Frog Modules

Character encoding

Frog assumes the input text to be plain text in the UTF-8 character encoding. However, Frog offers the option to specify another character encoding as input with the option t -e. This options is passed on to the Ucto Tokenizer. It has some limitations, (see [tokenizer]) and will be ignored when the Tokenizer is disabled. The character encodings are derived from the ubiquitous unix tool iconv [12]. The output of Frog will always be in UTF-8 character encoding. Likewise, FoLiA XML defaults to UTF-8 as well.


Frog uses the tokenization software Ucto [UCTO] for sentence boundary detection and to separate punctuation from words. In general, recognizing sentence boundaries and punctuation is a simple task but recognizing names and abbreviations is essential to perfom this task well. As shown in example [ex-frog-out], the tokenizer recognizes abbreviations such as z.g. and ‘41 and considers them to be one token. Ucto uses manually constructed rules and lists of Dutch names and abbreviations. Detailed information on Ucto can be found on

The tokenizer module in Frog can be adjusted in several ways. If the input text is already split on sentence boundaries and has one sentence per line, the -n option can be used to prevent Frog from changing the existing sentence boundaries. When sentence boundaries were already marked with a specific marker, one can specify this marker as –uttmarker “marker”. The marker strings will be ignored and their positions will be taken as sentence boundaries.

If the input text is already fully tokenized, the tokenization step in Frog can be skipped altogether using the skip parameter –skip=t. [13]

Multi-word units

Frog recognizes certain special multi-word units (mwu) where a group of consecutive, related tokens is treated as one token. This behavior accommodates, and is in fact required for Frog’s dependency parser as it is trained on a data set with such multi-word units. In the output the parts of the multi-word unit will be connected with an underscore. The PoS-tag, morphological analysis, named entity label and chunk label are concatenated in the same manner.

This multi-word detection can be disabled using the option –skip=m. When using this option, each element of the MWU is treated as separate token. We shown an example sentence in [ex_mwu] that has two multi-word units: Albert Heijn and ’s avonds.
[ex_mwu] Sentence Supermarkt Albert Heijn is tegenwoordig tot ’s avonds laat open.
1 Supermarkt supermarkt [super][markt] N(soort,ev,basis,zijd,stan) 0.542056 O B_NP
2 Albert_Heijn Albert_Heijn [Albert]_[Heijn] SPEC(deeleigen)_SPEC(deeleigen) 1.000000 B-ORG_I-ORG B-NP_I-NP
3 is zijn [zijn] WW(pv,tgw,ev) 0.999150 O B-VP
4 tegenwoordig tegenwoordig [tegenwoordig] ADJ(vrij,basis,zonder) 0.994033 O B-ADVP
5 tot tot [tot] VZ(init) 0.964286 O B-PP
6 ’s_avonds ’s_avond [’s]_[avond][s] LID(bep,gen,evmo)_N(soort,ev,basis,gen) 0.962560 O_O O_B-ADVP
7 laat laat [laat] ADJ(vrij,basis,zonder) 1.000000 O B-VP
8 open open [open] ADJ(vrij,basis,zonder) 0.983755 O B-ADJP
9 . . [.] LET() 1.000000 O O


The lemmatizer assigns the canonical form of a word to each word. For verbs the canonical form is the infinitive, and for nouns it is the singular form. The lemmatizer trained on the e-Lex lexicon [ELEX]. It is dependent on the Part-of-Speech tagger as it uses both the word form and the assigned PoS tag to disambiguate between different candidate lemmas. For example the word zakken used as a noun has the lemma zak while the verb has lemma zakken. Section [sec-bg-lem] presents further details on the lemmatizer.

Morphological Analyzer

The morphological Analyzer (MBMA) cuts each word into its morphemes and shows the spelling changes that took place to create the word form. The fourth column in example [ex-frog-out] shows the morphemes of the example sentence. MBMA tries to decompose every token into morphemes, except for punctuation marks and names. Note that MBMA sometimes makes mistakes with unknown words such as abbreviations that are not included in the MBMA lexicon. The abbreviation z.g. in the example is wrongly analyzed as consisting of two parts. As shown in the earlier XML example [ex-xml-tok] the past particle aangesneden is split into [aan][ge][snijd][en] where the morpheme [snijd] is the root form of sned. More information about the MBMA architecture can be found in [sec-bg-morf].

Part-of-Speech Tagger

The Part-of-Speech tagger uses the tag set of Corpus Gesproken Nederlands (CNG) [POS2004]. It has 12 main PoS tags (shown in table [tab-pos-tags]) and detailed features for type, gender, number, case, position, degree, and tense.

We show an example of the PoS tagger output in table [tab-pos-conf]. The tagger also expresses how certain it was about its tag label in a confidence score between 0 (not sure) and 1 (absolutely sure). In the example the PoS tagger is very sure about the first four tokens but not about the label N(soort,ev,basis,zijd,stan) for the token Psychologie as it only has a confidence score of 0.67. Psychologie is an ambiguous token and can also be used as a name (tag SPEC).

