Swift: extract string with regex and range

Question

In Swift, I have a string like this

http://mnc-hdqp.oss-cn-shanghai.aliyuncs.com/user%2Fheat%2Fdefault.jpg?Signature= 2BI%2BauSvy&Expires=1568682491&OSSAccessKeyId=LTAIQ8Lif1HHVkXd

Need to extract 2BI%2BauSvy, the value of key Signature

here is code: regex match, then use range to subtract the key ahead.

let key = "Signature"
let signatures = icon.matches(for: "\(key)[^&]+")
guard !signatures.isEmpty else{
 return
}
if let range = signatures[0].range(of: "\(key)="){
 let signature = String(signatures[0][range.upperBound...])
 print(signature)
}

Any way to implement it conveniently?

---

PS: func matches(for:)

extension String{
 func matches(for regex: String) -> [String] {
 do {
 let regex = try NSRegularExpression(pattern: regex)
 let results = regex.matches(in: self,
 range: NSRange(self.startIndex..., in: self))
 return results.map {
 String(self[Range(0ドル.range, in: self)!])
 }
 } catch let error {
 print("invalid regex: \(error.localizedDescription)")
 return []
 }
 }

• dengApro: (?<=Signature=)[^&]+, is exactly what I want
• Oh My Goodness: [&?]Signature=([^&#?]+) is probably closer, using a capture group, or [&?]Signature=(?<sig>[^&#?]+) to use a named capture group.
• dengApro: @Oh My Goodness, you can give an answer

Answer 1

Simplify the code

The guard statement and the following if let can be combined into a single statement:

let key = "Signature"
let signatures = iconURL.matches(for: "\(key)[^&]+")
if let firstSignature = signatures.first,
 let range = firstSignature.range(of: "\(key)=") {
 let signature = String(firstSignature[range.upperBound...])
 print(signature)
}

Improve the regex pattern

Your method is fragile because the key "Signature" may occur in the host part of the URL. Here is an example where it fails:

let iconURL = "http://Signature.com/?a=b&Signature=sig&c=d"

As mentioned in the comments, you can use a positive look-behind which includes the "=" character:

let key = "Signature"
let signatures = iconURL.matches(for: "(?<=\(key)=)[^&]+")
if let signature = signatures.first {
 print(signature)
}

However, this would still fail for

let iconURL = "http://foo.com/Signature=bar?a=b&Signature=sig&c=d"

because the "=" character is valid in the path part of an URL.

You also must ensure that the key does not contain any characters which have a special meaning in a regex pattern.

And now for something completely different

The Foundation framework has a dedicated URLComponents type to parse URLs into their parts. It does exactly what you need here:

let iconURL = "http://Signature.com/Signature=foo?a=b&Signature=2BI%2BauSvy&c=d"
let key = "Signature"
if let urlComponents = URLComponents(string: iconURL),
 let queryItems = urlComponents.queryItems {
 for queryItem in queryItems where queryItem.name == key {
 if let value = queryItem.value {
 print(value) // 2BI+auSvy
 }
 }
}

In addition, the value is already percent-decoded.