Diacritic-insensitive search in C#

When you’re dealing with multiple languages, searching for text can be a little tricky. Using normal string comparison techniques, a search for “Malmo” will not match “Malmö”. Technically it shouldn’t, because the characters are actually different, but it’s a great usability feature to allow people to search for text regardless of diacritics (accents and such).

The Normalization Method

The first idea I had was to strip off the diacritics and simply compare the simplified version of both the query and the text being searched. Using the same example, “Malmö” would become “Malmo” in the text, and so the query would match, since RemoveDiacritics(query) == RemoveDiacritics(text).

The RemoveDiacritics() method is defined in this StackOverflow answer:

static string RemoveDiacritics(string text) 
{
    var normalizedString = text.Normalize(NormalizationForm.FormD);
    var stringBuilder = new StringBuilder();

    foreach (var c in normalizedString)
    {
        var unicodeCategory = CharUnicodeInfo.GetUnicodeCategory(c);
        if (unicodeCategory != UnicodeCategory.NonSpacingMark)
        {
            stringBuilder.Append(c);
        }
    }

    return stringBuilder.ToString().Normalize(NormalizationForm.FormC);
}

As I pointed out in my followup question, this approach doesn’t work very well for search. If we run a simple test using the same words from my question…

            var words = new List<string>()
            {
                "Malmö",
                "München",
                "Åge",
                "Strømsgodset",
                "Kulħadd"
            };
            var simplifiedWords = words.Select(word => RemoveDiacritics(word)).ToList();

…you’ll notice that it works for basic accents that seem to be external to the base character, but not for others where it is embedded. Below is the output I got in the immediate window (since the Console can’t handle some of the characters with the default encoding):

simplifiedWords
Count = 5
    [0]: "Malmo"
    [1]: "Munchen"
    [2]: "Age"
    [3]: "Strømsgodset"
    [4]: "Kulħadd"

Apart from this, there is no way to simplify combined characters such as æ into a graphically similar ae.

This all makes sense, because technically æ and ae are different characters, as are ħ and h. But from a user’s perspective, it feels pretty natural to be able to interchange them when searching.

The Collation Method

The answer to my question shows that it is actually pretty easy to have diacritic-insensitive search in C#, even without doing any stripping operations. It is necessary only to specify CompareOptions.IgnoreNonSpace in string comparison methods. Here’s an example from that same answer:

int ix = CultureInfo.CurrentCulture.CompareInfo.IndexOf(
    "Ad aeternitatem", 
    "æter", 
    CompareOptions.IgnoreNonSpace); // 3

Here’s the same thing applied to one of my original examples:

            int ix = CultureInfo.CurrentCulture.CompareInfo.IndexOf(
                "Kulħadd",
                "hadd",
                CompareOptions.IgnoreNonSpace); // returns 3

This other answer shows the string.Compare() being used instead, using the same flag:

string s1 = "hello";
string s2 = "héllo";

if (String.Compare(s1, s2, CultureInfo.CurrentCulture,
    CompareOptions.IgnoreNonSpace) == 0)
{
    // both strings are equal
}

In either case, just add the CompareOptions.IgnoreCase flag to make it case insensitive as well.