ADJ Adjective
BW Adverb
LET Punctuation
LID Determiner
N Noun
SPEC Names and unknown
TSW Interjection
TW Numerator
VG Conjunction
VNW Pronoun
VZ Preposition
WW Verb

Table: [tab-pos-tags] The main tags in the CGN PoS-tag set.

34 Ik VNW(pers,pron,nomin,vol,1,ev) 0.999791
35 ben WW(pv,tgw,ev) 0.999589
36 ook BW() 0.999979
37 professor N(soort,ev,basis,zijd,stan) 0.997691
38 Psychologie N(soort,ev,basis,zijd,stan) 0.666667

Table: [tab-pos-conf] The PoS tagger assigns a confidence score to each tag.

Named Entity Recognition

The Named Entity Recognizer (NER) detects names in the text and labels them as location (LOC), person (PER), organization (ORG), product (PRO), event (EVE) or miscellaneous (MISC).

Internally and in Frog’s columned output, the tags use a so-called BIO paradigm where B stands for the beginning of the name, I signifies Inside the name, and O outside the name.

More detailed information about the NER module can be found in [sec-bg-ner].

Phrase Chunker

The phrase chunker represents an intermediate step between part-of-speech tagging and full parsing as it produces a non-recursive, non-overlapping flat structure of recognized phrases in the text and classifies them with their grammatical function such as adverbial phrase (ADVP), verb phrase (VP) or noun phrase (NP). The tag labels produced by the chunker use the same type of BIO-tags (Beginning-Inside-Outside) as the named entity recognizer. We show an example sentence in [ex-chunk] where the four-word noun phrase het cold case team is recognized as one phrase. The prepositional phrases (PP) consist only of the preposition themselves due to the flat structure in which the relation between prepositions and noun phrases is not expressed (note that the dependency parse labels, section [sec-dep] do express these relations). Here Midden-Nederland is recognized by the PoS tagger as name and therefor marked as a separate noun phrase that follows the noun phrase de politie.

\([\)Dat\(]_{NP} [\)bevestigt\(]_{VP} [\)het cold case team\(]_{NP} [\)van\(]_{PP}] [\)de politie\(]_{NP} [\)Midden-Nederland\(]_{NP} [\) aan \(]_{PP} [\)de Telegraaf\(]_{NP} [\) .

1 Dat B-NP
2 bevestigt B-VP
3 het B-NP
4 cold I-NP
5 case I-NP
6 team I-NP
7 van B-PP
8 de B-NP
9 politie I-NP
10 Midden-Nederland B-NP
11 aan B-PP
12 de B-NP
13 Telegraaf I-NP
14 . O

Table: [ex-chunk] The phrase chunker detects phrase boundaries and labels the phrases with their grammatical information.

Dependency Parser

The Constraint-satisfaction inference-based dependency parser (CSI-DP) [Canisius+2006] predicts grammatical relations between pairs of tokens. In each token pair relation, one token is the head and the other is the dependent. Together these relations represent the syntactic tree of the sentence. One token, usually the main verb in he sentence, forms the root of the tree and the other tokens depend on the root in a direct or indirect relation. CSI-DP is trained on the Alpino treebank [Alpino] for Dutch and uses the Alpino syntactic labels listed in appendix [app-dep]. In the plain text output of Frog ( example [ex-frog-out]) the dependency information is presented in the last two columns. The one-but-last column shows number of the token number of the head word of the dependency relation and the last column shows the grammatical relation type. We show the last two columns of the CSI-DP output in table [ex-dep]. The main verb bevestigt is root element of the sentence, the head of the subject relation (su) with the pronoun Dat and head in the object relation (obj1) with team. The noun team is the head in three relations: the determiner(det) het and the two modifiers(mod) cold case. The name Midden-Nederland is linked as an apposition to the noun politie. The prepositional phrase van is correctly assigned to the head noun team but the phrase aan is mistakenly linked to politie instead of the root verb bevestigt. Linking prepositional phrases is a hard task for parsers [atterer2007]. More details on the architecture of the CSI-DP can be found in section [sec-bg-dep]

1 Dat 2 su
2 bevestigt 0 ROOT
3 het 6 det
4 cold 5 mod
5 case 6 mod
6 team 2 obj1
7 van 6 mod
8 de 9 det
9 politie 7 obj1
10 Midden-Nederland 9 app
11 aan 9 mod
12 de 13 det
13 Telegraaf 11 obj1
14 . 13 punct

Table: [ex-dep] The dependency parser labels each token with a dependency relation to its head token and assigns the grammatical relation.

[12]In the current Frog version UTF-16 is not accepted as input in Frog.
[13]In fact the tokenizer still is used, but in PassThru mode. This allows for conversion to FoLiA XML and sentence detection.