Extending Patterns

For example, the Regex pattern in Moshi is defined as following

function decons_call(::Type{Regex}, ctx::PatternContext, pat::Pattern.Type)
    Data.isa_variant(pat.args[1], Pattern.Quote) || error("Regex head must be a string")
    re = Regex(pat.args[1].:1)
    return function regex(x)
        return quote
            $Base.occursin($re, $x)
        end
    end
end