https://stackoverflow.com/a/54487077/1187415

stringmatch.swift

import Foundation

// Swift 4.2.1 syntax

public struct SubstringMatchSource<S: StringProtocol> where S.Index == String.Index {
    private let wrapped: S
    
    public init(wrapping wrapped: S) {
        self.wrapped = wrapped
    }
    
    public static func ~=<T: StringProtocol> (substring: T, source: SubstringMatchSource)  -> Bool   {
        return source.wrapped.contains(substring)
    }
}

extension StringProtocol where Index == String.Index {
    var substrings: SubstringMatchSource<Self> {
        return SubstringMatchSource(wrapping: self)
    }
}