# Natural Language Toolkit: CISTEM Stemmer for German # Copyright (C) 2001-2023 NLTK Project # Author: Leonie Weissweiler # Tom Aarsen <> (modifications) # Algorithm: Leonie Weissweiler # Alexander Fraser # URL: # For license information, see LICENSE.TXT import re from typing import Tuple from nltk.stem.api import StemmerI class Cistem(StemmerI): """ CISTEM Stemmer for German This is the official Python implementation of the CISTEM stemmer. It is based on the paper Leonie Weissweiler, Alexander Fraser (2017). Developing a Stemmer for German Based on a Comparative Analysis of Publicly Available Stemmers. In Proceedings of the German Society for Computational Linguistics and Language Technology (GSCL) which can be read here: https://www.cis.lmu.de/~weissweiler/cistem/ In the paper, we conducted an analysis of publicly available stemmers, developed two gold standards for German stemming and evaluated the stemmers based on the two gold standards. We then proposed the stemmer implemented here and show that it achieves slightly better f-measure than the other stemmers and is thrice as fast as the Snowball stemmer for German while being about as fast as most other stemmers. case_insensitive is a a boolean specifying if case-insensitive stemming should be used. Case insensitivity improves performance only if words in the text may be incorrectly upper case. For all-lowercase and correctly cased text, best performance is achieved by setting case_insensitive for false. :param case_insensitive: if True, the stemming is case insensitive. False by default. :type case_insensitive: bool """ strip_ge = re.compile(r"^ge(.{4,})") repl_xx = re.compile(r"(.)\1") strip_emr = re.compile(r"e[mr]$") strip_nd = re.compile(r"nd$") strip_t = re.compile(r"t$") strip_esn = re.compile(r"[esn]$") repl_xx_back = re.compile(r"(.)\*") def __init__(self, case_insensitive: bool = False): self._case_insensitive = case_insensitive @staticmethod def replace_to(word: str) -> str: word = word.replace("sch", "$") word = word.replace("ei", "%") word = word.replace("ie", "&") word = Cistem.repl_xx.sub(r"\1*", word) return word @staticmethod def replace_back(word: str) -> str: word = Cistem.repl_xx_back.sub(r"\1\1", word) word = word.replace("%", "ei") word = word.replace("&", "ie") word = word.replace("$", "sch") return word def stem(self, word: str) -> str: """Stems the input word. :param word: The word that is to be stemmed. :type word: str :return: The stemmed word. :rtype: str >>> from nltk.stem.cistem import Cistem >>> stemmer = Cistem() >>> s1 = "Speicherbehältern" >>> stemmer.stem(s1) 'speicherbehalt' >>> s2 = "Grenzpostens" >>> stemmer.stem(s2) 'grenzpost' >>> s3 = "Ausgefeiltere" >>> stemmer.stem(s3) 'ausgefeilt' >>> stemmer = Cistem(True) >>> stemmer.stem(s1) 'speicherbehal' >>> stemmer.stem(s2) 'grenzpo' >>> stemmer.stem(s3) 'ausgefeil' """ if len(word) == 0: return word upper = word[0].isupper() word = word.lower() word = word.replace("ü", "u") word = word.replace("ö", "o") word = word.replace("ä", "a") word = word.replace("ß", "ss") word = Cistem.strip_ge.sub(r"\1", word) return self._segment_inner(word, upper)[0] def segment(self, word: str) -> Tuple[str, str]: """ This method works very similarly to stem (:func:'cistem.stem'). The difference is that in addition to returning the stem, it also returns the rest that was removed at the end. To be able to return the stem unchanged so the stem and the rest can be concatenated to form the original word, all subsitutions that altered the stem in any other way than by removing letters at the end were left out. :param word: The word that is to be stemmed. :type word: str :return: A tuple of the stemmed word and the removed suffix. :rtype: Tuple[str, str] >>> from nltk.stem.cistem import Cistem >>> stemmer = Cistem() >>> s1 = "Speicherbehältern" >>> stemmer.segment(s1) ('speicherbehält', 'ern') >>> s2 = "Grenzpostens" >>> stemmer.segment(s2) ('grenzpost', 'ens') >>> s3 = "Ausgefeiltere" >>> stemmer.segment(s3) ('ausgefeilt', 'ere') >>> stemmer = Cistem(True) >>> stemmer.segment(s1) ('speicherbehäl', 'tern') >>> stemmer.segment(s2) ('grenzpo', 'stens') >>> stemmer.segment(s3) ('ausgefeil', 'tere') """ if len(word) == 0: return ("", "") upper = word[0].isupper() word = word.lower() return self._segment_inner(word, upper) def _segment_inner(self, word: str, upper: bool): """Inner method for iteratively applying the code stemming regexes. This method receives a pre-processed variant of the word to be stemmed, or the word to be segmented, and returns a tuple of the word and the removed suffix. :param word: A pre-processed variant of the word that is to be stemmed. :type word: str :param upper: Whether the original word started with a capital letter. :type upper: bool :return: A tuple of the stemmed word and the removed suffix. :rtype: Tuple[str, str] """ rest_length = 0 word_copy = word[:] # Pre-processing before applying the substitution patterns word = Cistem.replace_to(word) rest = "" # Apply the substitution patterns while len(word) > 3: if len(word) > 5: word, n = Cistem.strip_emr.subn("", word) if n != 0: rest_length += 2 continue word, n = Cistem.strip_nd.subn("", word) if n != 0: rest_length += 2 continue if not upper or self._case_insensitive: word, n = Cistem.strip_t.subn("", word) if n != 0: rest_length += 1 continue word, n = Cistem.strip_esn.subn("", word) if n != 0: rest_length += 1 continue else: break # Post-processing after applying the substitution patterns word = Cistem.replace_back(word) if rest_length: rest = word_copy[-rest_length:] return (word, rest)