Regex only if group sequence not followed by?

Question

Using Swift 4.2, trying to match a regex similar to 1980 / 1989 / 2019 etc., but what I need is a match only if sequence is not followed by "p" ? what I'm trying ... "(?:[1-2]{1}[0,9]{1}[0-9]{1,2})\1(?![p])"

Assuming Swift regex supports it, your negative lookahead at the end looks correct. What is wrong with your current pattern? — Tim Biegeleisen, Apr 25 '19 at 14:15
Possible duplicate of [Regular expression to match a line that doesn't contain a word](https://stackoverflow.com/questions/406230/regular-expression-to-match-a-line-that-doesnt-contain-a-word) — Paul Benn, Apr 25 '19 at 14:51
Is this about regular expressions or swift? Depending on which it is could you clarify your question with some sample data (and code if relevant) and given result vs expected result — Joakim Danielson, Apr 25 '19 at 15:16

score 0 · Answer 1 · answered Apr 25 '19 at 15:19

0

After plenty of testing I found a solution ... "(?:([1-2]{1}[0,9]{1}[0-9]{2})(?![p]))"

answered Apr 25 '19 at 15:19

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This matches `1975`, `2020` `2999` – ielyamani Apr 25 '19 at 17:32
... and `1,80` because of `[0,9]` – ielyamani Apr 25 '19 at 17:42

ielyamani · Answer 2 · 2019-04-25T18:30:08.183

Let's consider this string :

let str = """
10 Hello 980 world,
1975 Hello 1980 world,
1985p Hello :1995 world,
2000 Hello 2005p world,
2010 Hello 2015 world,
2019 Hello 2020 world,
2999
"""

Let's declare this regex :

let pattern = "19[89]{1}[0-9]{1}(?![p])|20[01]{1}[0-9]{1}(?![p])"
let regex = try! NSRegularExpression(pattern: pattern)

Here are the different parts of the pattern :

19 matches the characters 19 literally,
[89]{1} matches a single character in the list 89, to limit the years to the 1980s and 90s,
[0-9]{1} one digit for the year,
(?![p]) negative lookahead, meaning: not followed by p,
| logical OR,
20 matches the characters 20 literally,
[01]{1} matches a single character in the list 01, to limit the years to the 2000s and 2010s,
[0-9]{1} one digit for the year,
(?![p]) negative lookahead, meaning: not followed by p.

Now, let's get the matches :

let matches =
    regex.matches(in    : str,
                  range : NSRange(location: 0, length: str.utf16.count))

for match in matches {
    let range = match.range
    if let swiftRange = Range(range, in: str) {
        let name = str[swiftRange]
        print(name)
    }
}

Which prints in the console :

^{Bear in mind that this would still match things like 1990s, 1999a, 19999999, since you've only asked to not be followed by p.}

Regex only if group sequence not followed by?

2 Answers2