String Operations (substring) in Swift

Question

If I have a string like follows:

"abc; expire=Thu, 16-Oct-2014 16:46:53 GMT; path=/;", and I am trying to extract everything between expire= and the next ; so I can get the timestamp, how would I do this in Swift? I am totally confused by the ideas of characters and StringIndex. I feel like the following code will give me the locations in the string of " expire=":

var range = cookie!.rangeOfString(" expires=");

I don't however understand where I should go from there.

In Java-like languages the algorithm would be something like:

1) Locate index of "expires=";

2) Take substring of everything after "expires=" to end of full string;

3) Look in substring for first occurrence of ";"

4) Everything from index 0 to index found in 3 is the expire timestamp.

How do you go about working with Swift strings?

Sergey Kalinichenko · Accepted Answer · 2014-10-02 18:19:37Z

3

In Swift you can use regex to search strings, without having to deal with NSRegularExpression explicitly:

let str = "abc; expire=Thu, 16-Oct-2014 16:46:53 GMT; path=/;"
let range = str.rangeOfString("(?<=expire=)[^;]*", options: .RegularExpressionSearch)
if let range = optRange? {
    print(str.substringWithRange(range))
} else {
    print("Not found")
}

The regular expression is straightforward: [^;]* means "a sequence of characters up to the first semicolon"; (?<=expire=) means "match only when preceded by expire= string".

Demo.

answered Oct 2, 2014 at 18:19

Sergey Kalinichenko

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2 Comments

thatidiotguy Over a year ago

I am not against using regular expressions if necessary, but shouldn't there be a couple basic string operations that would enable me to do this without them?

Sergey Kalinichenko Over a year ago

@thatidiotguy Absolutely, it is possible to rewrite this as a sequence of finding substrings, arriving at the same result after a sequence of simple subtractions. This would make a four-line solution out of a one-line solution, which is suboptimal from the point of view of the reader of the code.

Mike S · Accepted Answer · 2014-10-02 18:25:26Z

1

Personally I'd go with @dasblinkenlight's solution of using a regular expression, but if you want to do something more like the algorithm in your question, you can:

let cookie: String = "abc; expires=Thu, 16-Oct-2014 16:46:53 GMT; path=/;"
if let range = cookie.rangeOfString(" expires=") {
    let expiresAndPath = cookie[range.endIndex..<cookie.endIndex]
    if let semicolonRange = expiresAndPath.rangeOfString(";") {
        let expires = expiresAndPath[expiresAndPath.startIndex..<semicolonRange.startIndex]
        /* do something with expires */
    }
}

Step-by-step:

Locate index of "expires="

let range = cookie.rangeOfString(" expires=")

Take substring of everything after "expires=" to end of full string

let expiresAndPath = cookie[range.endIndex..<cookie.endIndex]

Look in substring for first occurrence of ";"

let semicolonRange = expiresAndPath.rangeOfString(";")

Everything from index 0 to index found in 3 is the expire timestamp.

let expires = expiresAndPath[expiresAndPath.startIndex..<semicolonRange.startIndex]

answered Oct 2, 2014 at 18:25

Mike S

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1 Comment

thatidiotguy Over a year ago

I am accepting your answer as this follows the algorithm I spelled out in OP. Although I am finding this range stuff to be a little wild coming from a Java background, this definitely helped me understand this a lot better. Thanks!

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