The English (Porter2) stemming algorithm

Links to resources

Here is a sample of vocabulary, with the stemmed forms that will be generated with the algorithm.

word stem          word stem
consign
consigned
consigning
consignment
consist
consisted
consistency
consistent
consistently
consisting
consists
consolation
consolations
consolatory
console
consoled
consoles
consolidate
consolidated
consolidating
consoling
consolingly
consols
consonant
consort
consorted
consorting
conspicuous
conspicuously
conspiracy
conspirator
conspirators
conspire
conspired
conspiring
constable
constables
constance
constancy
constant
consign
consign
consign
consign
consist
consist
consist
consist
consist
consist
consist
consol
consol
consolatori
consol
consol
consol
consolid
consolid
consolid
consol
consol
consol
conson
consort
consort
consort
conspicu
conspicu
conspiraci
conspir
conspir
conspir
conspir
conspir
constabl
constabl
constanc
constanc
constant
knack
knackeries
knacks
knag
knave
knaves
knavish
kneaded
kneading
knee
kneel
kneeled
kneeling
kneels
knees
knell
knelt
knew
knick
knif
knife
knight
knightly
knights
knit
knits
knitted
knitting
knives
knob
knobs
knock
knocked
knocker
knockers
knocking
knocks
knopp
knot
knots
knack
knackeri
knack
knag
knave
knave
knavish
knead
knead
knee
kneel
kneel
kneel
kneel
knee
knell
knelt
knew
knick
knif
knife
knight
knight
knight
knit
knit
knit
knit
knive
knob
knob
knock
knock
knocker
knocker
knock
knock
knopp
knot
knot

Developing the English stemmer

(Revised slightly, December 2001)
(Further revised, September 2002)

I have made more than one attempt to improve the structure of the Porter algorithm by making it follow the pattern of ending removal of the Romance language stemmers. It is not hard to see why one should want to do this: step 1b of the Porter stemmer removes ed and ing, which are i-suffixes (*) attached to verbs. If these suffixes are removed, there should be no need to remove d-suffixes which are not verbal, although it will try to do so. This seems to be a deficiency in the Porter stemmer, not shared by the Romance stemmers. Again, the divisions between steps 2, 3 and 4 seem rather arbitrary, and are not found in the Romance stemmers.

Nevertheless, these attempts at improvement have been abandoned. They seem to lead to a more complicated algorithm with no very obvious improvements. A reason for not taking note of the outcome of step 1b may be that English endings do not determine word categories quite as strongly as endings in the Romance languages. For example, condition and position in French have to be nouns, but in English they can be verbs as well as nouns,

We are all conditioned by advertising
They are positioning themselves differently today
A possible reason for having separate steps 2, 3 and 4 is that d-suffix combinations in English are quite complex, a point which has been made elsewhere.

But it is hardly surprising that after twenty years of use of the Porter stemmer, certain improvements did suggest themselves, and a new algorithm for English is therefore offered here. (It could be called the ‘Porter2’ stemmer to distinguish it from the Porter stemmer, from which it derives.) The changes are not so very extensive: (1) terminating y is changed to i rather less often, (2) suffix us does not lose its s, (3) a few additional suffixes are included for removal, including (4) suffix ly. In addition, a small list of exceptional forms is included. In December 2001 there were two further adjustments: (5) Steps 5a and 5b of the old Porter stemmer were combined into a single step. This means that undoubling final ll is not done with removal of final e. (6) In Step 3 ative is removed only when in region R2. (7) In July 2005 a small adjustment was made (including a new step 0) to handle apostrophe.

To begin with, here is the basic algorithm without reference to the exceptional forms. An exact comparison with the Porter algorithm needs to be done quite carefully if done at all. Here we indicate by * points of departure, and by + additional features. In the sample vocabulary, Porter and Porter2 stem slightly under 5% of words to different forms.

Definition of the English stemmer

Define a vowel as one of

a   e   i   o   u   y
Define a double as one of
bb   dd   ff   gg   mm   nn   pp   rr   tt
Define a valid li-ending as one of
c   d   e   g   h   k   m   n   r   t
R1 is the region after the first non-vowel following a vowel, or the end of the word if there is no such non-vowel. (This definition may be modified for certain exceptional words — see below.)

R2 is the region after the first non-vowel following a vowel in R1, or the end of the word if there is no such non-vowel. (See note on R1 and R2.)

Define a short syllable in a word as either (a) a vowel followed by a non-vowel other than w, x or Y and preceded by a non-vowel, or * (b) a vowel at the beginning of the word followed by a non-vowel.

So rap, trap, entrap end with a short syllable, and ow, on, at are classed as short syllables. But uproot, bestow, disturb do not end with a short syllable.

