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Editorial Team
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Asked: 2026-05-15T01:02:08+00:00

Is there a nice way to evaluate a regular expression range, say, for a

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Is there a nice way to evaluate a regular expression range, say, for a url such as 

http://example.com/[a-z]/[0-9].htm

This would be converted into:

http://example.com/a/0.htm
http://example.com/a/1.htm
http://example.com/a/2.htm
...
http://example.com/a/9.htm
...
http://example.com/z/0.htm
http://example.com/z/1.htm
http://example.com/z/2.htm
...
http://example.com/z/9.htm

I’ve been scratching my head about this, and there’s no pretty way of doing it without going through the alphabet and looping through numbers.

Thanks in advance!

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1 Answer

  1. Editorial Team

    If you really need to do this, it’s not that hard to generate the strings using recursion. Here’s a snippet to do just that in Java:

    public class Explode {
    static void dfs(String prefix, String suffix) {
        final int k = suffix.indexOf('[');
        if (k == -1) {
            System.out.println(prefix + suffix);
        } else {
            prefix += suffix.substring(0, k);
            char from = suffix.charAt(k+1);
            char to = suffix.charAt(k+3);
            suffix = suffix.substring(k+5);
            for (char ch = from; ch <= to; ch++) {
                dfs(prefix + ch, suffix);               
            }
        }
    }
    public static void main(String[] args) {
        String template = "http://example.com/[a-c]/[0-2][x-z].htm";
        dfs("", template);
    }
}

    (see full output)

    This is a standard recursive tuple generator, but with some string infixing in between. It’s trivial to port to C#. You’d want to use a mutable StringBuilder-like class for better performance.

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