Double Metaphone

The Double Metaphone search algorithm is a phonetic algorithm written by Lawrence Philips and is the second generation of his Metaphone algorithm. Its implementation was described in the June 2000 issue of "C/C++ Users Journal".

It is called "Double" because it can return both a primary and a secondary code for a string; this accounts for some ambiguous cases as well as for multiple variants of surnames with common ancestry. For example, encoding the name "Smith" yields a primary code of "SM0" and a secondary code of "XMT", while the name "Schmidt" yields a primary code of "XMT" and a secondary code of "SMT"--both have "XMT" in common.

Double Metaphone tries to account for myriad irregularities in English of Slavic, Germanic, Celtic, Greek, French, Italian, Spanish, Chinese, and other origin. Thus it uses a much more complex ruleset for coding than its predecessor; for example, it tests for approximately 100 different contexts of the use of the letter C alone. In the introduction to his original journal article, Mr. Philips defended this complexity:

Albert Einstein once said, "Everything should be made as simple as possible--but no simpler!" Simplicity, of course, is a fundamental touchstone of quality in engineering and science. If an algorithm accomplishes its task as simply as possible, and demonstrates a touch of intuitive inspiration as well, we compliment it as "elegant." Unfortunately for engineers, human activity can rarely be described elegantly. And although text processing is a critical technology at a time when millions of people are searching the web, the unsystematic and exception-laden English language often demands algorithms that look ugly to engineers.

Implementations of Double Metaphone

* C++ implementation : http://www.cuj.com/documents/s=8038/cuj0006philips/
* C# implementation: http://www.codeproject.com/KB/recipes/dmetaphone5.aspx
* PHP implementation: http://swoodbridge.com/DoubleMetaPhone/
* PHP implementation (native, in C): http://pecl.php.net/package/doublemetaphone
* Ruby implementation included in http://english.rubyforge.org
* Ruby implementation included in http://rubyforge.org/projects/text/
* Perl implementation: http://www.cpan.org/modules/by-authors/id/MAURICE/
* Java implementation: http://commons.apache.org/codec/userguide.html
* Transact SQL implementation: http://www.sqlmag.com/Articles/ArticleID/26094/pg/1/1.html (full-text access requires subscription)
* Python and MySQL implementations: http://atomboy.isa-geek.com/plone/Members/acoil/programing/double-metaphone/

ee also

* Metaphone
* Soundex

External links

* [http://www.cuj.com/documents/s=8038/cuj0006philips/ "The Double Metaphone Search Algorithm"] , "C/C++ Users Journal", June 2000 (full-text access requires registration)
* [ftp://ftp.cuj.com/sourcecode/cuj/2000/cujjun2000.zip Source code from the above issue] (73 kilobytes, ZIP), including the Double Metaphone source in C++
* Code project article on double metaphone: http://www.codeproject.com/string/dmetaphone1.asp