A word is called short if it ends in a short syllable, and if R1 is null.

So bed, shed and shred are short words, bead, embed, beds are not short words.

An apostrophe (') may be regarded as a letter. (See note on apostrophes in English.)

If the word has two letters or less, leave it as it is.

Otherwise, do each of the following operations,

Remove initial ', if present. + Then,

Set initial y, or y after a vowel, to Y, and then establish the regions R1 and R2. (See note on vowel marking.)

Step 0: +

Search for the longest among the suffixes,

'
's
's'
and remove if found.
Step 1a:

Search for the longest among the following suffixes, and perform the action indicated.

sses
replace by ss
ied+   ies*
replace by i if preceded by more than one letter, otherwise by ie (so tiestie, criescri)
s
delete if the preceding word part contains a vowel not immediately before the s (so gas and this retain the s, gaps and kiwis lose it)
us+   ss
do nothing
Step 1b:

Search for the longest among the following suffixes, and perform the action indicated.

eed   eedly+
replace by ee if in R1
ed   edly+   ing   ingly+
delete if the preceding word part contains a vowel, and after the deletion:
if the word ends at, bl or iz add e (so luxuriatluxuriate), or
if the word ends with a double remove the last letter (so hopphop), or
if the word is short, add e (so hophope)
Step 1c: *
replace suffix y or Y by i if preceded by a non-vowel which is not the first letter of the word (so crycri, byby, saysay)
Step 2:

Search for the longest among the following suffixes, and, if found and in R1, perform the action indicated.

tional:   replace by tion
enci:   replace by ence
anci:   replace by ance
abli:   replace by able
entli:   replace by ent
izer   ization:   replace by ize
ational   ation   ator:   replace by ate
alism   aliti   alli:   replace by al
fulness:   replace by ful
ousli   ousness:   replace by ous
iveness   iviti:   replace by ive
biliti   bli+:   replace by ble
ogi+:   replace by og if preceded by l
fulli+:   replace by ful
lessli+:   replace by less
li+:   delete if preceded by a valid li-ending
Step 3:

Search for the longest among the following suffixes, and, if found and in R1, perform the action indicated.

tional+:   replace by tion
ational+:   replace by ate
alize:   replace by al
icate   iciti   ical:   replace by ic
ful   ness:   delete
ative*:   delete if in R2
Step 4:

Search for the longest among the following suffixes, and, if found and in R2, perform the action indicated.

al   ance   ence   er   ic   able   ible   ant   ement   ment   ent   ism   ate   iti   ous   ive   ize
delete
ion
delete if preceded by s or t
Step 5: *

Search for the following suffixes, and, if found, perform the action indicated.

e
delete if in R2, or in R1 and not preceded by a short syllable
l
delete if in R2 and preceded by l

Finally, turn any remaining Y letters in the word back into lower case.

Exceptional forms in general

It is quite easy to expand a Snowball script so that certain exceptional word forms get special treatment. The standard case is that certain words W1,  W2  ..., instead of passing through the stemming process, are mapped to the forms  X1,  X2  ... respectively. If the script does the stemming by means of the call

    define stem as C

where  C  is a command, the exceptional cases can be dealt with by extending this to

    define stem as ( exception or C )

and putting in a routine  exception:

    define exception as (
        [substring] atlimit among(
            'W1'  ( <- 'X1' )
            'W2'  ( <- 'X2' )
            ...
        )
    )

atlimit  causes the whole string to be tested for equality with one of the  Wi, and if a match is found, the string is replaced with Xi.

More precisely we might have a group of words  W11,  W12  ... that need to be mapped to  X1, another group  W21,  W22 ... that need to be mapped to  X2, and so on, and a list of words V1,  V2  ...  Vk  that are to remain invariant. The exception  routine may then be written as follows:

    among( 'W11' 'W12' ... (<- 'X1')
           'W21' 'W22' ... (<- 'X2')
           ...
           'Wn1' 'Wn2' ... (<- 'Xn')
           'V1' 'V2' ... 'Vk'
         )

And indeed the  exception1  routine for the English stemmer has just that shape:

    define exception1 as (

         [substring] atlimit among(

            /* special changes: */

            'skis'      (<-'ski')
            'skies'     (<-'sky')
            'dying'     (<-'die')
            'lying'     (<-'lie')
            'tying'     (<-'tie')

            /* special -LY cases */

            'idly'      (<-'idl')
            'gently'    (<-'gentl')
            'ugly'      (<-'ugli')
            'early'     (<-'earli')
            'only'      (<-'onli')
            'singly'    (<-'singl')

            // ... extensions possible here ...

