Substitute: add support for $& $` $' $_ and \b \v

[NWilson]

The $& replacement, and $` and $' are common sequences for $0, and "left of $0" / "right of $0" respectively. Perl supports these, so PCRE2 surely should.

The $_ is less common but supported by some engines.

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Finally, very very minor:

The \b escape is supported by Perl, so we should add it too.

The \v escape is there for completeness - it's not apparently supported by Perl, but many other dialects such as JavaScript or .NET use this C-inspired escape sequence for "vertical tab". It's obviously not possible to support this in patterns, but in the replacement string I can't see a reason not support it.

[carenas]

would be nice to see updated documentation as well (ex: doc/pcre2syntax.3 and maybe even doc/pcre2pattern.3)

the html is generated with the help of some scripts that are called from PrepareRelease, so those files might not need to be included, but sure are nicer to generate locally for a quick inspection.

[NWilson]

@PhilipHazel Thank you for merging, and adding documentation!

I saw in your documentation that you have documented the extra substitutions $& etc to require PCRE2_SUBSTITUTE_EXTENDED. But actually, I implemented them to be available always (like $0 is). They are intended to be Perl-compatible, unlike the ${name:+yes:no} syntax.

Would you like me to open a PR to change the code to require PCRE2_SUBSTITUTE_EXTENDED, or should I update the documentation?

[carenas]

They are intended to be Perl-compatible

I think it will be nice to keep the code as-is and update the Documentation, but what was really missing was a way to put the test (which was also using by mistake the substitute_extended modifier) and that is Perl compatible on testinput1, which means we need to add functionality to perltest.sh to add support for the replace modifier at least, but which will also help in the future testing substitutions better IMHO.

[PhilipHazel]

Oh , sorry I missed that. Obvious, really, but as your test of these had SUBSTITUTE_EXTENDED set, I assumed that was needed, without checking. I will fix the documentation , and also update the test. // When I implemented substitution I wasn't really thinking about Perl-compatibility at all, I have to confess. That's why I never extended perltest.sh.