Question

I had a regex, like so:

(?<one-1>cat)|(?<two-2>dog)|(?<three-3>mouse)|(?<four-4>fish)

When I tried to use this pattern in a .Net app, it failed, because the group name contained a ‘-‘ in it.

So, as a workaround, I tried to use two regexes, the first:

(?<A>cat)|(?<Be>dog)|(?<C>mouse)|(?<D>fish)

would match the original cases I was looking for into group names I could control.
And then, I intended to use the correctly matched group name from that regex in one like this:

(?<A>one-1)|(?<Be>two-2)|(?<C>three-3)|(?<D>four-4)

I would do so, by finding the string that matched this pattern and determining if the group names were equal.

I know this seems a bit convoluted. Thanks of any help offered.

Answer 1

Something along the lines of the following?

string[,] patterns = {
    { "one-1", "cat" },
    { "two-2", "dog" },
    { "three-3", "mouse" },
    { "four-4", "fish" },
};

var regex = buildRegex(patterns);

string[] tests = { "foo", "dog", "bar", "fish" };
foreach (var t in tests) {
    var m = regex.Match(t);
    Console.WriteLine("{0}: {1}", t, reportMatch(regex, m));
}

Output

foo: no match
dog: two-2 = dog
bar: no match
fish: four-4 = fish

First we build up a Regex instance by escaping the group names and combining them with the patterns. Any non-word character is replaced with the sequence _nnn_ where nnn is its UTF-32 value.

private static Regex buildRegex(string[,] inputs)
{   
    string regex = ""; 
    for (int i = 0; i <= inputs.GetUpperBound(0); i++) {
        var part = String.Format(
            "(?<{0}>{1})",
            Regex.Replace(inputs[i,0], @"([\W_])", new MatchEvaluator(escape)),
            inputs[i,1]);

        regex += (regex.Length != 0 ? "|" : "") + part;
    }   

    return new Regex(regex);
}   

private static string escape(Match m)
{
    return "_" + Char.ConvertToUtf32(m.Groups[1].Value, 0) + "_";
}   

For matches, the .NET library doesn’t give us an easy way to get a group’s name, so we have to go the other way: for each group name, we check whether that group matched and if so unescape its name and let the caller know both name and captured substring.

private static string reportMatch(Regex regex, Match m)
{   
    if (!m.Success)
        return "no match";

    foreach (var name in regex.GetGroupNames()) {
        if (name != "0" && m.Groups[name].Value.Length > 0)
            return String.Format(
                       "{0} = {1}",
                       Regex.Replace(name, @"_(\d+)_",
                           new MatchEvaluator(unescape)),
                       m.Groups[name].Value);
    }

    return null;
}   

private static string unescape(Match m)
{   
    return Char.ConvertFromUtf32(int.Parse(m.Groups[1].Value));
}