How to split a regular expression into parts?

Yuval Published at Dev

Yuval

I need to break down a regular expression into its basic parts. For instance, given the regex [a-d]+[r-z]* I need to split it into [a-d]+ and [r-z]*. This is of course a very simple example, and regex syntax can get very complex...

Is there a (relatively) simple way to achieve this, or am I doomed to reverse-engineer a regex parser?

_{I need this to find out if a given string is a part of matching input for a given regular expression.}

Danny Daglas

You can brute-force it this way:

import java.util.ArrayList;
import java.util.List;
import java.util.regex.Pattern;
import java.util.regex.PatternSyntaxException;

public class RegexSplitter {    
    private static boolean tryRegex(String regex) {
        try {
            Pattern.compile(regex);
            return true;
        } catch(PatternSyntaxException pse) {
            return false;
        }
    }

    public static void main(String args[]) {
        String input = "[a-d]+[r-z]*";

        List<String> results = new ArrayList<>();

        int start = 0;
        int end = 1;
        boolean good = false;
        while(end < input.length()) {
            String part = input.substring(start, end);
            if(!tryRegex(part)) {
                if(good) {
                    good = false;
                    results.add(input.substring(start, end - 1));
                    start = end-1;
                }
            } else {
                good = true;
            }
            ++end;
        }
        if(tryRegex(input))
            results.add(input.substring(start,end));

        System.out.println(results);
    }
}

// Output: [[a-d]+, [r-z]*]

It's hacky and heuristic, but it may work for your purposes.

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