map: Replace placeholders with regex groups

[rajat315315]

Fixes #3971

[CLAassistant]

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[francislavoie]

I don't think this is quite right. The linked issue is asking to essentially do the same as the path_regexp matcher, where it sets regexp results in the replacer to be used later. Basically, this code needs to be ported to the map handler when a regexp matches:

https://github.com/caddyserver/caddy/blob/deedf8a/modules/caddyhttp/matchers.go#L932-L944

[rajat315315]

@francislavoie it good? Or better?

[francislavoie]

Hmm. I'm not sure that's still quite right. The typical regexp placeholder involves a name, because you could have more than one regexp in your handler chain (i.e. a path_regexp or header_regexp matchers, etc). Your implementation will clobber those since it's placing the results at the same placeholder depth as where the names would normally be.

I think we need to figure out how to tell the map handler the regexp name to be used. I'm not sure what's right there. @mholt do you have any ideas?

[mholt]

@francislavoie I'm just getting to this now, but I'm confused. What was wrong with the initial commit? The current commit looks way complicated. ReplaceAllString() is what allows you to use capture groups like \1 in replacements. Placeholders are a separate concept. Why does this have anything to do with matchers?

[mholt]

Hmm are you sure? Linked issue says:

I want to refer to the captured parts of the regex in destination

[francislavoie]

Oh, I see, my bad. I misunderstood how the first implementation was meant to be used because there was no explanation or associated test. I figured we would use the same regexp placeholders like {re.*.*} here but it makes sense to use \* for using the capture groups inside the map case.

[mholt]

I'm pretty sure we can restore the first commit @rajat315315 -- but keep the later commits.

[rajat315315]

Sure
@mholt Should I revert later ones? and keep first one?

[mholt]

I believe we want to keep this one: acb5ff3

And any related test cases. :)

[rajat315315]

-- but keep the later commits.

I'm not sure about this statement..

[francislavoie]

By "later commits", Matt means the changes to the tests. Basically just revert the changes after the first commit, then add tests.

[mholt]

Cool, let's try this out. Hopefully nobody wants literal ${1} strings in their outputs. :)

[rajat315315]

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