regexp: add ErrLarge error

[rsc]

The fix for #55949 introduced various regexp size limits in the regexp parser.
To avoid adding new API, the fix returns ErrInternalError on regexps that are too large.
For Go 1.20 we want to give this error a proper value with a good message.

I propose to add a new ErrorCode:

ErrLarge          ErrorCode = "expression too large"

See also #51684 which added a similar error after a similar security fix.

[rsc]

This proposal has been added to the active column of the proposals project
and will now be reviewed at the weekly proposal review meetings.
— rsc for the proposal review group

[rsc]

Based on the discussion above, this proposal seems like a likely accept.
— rsc for the proposal review group

[rsc]

No change in consensus, so accepted. 🎉
This issue now tracks the work of implementing the proposal.
— rsc for the proposal review group

[gopherbot]

Change https://go.dev/cl/444817 mentions this issue: regexp: add ErrLarge error

[zephyrtronium]

I noticed this is still open while looking for something unrelated on https://goissues.org/regexp. Is there something else to do here?

[ianlancetaylor]

There is nothing else to do. Thanks for pointing that out.

[dmitshur]

Indeed—this was both implemented (CL 444817) and covered in release notes (https://go.dev/doc/go1.20#regexp). Placing this issue in the corresponding milestone retroactively so it'll be easier to determine when this change happened if looking at this in the future.