use string equality instead of regexps to compare constant strings

[erik-krogh]

Regexps have previously been used to encode a set of strings, but now that we have set literals we should use those instead.
And sometimes regexps seem to be used without any good reason (second example below).

Consider the below examples.

str.regexpMatch("foo|bar|baz") or // is equivalent to the below, but less efficient. 
str = ["foo", "bar", "baz"]
or
str.matches("foo") or // is equivalent to the below, but less efficient. 
str = "foo"

Rewriting from a regular expression to comparison against string constants means that there are more possible join-orders for the optimizer to choose from, so we need to run performance evaluations.

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Evaluations all look good: CPP, C#, JavaScript, Java, Python, Ruby.
I had to do a bunch of retries in C++ before that turned out OK, but I think that's due to big variations in DB creation time
(DB caching in DCA 🙏).

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Inspired by this comment: #5160 (reply in thread)

[alexet]

Underscore is also special in match strings and it matches any character. However I also note that most cases with underscores seem to be bugs where an escaped underscore is intended.

[erik-krogh]

However I also note that most cases with underscores seem to be bugs where an escaped underscore is intended.

I found two uses of underscores, both were bugs.
I've fixed them.

[hvitved]

C# and Ruby 👍

[aschackmull]

Java LGTM

[jketema]

C++ 👍

[esbena]

JS 👍🏻

[tausbn]

Python/QL 👍