Sunday, 7 July 2013

C# How To: Implement the Soundex Algorithm

I caught the end of a conversation about the Soundex algorithm at work the other day which inspired me to write an implementation of it in C#. If you are not familiar with what Soundex is then the Wikipedia article on Soundex is a good place to start. I first came across this algorithm in a Natural Language Processing module during my university education. In a nutshell, when the Soundex algorithm is applied to a word, a Soundex Code is produced as output. Words that differ in spelling but sound the same (homophones) should produce the same Soundex Codes. For instance, "to" and "two" are spelt differently, but sound the same and therefore produce the same Soundex Code of "T000".

This is a useful little algorithm and I particularly like it for its simplicity and the fact that the heuristics used in the algorithm work well in most cases (one limitation of Soundex is that it falls short of covering words that sound the same but have a different first letter, e.g. "site" and "cite" produce different Soundex codes). Soundex is useful when writing search functionality where you want to account for misspellings in the users query. It's worth pointing out that SQL Server natively supports Soundex (see the Soundex function in T-SQL, for example).

My C# implementation is below - I opted to implement it in a static class that exposes one public method "For". The example source code is available on GitHub -

public static class Soundex
    public static string For(string word)
        const int MaxSoundexCodeLength = 4;

        var soundexCode = new StringBuilder();
        var previousWasHOrW = false;

        word = Regex.Replace(
            word == null ? string.Empty : word.ToUpper(),

        if (string.IsNullOrEmpty(word))
            return string.Empty.PadRight(MaxSoundexCodeLength, '0');


        for (var i = 1; i < word.Length; i++)
            var numberCharForCurrentLetter =

            if (i == 1 &&
                    numberCharForCurrentLetter ==

            if (soundexCode.Length > 2 && previousWasHOrW &&
                    numberCharForCurrentLetter ==
                        soundexCode[soundexCode.Length - 2])

            if (soundexCode.Length > 0 &&
                    numberCharForCurrentLetter ==
                        soundexCode[soundexCode.Length - 1])


            previousWasHOrW = "HW".Contains(word[i]);

        return soundexCode
                        .PadRight(MaxSoundexCodeLength, '0')
                            .Substring(0, MaxSoundexCodeLength);

    private static char GetCharNumberForLetter(char letter)
        if ("BFPV".Contains(letter)) return '1';
        if ("CGJKQSXZ".Contains(letter)) return '2';
        if ("DT".Contains(letter)) return '3';
        if ('L' == letter) return '4';
        if ("MN".Contains(letter)) return '5';
        if ('R' == letter) return '6';

        return '0';

// Both lines below output R100