alpha for \\k<.

I'm fine with the proposed change.

Thanks,
Sherman


On 2/8/18, 8:32 PM, Ivan Gerasimov wrote:
> Hello!
>
> Capturing group name can be used in a regular expression in two 
> contexts:  When introducing a group (?<name>...) or when referring it 
> \k<name>.
> If the name is invalid (i.e. does not start with a Latin letter, or 
> contains wrong chars) then we may see different error messages, some 
> of which look confusing.
>
> Here are examples of the messages produced by the current JDK:
> Unknown look-behind group near index 3
> (?<>)
>    ^
> named capturing group is missing trailing '>' near index 4
> \\k<>
>     ^
> Unknown look-behind group near index 4
> (?<.>)
>     ^
> (named capturing group <.> does not exit near index 4
> \\k<.>
>     ^
> named capturing group is missing trailing '>' near index 4
> (?<a.>)
>     ^
> named capturing group is missing trailing '>' near index 4
> \\k<a.>
>     ^
>
> In particular, this diversity is caused by that the internal 
> Pattern.groupname() function lacks a check for the very first 
> character of the name.
> So that when \k<name> is parsed, the first char is always accepted, no 
> matter what it was.
>
> Some cleanup was also done along the way.
>
> Would you please help review the fix?
>
> BUGURL: https://bugs.openjdk.java.net/browse/JDK-8197462
> WEBREV: http://cr.openjdk.java.net/~igerasim/8197462/00/webrev/
>
> Thanks in advance!
>