The English (Porter2) stemming algorithm

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The English (Porter2) stemming algorithm

The English (Porter2) stemming algorithm

 

Links to resources

Snowball main page

The stemmer in Snowball

The ANSI C stemmer

— and its header

Sample English vocabulary

Its stemmed equivalent

Vocabulary + stemmed equivalent

Tar-gzipped file of all of the above

A stop word list

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 ties -> tie, cries -> cri)

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 luxuriat -> luxuriate), or

if the word ends with a double remove the last letter (so hopp -> hop), or

if the word is short, add e (so hop -> hope)

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 cry -> cri, by -> by, say -> say)

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 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  W_i, and if a match is found, the string is replaced with X_i.

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  ...  V_k  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 p1

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
    )
)