[swift-evolution] Draft: Regular Expression in Swift

Charlie Monroe charlie at charliemonroe.net
Thu Aug 10 01:43:26 CDT 2017

Hi Joshua,

while this is a great idea, I'm afraid that this needs to be more thought-through. For example, which regex standards would you like to include? I really hope that Swift's regex support will not get stuck with plain ICU regexes like NSRegularExpression. I personally would love to see Perl-syntax improvements such as named groups (any many others):

let regexString = "NS_ENUM\((?P<TYPE>\w+), (?P<NAME>\w+)\)"

let mySource = "NS_ENUM(NSInteger, NSComparisonResult)"
mySource.match("TYPE", in: regexString) // NSInteger

Of course, it would be great if the compiler support could be extended so that the TYPE and NAME variables can be extracted without using string literals, e.g. Regex(regexString).firstMatch(in: mySource)?.TYPE

I have written myself an ObjC wrapper arround C++ implementation of such regex (re2) and use that from Swift instead of using NSRegularExpression.

Regarding your snippet:

- it's always good to keep in mind that in many usecases you only need the first match, not all of them
- instead of search(_:), consider matches(_:)
- huge part of regex power is in replacing occurrences, which is missing

> On Aug 10, 2017, at 8:25 AM, Joshua Alvarado via swift-evolution <swift-evolution at swift.org> wrote:
> Hey everyone,
> I would like to pitch an implementation of Regex in Swift and gather all of your thoughts.
> Motivation:
> In the String Manifesto for Swift 4, addressing regular expressions was not in scope. Swift 5 would be a more fitting version to address the implementation of Regex in Swift. NSRegularExpression is a suitable solution for pattern matching but the API is in unfitting for the future direction of Swift.
> Implementation:
> The Regular expression API will be implemented by a Regex structure object which is a regular expression that you can apply to Unicode strings. The Regex struct will conform to the RegexProtocol, which is a type that can represent a regular expression. ExpressibleByRegexLiteral will be used to initialize a regex literal creating an easy to use syntax and a Match structure will be used to represent a match found with a Regex.
> Draft of implementation:
> protocol ExpressibleByRegexLiteral {
>     associatedtype RegexLiteralType
>     init(regexLiteral value: Self.RegexLiteralType)
> }
> // Structure of information about a match of regex on a string
> struct Match {
>     var regex: Regex
>     var start: String.Index
>     var end: String.Index
> }
> protocol RegexProtocol {
>     init(pattern: String) throws
>     var pattern: String { get } // string representation of the pattern
>     func search(string: String) -> Bool // used to check if a match is found at all in the string
>     func match(string: String) -> [Match] // returns an array of all the matches
>     func match(string: String, using: ((Match) -> Void)) // enmuerate over matches
> }
> struct Regex: RegexProtocol {
>     init(pattern: Regex, options: Regex.Options)
>     let options: [Regex.Options]
>     static let word: Regex // \w
>     // other useful regexes can be added as well
> }
> // Examples
> let regex = \[a-zA-Z]+\
> let matches = regex.match("Matching words in text.")
> for match in matches {
>     print("Found a match at in string at \(match.start) to \(match.end)")
> }
> let helloStr = "Hello world"
> Regex.word.match(helloStr) { match in
>     print("Matched \(helloStr[match.start..<match.end])")
> }
> Of course this is a scratch implementation I made but it is to open discussion on the topic. I feel the Regex struct itself will need more methods and variables such as for flags and number of groups. Please provide feedback with improvements to the code, concerns on the topic, or just open up discussion. Thank you!
> Joshua Alvarado
> alvaradojoshua0 at gmail.com <mailto:alvaradojoshua0 at gmail.com>
> _______________________________________________
> swift-evolution mailing list
> swift-evolution at swift.org
> https://lists.swift.org/mailman/listinfo/swift-evolution

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