# Natural Language Toolkit: Stemmers
#
# Copyright (C) 2001-2019 NLTK Project
# Author: Steven Tomcavage <[email protected]>
# URL: <http://nltk.org/>
# For license information, see LICENSE.TXT
"""
A word stemmer based on the Lancaster (Paice/Husk) stemming algorithm.
Paice, Chris D. "Another Stemmer." ACM SIGIR Forum 24.3 (1990): 56-61.
"""
from __future__ import unicode_literals
import re
from nltk.stem.api import StemmerI
from nltk.compat import python_2_unicode_compatible
[docs]@python_2_unicode_compatible
class LancasterStemmer(StemmerI):
"""
Lancaster Stemmer
>>> from nltk.stem.lancaster import LancasterStemmer
>>> st = LancasterStemmer()
>>> st.stem('maximum') # Remove "-um" when word is intact
'maxim'
>>> st.stem('presumably') # Don't remove "-um" when word is not intact
'presum'
>>> st.stem('multiply') # No action taken if word ends with "-ply"
'multiply'
>>> st.stem('provision') # Replace "-sion" with "-j" to trigger "j" set of rules
'provid'
>>> st.stem('owed') # Word starting with vowel must contain at least 2 letters
'ow'
>>> st.stem('ear') # ditto
'ear'
>>> st.stem('saying') # Words starting with consonant must contain at least 3
'say'
>>> st.stem('crying') # letters and one of those letters must be a vowel
'cry'
>>> st.stem('string') # ditto
'string'
>>> st.stem('meant') # ditto
'meant'
>>> st.stem('cement') # ditto
'cem'
>>> st_pre = LancasterStemmer(strip_prefix_flag=True)
>>> st_pre.stem('kilometer') # Test Prefix
'met'
>>> st_custom = LancasterStemmer(rule_tuple=("ssen4>", "s1t."))
>>> st_custom.stem("ness") # Change s to t
'nest'
"""
# The rule list is static since it doesn't change between instances
default_rule_tuple = (
"ai*2.", # -ia > - if intact
"a*1.", # -a > - if intact
"bb1.", # -bb > -b
"city3s.", # -ytic > -ys
"ci2>", # -ic > -
"cn1t>", # -nc > -nt
"dd1.", # -dd > -d
"dei3y>", # -ied > -y
"deec2ss.", # -ceed >", -cess
"dee1.", # -eed > -ee
"de2>", # -ed > -
"dooh4>", # -hood > -
"e1>", # -e > -
"feil1v.", # -lief > -liev
"fi2>", # -if > -
"gni3>", # -ing > -
"gai3y.", # -iag > -y
"ga2>", # -ag > -
"gg1.", # -gg > -g
"ht*2.", # -th > - if intact
"hsiug5ct.", # -guish > -ct
"hsi3>", # -ish > -
"i*1.", # -i > - if intact
"i1y>", # -i > -y
"ji1d.", # -ij > -id -- see nois4j> & vis3j>
"juf1s.", # -fuj > -fus
"ju1d.", # -uj > -ud
"jo1d.", # -oj > -od
"jeh1r.", # -hej > -her
"jrev1t.", # -verj > -vert
"jsim2t.", # -misj > -mit
"jn1d.", # -nj > -nd
"j1s.", # -j > -s
"lbaifi6.", # -ifiabl > -
"lbai4y.", # -iabl > -y
"lba3>", # -abl > -
"lbi3.", # -ibl > -
"lib2l>", # -bil > -bl
"lc1.", # -cl > c
"lufi4y.", # -iful > -y
"luf3>", # -ful > -
"lu2.", # -ul > -
"lai3>", # -ial > -
"lau3>", # -ual > -
"la2>", # -al > -
"ll1.", # -ll > -l
"mui3.", # -ium > -
"mu*2.", # -um > - if intact
"msi3>", # -ism > -
"mm1.", # -mm > -m
"nois4j>", # -sion > -j
"noix4ct.", # -xion > -ct
"noi3>", # -ion > -
"nai3>", # -ian > -
"na2>", # -an > -
"nee0.", # protect -een
"ne2>", # -en > -
"nn1.", # -nn > -n
"pihs4>", # -ship > -
"pp1.", # -pp > -p
"re2>", # -er > -
"rae0.", # protect -ear
"ra2.", # -ar > -
"ro2>", # -or > -
"ru2>", # -ur > -
"rr1.", # -rr > -r
"rt1>", # -tr > -t
"rei3y>", # -ier > -y
"sei3y>", # -ies > -y
"sis2.", # -sis > -s
"si2>", # -is > -
"ssen4>", # -ness > -
"ss0.", # protect -ss
"suo3>", # -ous > -
"su*2.", # -us > - if intact
"s*1>", # -s > - if intact
"s0.", # -s > -s
"tacilp4y.", # -plicat > -ply
"ta2>", # -at > -
"tnem4>", # -ment > -
"tne3>", # -ent > -
"tna3>", # -ant > -
"tpir2b.", # -ript > -rib
"tpro2b.", # -orpt > -orb
"tcud1.", # -duct > -duc
"tpmus2.", # -sumpt > -sum
"tpec2iv.", # -cept > -ceiv
"tulo2v.", # -olut > -olv
"tsis0.", # protect -sist
"tsi3>", # -ist > -
"tt1.", # -tt > -t
"uqi3.", # -iqu > -
"ugo1.", # -ogu > -og
"vis3j>", # -siv > -j
"vie0.", # protect -eiv
"vi2>", # -iv > -
"ylb1>", # -bly > -bl
"yli3y>", # -ily > -y
"ylp0.", # protect -ply
"yl2>", # -ly > -
"ygo1.", # -ogy > -og
"yhp1.", # -phy > -ph
"ymo1.", # -omy > -om
"ypo1.", # -opy > -op
"yti3>", # -ity > -
"yte3>", # -ety > -
"ytl2.", # -lty > -l
"yrtsi5.", # -istry > -
"yra3>", # -ary > -
"yro3>", # -ory > -
"yfi3.", # -ify > -
"ycn2t>", # -ncy > -nt
"yca3>", # -acy > -
"zi2>", # -iz > -
"zy1s.", # -yz > -ys
)
def __init__(self, rule_tuple=None, strip_prefix_flag=False):
"""Create an instance of the Lancaster stemmer.
