metaphone

Metaphone is a phonetic code algorithm developed in 1990 by Lawrence Philips, and presented in the December 1990 issue of Computer Language.

The goal is to reduce an input word to a short 1-4 characters long code using a set of rules that approximate spoken English. This results in similar codes for similar sounding words.

Rules

The alphabet is reduced to the following set of "codes":

B X S K J T F H L M N P R 0 W Y

(The third letter from the end is a zero, representing the th-sound, or theta.)

This is accomplished by performing the following transformations:

B -> B   unless at the end of a word after "m" as in "dumb"
C -> X    (sh) if -cia- or -ch-
     S   if -ci-, -ce- or -cy-
     K   otherwise, including -sch-
D -> J   if in -dge-, -dgy- or -dgi-
     T   otherwise
F -> F
G -> silent if in -gh- and not at end or before a vowel
         in -gn- or -gned- (also see dge etc. above)
     J   if before i or e or y if not double gg
     K   otherwise
H -> silent if after vowel and no vowel follows
     H   otherwise
J -> J
K -> silent if after "c"
     K   otherwise
L -> L   
M -> M
N -> N
P -> F   if before "h"
     P   otherwise
Q -> K
R -> R
S -> X   (sh) if before "h" or in -sio- or -sia-
     S   otherwise
T -> X   (sh) if -tia- or -tio-
     0   (th) if before "h"
         silent if in -tch-
     T   otherwise
V -> F
W -> silent if not followed by a vowel
     W   if followed by a vowel
X -> KS
Y -> silent if not followed by a vowel
     Y   if followed by a vowel
Z -> S 

Then the initial letters are inspected:

Initial  kn-, gn- pn, ae- or wr-   -> drop first letter
Initial  x-                        -> change to "s"
Initial  wh-                       -> change to "w"

The original algoritm then truncates the resulting code to 4 characters, but more could be used.