            /* invariant forms: */

            'sky'
            'news'
            'howe'

            'atlas' 'cosmos' 'bias' 'andes' // not plural forms

            // ... extensions possible here ...
        )
    )

(More will be said about the words that appear here shortly.)

Here we see words being treated exceptionally before stemming is done, but equally we could treat stems exceptionally after stemming is done, and so, if we wish, map absorpt to absorb, reduct to reduc etc., as in the Lovins stemmer. But more generally, throughout the algorithm, each significant step may have recognised exceptions, and a suitably placed  among  will take care of them. For example, a point made at least twice in the literature is that words beginning gener are overstemmed by the Porter stemmer:

generate
generates
generated
generating
general
generally
generic
generically
generous
generously
  →   gener

To fix this over-stemming, we make an exception to the usual setting of p1, the left point of R1, and therefore replace

    gopast v  gopast non-v  setmark p1

with

    among (
        'gener'
        // ... and other stems may be included here ...
    ) or (gopast v  gopast non-v)
    setmark p

after which the words beginning gener stem as follows:

generate
generates
generated
generating
  →   generat
general
generally
  →   general
generic
generically
  →   generic
generous
generously
  →   generous

Another example is given by the  exception2  routine, which is similar to  exception1, but placed after the call of  Step_1a, which may have removed terminal s,

    define exception2 as (

        [substring] atlimit among(
            'inning' 'outing' 'canning' 'herring'
            'proceed' 'exceed' 'succeed'

            // ... extensions possible here ...

        )
    )

Snowball makes it easy therefore to add in lists of exceptions. But deciding what the lists of exceptions should be is far from easy. Essentially there are two lines of attack, the systematic and the piecemeal. One might systematically treat as exceptions the stem changes of irregular verbs, for example. The piecemeal approach is to add in exceptions as people notice them — like gener above. The problem with the systematic approach is that it should be done by investigating the entire language vocabulary, and that is more than most people are prepared to do. The problem with the piecemeal approach is that it is arbitrary, and usually yields little.

The exception lists in the English stemmer are meant to be illustrative (‘this is how it is done if you want to do it’), and were derived piecemeal.

a) The new stemmer improves on the Porter stemmer in handling short words ending e and y. There is however a mishandling of the four forms sky, skies, ski, skis, which is easily corrected by treating three of these words as special cases.

b) Similarly there is a problem with the ing form of three letter verbs ending ie. There are only three such verbs: die, lie and tie, so a special case is made for dying, lying and tying.

c) One has to be a little careful of certain ing forms. inning, outing, canning, which one does not wish to be stemmed to in, out, can.

d) The removal of suffix ly, which is not in the Porter stemmer, has a number of exceptions. Certain short-word exceptions are idly, gently, ugly, early, only, singly. Rarer words (bristly, burly, curly, surly ...) are not included.

e) The remaining words were included following complaints from users of the Porter algorithm. news is not the plural of new (noticed when IR systems were being set up for Reuters). Howe is a surname, and needs to be separated from how (noticed when doing a search for ‘Sir Geoffrey Howe’ in a demonstration at the House of Commons). succeed etc are not past participles, so the ed should not be removed (pointed out to me in an email from India). herring should not stem to her (another email from Russia).

f) Finally, a few non-plural words ending s have been added.

Incidentally, this illustrates how much feedback to expect from the real users of a stemming algorithm: seven or eight words in twenty years!

The definition of the English stemmer above is therefore supplemented by the following:

Exceptional forms in the English stemmer

If the words begins gener, commun or arsen, set R1 to be the remainder of the word.

Stem certain special words as follows,

skis   →   ski
skies   →   sky
dying
lying
tying
die
lie
tie
idly
gently
ugly
early
only
singly
idl
gentl
ugli
earli
onli
singl

If one of the following is found, leave it invariant,

sky
news
howe
atlas     cosmos     bias     andes

Following step 1a, leave the following invariant,

inning     outing     canning     herring     earring
proceed     exceed     succeed

