An Introduction to Regular Expressions | Ray Wenderlich

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This introduction to regular expressions teaches you the basics of regular expressions and how to use them.

This is a companion discussion topic for the original entry at https://www.raywenderlich.com/5767-an-introduction-to-regular-expressions
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Hi
Just wanted to thank you for the tutorial. It’s awesome.
Also thank you for highligtMatches functions it’s toooo goood.
On my own could not have made it. You guys rocks :+1:t2:

Cheers. Nanu Jogi

1 Like
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@nanujogi Really glad to hear it helped so much. Take care and keep up the good work! :]

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Hello, thank you for really great tutorial for iOS beginners! :slight_smile: Maybe I have find bug in function listGroups(). I have tried it in Playground this way:

let person = "54 male Russell Crowe"
listGroups(for: "([0-9]+)\\s?(male|female)?\\s?", inString: person)

let person2 = "54 Sandra Bullock"
listGroups(for: "([0-9]+)\\s?(male|female)?\\s?", inString: person2)

By the second usage, Playground wrote: Fatal error: Unexpectedly found nil while unwrapping an Optional value because (male|female)? was return nil for let range = Range(match.range(at: group), in: string)

I have changed
let range = Range(match.range(at: group), in: string)!
groupMatches.append(String(string[range]))

for

if let range = Range(match.range(at: group), in: string) {
    groupMatches.append(String(string[range]))
} else {
    groupMatches.append("")
}

Is it correct reasoning? I am programming in PHP and actually reading iOS Apprentice.

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@tomelliott Can you please help with this when you get a chance? Thank you - much appreciated! :]

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Hi @internetstream, you are right that the listGroups function won’t work in the example above.

The reason is because your regex contains the group (male|female)?, which is matched zero or more times. So when the text ‘male’ or ‘female’ is not present in the text, the regex engine is returning a capture group with a length of 0 and a location of NSNotFound.

You are also correct that guarding against this using if let will work. Although if your case I think I would also make sure to specifically guard against the location being NSNotFound as well.

Finally, one thing I should probably mention as well if you want to make this generic… The result of NSRegularExpression.matches(in:options:range:) returns the regex match as the first element in the array, and any Groups matched from element 1 onwards. So you should modify the function to start from index 1 not 0 if you only want groups.

I’ve tidied up ViewController.listGroups(for:inString:) to make it more generically applicable:

func listGroups(for pattern: String, inString string: String) -> [String] {
    guard let regex = try? NSRegularExpression(pattern: pattern, options: []) else {
      return []
    }

    let range = NSRange(string.startIndex..., in: string)
    let matches = regex.matches(in: string, options: [], range: range)

    var groupMatches: [String] = []
    for match in matches {
      let numberOfRangesInMatch = match.numberOfRanges
  
      for rangeIndex in 1..<numberOfRangesInMatch {
        let range = match.range(at: rangeIndex);
        if range.location != NSNotFound {
          if let rangeInString = Range(range, in: string) {
            groupMatches.append(String(string[rangeInString]))
          }
        }
      }
    }

    return groupMatches
  }

Hope that helps!

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Hello @tomelliott, thanks a lot! Helped a lot. Have a nice day

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This tutorial is more than six months old so questions are no longer supported at the moment for it. Thank you!