Require unique names for named captures

[rxwei]

According to regex101, PCRE2 requires a unique name for each capture, but the current implementation of Regex.init(compiling:) doesn't throw an error when there's duplicate names.

[hamishknight]

Note that there is a mode (?J) that allows duplicate capture names, so we will also need to handle the coalescing of names for the purposes of typed captures. Perhaps the parser could mark capture groups that have names that have been used before.

[benlings]

A slight relaxation of requiring unique names for captures would be to only allow a name to correspond to a single capture index.

This would be following the advice given here, and would be consistent with how captures are available in Tuple outputs.

In Perl 5.10, PCRE 8.00, PHP 5.2.14, and Boost 1.42 (or later versions of these) it is best to use a branch reset group when you want groups in different alternatives to have the same name, as in (?|a(?[0-5])|b(?[4-7]))c\k. With this special syntax—group opened with (?| instead of (?:—the two groups named “digit” really are one and the same group. Then backreferences to that group are always handled correctly and consistently between these flavors.

[milseman]

A slight relaxation of requiring unique names for captures would be to only allow a name to correspond to a single capture index.

This would be following the advice given here, and would be consistent with how captures are available in Tuple outputs.

In Perl 5.10, PCRE 8.00, PHP 5.2.14, and Boost 1.42 (or later versions of these) it is best to use a branch reset group when you want groups in different alternatives to have the same name, as in (?|a(?[0-5])|b(?[4-7]))c\k. With this special syntax—group opened with (?| instead of (?:—the two groups named “digit” really are one and the same group. Then backreferences to that group are always handled correctly and consistently between these flavors.

@hamishknight are we tracking branch reset groups anywhere? I believe we were parsing them but I don't know if we implemented renumbering logic or not.

[hamishknight]

They're in #370