The full algorithm in Snowball

integers ( p1 p2 )
booleans ( Y_found )

routines (
    prelude postlude
    mark_regions
    shortv
    R1 R2
    Step_1a Step_1b Step_1c Step_2 Step_3 Step_4 Step_5
    exception1
    exception2
)

externals ( stem )

groupings ( v v_WXY valid_LI )

stringescapes {}

define v        'aeiouy'
define v_WXY    v + 'wxY'

define valid_LI 'cdeghkmnrt'

define prelude as (
    unset Y_found
    do ( ['{'}'] delete)
    do ( ['y'] <-'Y' set Y_found)
    do repeat(goto (v ['y']) <-'Y' set Y_found)
)

define mark_regions as (
    $p1 = limit
    $p2 = limit
    do(
        among (
            'gener'
            'commun'  //  added May 2005
            'arsen'   //  added Nov 2006 (arsenic/arsenal)
            // ... extensions possible here ...
        ) or (gopast v  gopast non-v)
        setmark p1
        gopast v  gopast non-v  setmark p2
    )
)

backwardmode (

    define shortv as (
        ( non-v_WXY v non-v )
        or
        ( non-v v atlimit )
    )

    define R1 as $p1 <= cursor
    define R2 as $p2 <= cursor

    define Step_1a as (
        try (
            [substring] among (
                '{'}' '{'}s' '{'}s{'}'
                       (delete)
            )
        )
        [substring] among (
            'sses' (<-'ss')
            'ied' 'ies'
                   ((hop 2 <-'i') or <-'ie')
            's'    (next gopast v delete)
            'us' 'ss'
        )
    )

    define Step_1b as (
        [substring] among (
            'eed' 'eedly'
                (R1 <-'ee')
            'ed' 'edly' 'ing' 'ingly'
                (
                test gopast v  delete
                test substring among(
                    'at' 'bl' 'iz'
                         (<+ 'e')
                    'bb' 'dd' 'ff' 'gg' 'mm' 'nn' 'pp' 'rr' 'tt'
                    // ignoring double c, h, j, k, q, v, w, and x
                         ([next]  delete)
                    ''   (atmark p1  test shortv  <+ 'e')
                )
            )
        )
    )

    define Step_1c as (
        ['y' or 'Y']
        non-v not atlimit
        <-'i'
    )

    define Step_2 as (
        [substring] R1 among (
            'tional'  (<-'tion')
            'enci'    (<-'ence')
            'anci'    (<-'ance')
            'abli'    (<-'able')
            'entli'   (<-'ent')
            'izer' 'ization'
                      (<-'ize')
            'ational' 'ation' 'ator'
                      (<-'ate')
            'alism' 'aliti' 'alli'
                      (<-'al')
            'fulness' (<-'ful')
            'ousli' 'ousness'
                      (<-'ous')
            'iveness' 'iviti'
                      (<-'ive')
            'biliti' 'bli'
                      (<-'ble')
            'ogi'     ('l' <-'og')
            'fulli'   (<-'ful')
            'lessli'  (<-'less')
            'li'      (valid_LI delete)
        )
    )

    define Step_3 as (
        [substring] R1 among (
            'tional'  (<- 'tion')
            'ational' (<- 'ate')
            'alize'   (<-'al')
            'icate' 'iciti' 'ical'
                      (<-'ic')
            'ful' 'ness'
                      (delete)
            'ative'
                      (R2 delete)  // 'R2' added Dec 2001
        )
    )

    define Step_4 as (
        [substring] R2 among (
            'al' 'ance' 'ence' 'er' 'ic' 'able' 'ible' 'ant' 'ement'
            'ment' 'ent' 'ism' 'ate' 'iti' 'ous' 'ive' 'ize'
                      (delete)
            'ion'     ('s' or 't' delete)
        )
    )

    define Step_5 as (
        [substring] among (
            'e' (R2 or (R1 not shortv) delete)
            'l' (R2 'l' delete)
        )
    )

    define exception2 as (

        [substring] atlimit among(
            'inning' 'outing' 'canning' 'herring' 'earring'
            'proceed' 'exceed' 'succeed'

            // ... extensions possible here ...

        )
    )
)

define exception1 as (

    [substring] atlimit among(

        /* special changes: */

        'skis'      (<-'ski')
        'skies'     (<-'sky')
        'dying'     (<-'die')
        'lying'     (<-'lie')
        'tying'     (<-'tie')

        /* special -LY cases */

        'idly'      (<-'idl')
        'gently'    (<-'gentl')
        'ugly'      (<-'ugli')
        'early'     (<-'earli')
        'only'      (<-'onli')
        'singly'    (<-'singl')

        // ... extensions possible here ...

        /* invariant forms: */

        'sky'
        'news'
        'howe'

        'atlas' 'cosmos' 'bias' 'andes' // not plural forms

        // ... extensions possible here ...
    )
)

define postlude as (Y_found  repeat(goto (['Y']) <-'y'))

define stem as (

    exception1 or
    not hop 3 or (
        do prelude
        do mark_regions
        backwards (

            do Step_1a

            exception2 or (

                do Step_1b
                do Step_1c

                do Step_2
                do Step_3
                do Step_4

                do Step_5
            )
        )
        do postlude
    )
)