Parse regex with regex

Question

I have pattern [a-z0-9]{9}|[a-z0-9]{11}

Here, I have to parse that regex to get 9 and 11 for future message that "U have to input 9 or 11 symbols"

Also I have regex like

[a-z0-9]{9}

[a-z0-9]{9,}

[a-z0-9]{9,11}

I need to write some method,which can parse all that strings to get Number of available symbols.

I did

Pattern.compile(".*\\{(\\d+)(,?)(\\d*)\\}(\\|.*\\{(\\d+)\\})?");

It find only last number in my first pattern. But other don't recognize.

How to check all strings?

Upd: I can not understand, but in Java first pattern from answer \{(\d+)(?:,(\d*))?\} doesn't match but find 3 my strings. But first and the longest string it doesn't find.

What is different between matches and find? And why that pattern can find matches in web, but can't in Java? And sometimes I do matcher.find(); // return 2 matcher.group(1); // No match found exception

Alexey R. · Accepted Answer · 2020-09-30 10:25:48Z

1

This regex should work for you: [^\\](?:\\\\)*\{(\d+)(?:,(\d*))?\}.

Part [^\\](?:\\\\)* addresses the case of slash escaping.

Below is Java code example:

public static void main(String[] args) {
    Pattern p = Pattern.compile("[^\\\\](?:\\\\\\\\)*\\{(\\d+)(?:,(\\d*))?\\}");
    String testInput = "[a-z0-9]{1,2}|[a-z0-9]{3,}|[a-z0-9]{4}";
    Matcher matcher = p.matcher(testInput);
    while (matcher.find()){
        System.out.println("Next matching found:");
        if(matcher.group(2) == null){
            System.out.println("Strict: " + matcher.group(1));
        }else {
            System.out.println("Min: "
                    + matcher.group(1)
                    + ", Max: "
                    + ("".equals(matcher.group(2)) ? "Infinte" : matcher.group(2)));
        }
    }
}

Output:

Next matching found:
Min: 1, Max: 2
Next matching found:
Min: 3, Max: Infinte
Next matching found:
Strict: 4

edited Sep 30, 2020 at 10:25

answered Sep 29, 2020 at 15:23

Alexey R.

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5 Comments

Dred Over a year ago

Thank you for your trying, but both variant was unsuccessful. Yes, I checked them by the link you provided, but in Java it doesn't work((( And, by the way, what is mean ?: Is it greedy quantifier? If I choose ? -it is mean 0/1 times, but what is about ?:

Alexey R. Over a year ago

Check update... Construction like (?:) meas non-capturing group, i.e. the group that you wouldn't like to parse but rather for use just as a part of your rule.

Dred Over a year ago

Why need that part [^\\\\](?:\\\\\\\\) ?

Alexey R. Over a year ago

because your expression might have slashes as the escape symbols. Since slash can be used to escape slashes, this part is used in order to check if \{ is escaped { or it is a part of bigger picture like \\{ where { defines the quantifier of `\` literal.

Dred Over a year ago

I would never thought about that :) and even after your explanation, I hardly understand that) but thank you

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