Raku - String invoking anonymous regex

Question

Why does the first expression interpret but not the second?
I understand them to be identical, that is, a string invoking an anonymous regex.

("foo").first: /foo/ andthen say "$_ bar";
> foo bar

"foo": /foo/ andthen say "$_ bar";
> Confused
> at /home/dmc7z/raku/foo.raku:2
> ------> "foo":⏏ /foo/ andthen say "$_ bar";
>     expecting any of:
>         colon pair

Answer 1

This is a method call:

 ("foo").first: /foo/

It's the same as if you had written:

("foo").first( /foo/ )

Or just:

"foo".first( /foo/ )

(Note that I used : at the end of the three above descriptions in English. That's where the idea to use : to mean that whatever following it is part of the same expression comes from.)

In this case it doesn't make a whole lot of sense to use first. Instead I would use ~~.

"foo" ~~ /foo/ andthen say "$_ bar";

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first is used to find the first item in a list that matches some description. If you use it on a single item it will either return the item, or return Nil. It always returns one value, and Nil is the most sensible single undefined value in this case.

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The reason it says it's expecting a colon pair is that is the only use of : that could be valid in that location. Honestly I halfway expected it to complain that it was an invalid label.