"""
# Setup an empty rule dictionary - this will be filled in later
self.rule_dictionary = {}
# Check if a user wants to strip prefix
self._strip_prefix = strip_prefix_flag
# Check if a user wants to use his/her own rule tuples.
self._rule_tuple = rule_tuple if rule_tuple else self.default_rule_tuple
[docs] def parseRules(self, rule_tuple=None):
"""Validate the set of rules used in this stemmer.
If this function is called as an individual method, without using stem
method, rule_tuple argument will be compiled into self.rule_dictionary.
If this function is called within stem, self._rule_tuple will be used.
"""
# If there is no argument for the function, use class' own rule tuple.
rule_tuple = rule_tuple if rule_tuple else self._rule_tuple
valid_rule = re.compile("^[a-z]+\*?\d[a-z]*[>\.]?$")
# Empty any old rules from the rule set before adding new ones
self.rule_dictionary = {}
for rule in rule_tuple:
if not valid_rule.match(rule):
raise ValueError("The rule {0} is invalid".format(rule))
first_letter = rule[0:1]
if first_letter in self.rule_dictionary:
self.rule_dictionary[first_letter].append(rule)
else:
self.rule_dictionary[first_letter] = [rule]
[docs] def stem(self, word):
"""Stem a word using the Lancaster stemmer.
"""
# Lower-case the word, since all the rules are lower-cased
word = word.lower()
word = self.__stripPrefix(word) if self._strip_prefix else word
# Save a copy of the original word
intact_word = word
# If rule dictionary is empty, parse rule tuple.
if not self.rule_dictionary:
self.parseRules()
return self.__doStemming(word, intact_word)
def __doStemming(self, word, intact_word):
"""Perform the actual word stemming
"""
valid_rule = re.compile("^([a-z]+)(\*?)(\d)([a-z]*)([>\.]?)$")
proceed = True
while proceed:
# Find the position of the last letter of the word to be stemmed
last_letter_position = self.__getLastLetter(word)
# Only stem the word if it has a last letter and a rule matching that last letter
if (
last_letter_position < 0
or word[last_letter_position] not in self.rule_dictionary
):
proceed = False
else:
rule_was_applied = False
# Go through each rule that matches the word's final letter
for rule in self.rule_dictionary[word[last_letter_position]]:
rule_match = valid_rule.match(rule)
if rule_match:
(
ending_string,
intact_flag,
remove_total,
append_string,
cont_flag,
) = rule_match.groups()
# Convert the number of chars to remove when stemming
# from a string to an integer
remove_total = int(remove_total)
# Proceed if word's ending matches rule's word ending
if word.endswith(ending_string[::-1]):
if intact_flag:
if word == intact_word and self.__isAcceptable(
word, remove_total
):
word = self.__applyRule(
word, remove_total, append_string
)
rule_was_applied = True
if cont_flag == '.':
proceed = False
break
elif self.__isAcceptable(word, remove_total):
word = self.__applyRule(
word, remove_total, append_string
)
rule_was_applied = True
if cont_flag == '.':
proceed = False
break
# If no rules apply, the word doesn't need any more stemming
if rule_was_applied == False:
proceed = False
return word
def __getLastLetter(self, word):
"""Get the zero-based index of the last alphabetic character in this string
"""
last_letter = -1
for position in range(len(word)):
if word[position].isalpha():
last_letter = position
else:
break
return last_letter
def __isAcceptable(self, word, remove_total):
"""Determine if the word is acceptable for stemming.
"""
word_is_acceptable = False
# If the word starts with a vowel, it must be at least 2
# characters long to be stemmed
if word[0] in "aeiouy":
if len(word) - remove_total >= 2:
word_is_acceptable = True
# If the word starts with a consonant, it must be at least 3
# characters long (including one vowel) to be stemmed
elif len(word) - remove_total >= 3:
if word[1] in "aeiouy":
word_is_acceptable = True
elif word[2] in "aeiouy":
word_is_acceptable = True
return word_is_acceptable
def __applyRule(self, word, remove_total, append_string):
"""Apply the stemming rule to the word
"""
# Remove letters from the end of the word
new_word_length = len(word) - remove_total
word = word[0:new_word_length]
# And add new letters to the end of the truncated word
if append_string:
word += append_string
return word
def __stripPrefix(self, word):
"""Remove prefix from a word.
This function originally taken from Whoosh.
"""
for prefix in (
"kilo",
"micro",
"milli",
"intra",
"ultra",
"mega",
"nano",
"pico",
"pseudo",
):
if word.startswith(prefix):
return word[len(prefix) :]
return word
def __repr__(self):
return '<LancasterStemmer>'