Tokenization – Different types of tokenizers and why it is used?

Tokenization, a multifaceted concept, serves as the cornerstone for processing and understanding textual data across various domains. While its applications extend to data security, our focus here is on its intricate role within the realms of Natural Language Processing (NLP) and Machine Learning (ML). Tokenization, when applied to data security, involves substituting a sensitive data element with a non-sensitive equivalent known as a token. However, in this exploration, our emphasis lies on the transformative impact of tokenization in NLP and ML tasks.

In the landscape of NLP and ML, tokenization is the initial and fundamental step in harnessing the power of language data. It involves breaking down raw, unstructured text into smaller units, referred to as tokens. Word tokenization is a conventional method, where sentences are segmented into individual words. This approach is not only foundational but also crucial for numerous NLP applications, contributing to the semantic understanding of language.

Reference – Embeddings


Different types of tokens based on breakdown of text.

1. Word Tokenization:

Word tokenization is the process of breaking down text into individual words. In this method, sentences are segmented, and each word becomes a distinct token. For instance, the sentence “Natural language processing is fascinating” would be tokenized into [“Natural”, “language”, “processing”, “is”, “fascinating”]. Word tokenization is foundational in NLP, forming the basis for various language processing tasks and analyses.

2. Character Tokenization:

Character tokenization breaks down text into individual characters. Each character, including letters and punctuation, becomes a separate token. This approach is beneficial when analysis needs to be performed at the granular level of individual characters, such as in text generation or handwriting recognition.

Input: “Embeddings”

Output: [“E”, “m”, “b”, “e”, “d”, “d”, “i”, “n”, “g”, “s”]

3. Sentence Tokenizer:

Sentence tokenization involves breaking down a paragraph or text into individual sentences. This tokenizer identifies sentence boundaries based on punctuation marks such as periods, exclamation points, and question marks.

Example: Input: “NLP is fascinating. It involves processing textual data!” Output: [“NLP is fascinating.”, “It involves processing textual data!”]

4. Punctuation Tokenizer:

Punctuation tokenizer focuses on isolating punctuation marks, separating them from the surrounding words. This tokenizer is useful for tasks where punctuation itself holds significance.

Example: Input: “Hello, world!” Output: [“Hello”, “,”, “world”, “!”]

5. Whitespace Tokenizer:

Whitespace tokenizer segments text based on spaces, tabs, or newlines, treating each space-separated segment as a distinct token. Simple yet effective for many tasks, especially when words are clearly separated by whitespace characters.

Example: Input: “Tokenization is crucial for NLP.” Output: [“Tokenization”, “is”, “crucial”, “for”, “NLP.”]

Despite the effectiveness of word, subword, and character tokenizations in addressing various linguistic challenges, the evolving landscape of natural language processing demands further advancements. These traditional methods encounter complexities with ambiguous boundaries, out-of-vocabulary terms, and languages rich in emojis and symbols. More advanced tokenization techniques, such as contextual embeddings and transformer-based models, are imperative to capture nuanced context and intricate linguistic structures.

The insufficiency lies in the need for adaptability to diverse languages, improved handling of domain-specific jargon, and enhanced contextual understanding for sophisticated NLP and ML tasks. Continuous advancements aim to overcome these limitations, ensuring tokenization methods evolve in tandem with the intricacies of language and the requirements of modern language processing systems.


Subwords are linguistic units smaller than complete words, often derived by breaking down words into smaller components, such as prefixes, suffixes, or character sequences. They are useful for handling morphologically rich languages and addressing out-of-vocabulary terms. subword tokenization methods such as BPE, WordPiece, and SentencePiece play a crucial role in breaking down words into smaller units, enhancing the adaptability and performance of language models across various linguistic challenges.

1. Byte Pair Encoding (BPE):

BPE is a subword tokenization algorithm that iteratively merges the most frequent pairs of characters. It adapts to the data, forming a vocabulary of subword units. BPE begins by initializing its vocabulary with individual characters. The algorithm iteratively identifies and merges the most frequent pairs of adjacent characters in the existing vocabulary, creating new subword units. This process continues for a predetermined number of iterations or until a convergence criterion is met.

For example, consider the words “apple,” “banana,” “apples,” and “bananas.” In the first iteration, BPE might merge the most frequent pair ‘ap’ into a new symbol, say ‘X’, resulting in “Xple,” “banana,” “Xles,” and “bananas.” In subsequent iterations, it could merge ‘Xp’ into ‘Y’, yielding “Yle,” “bYna,” “Yles,” and “bYnas.” This process continues, creating subword units that effectively represent the linguistic variations present in the data.

BPE merges the most frequent pairs of individual charaters, forming subword units.Adapts tp linguistic structures, creating subword units that may not align with word boundaries.

Reference – Byte pair Encoding

2. WordPiece:

WordPiece is a subword tokenization algorithm that, like BPE, starts with a vocabulary of individual characters. It iteratively merges the most frequent pairs of characters, adapting to the data and forming a vocabulary of subword units. This process allows WordPiece to capture linguistic variations and handle rare or out-of-vocabulary terms effectively.

For example, let’s consider the words “unbelievable” and “incredible.” In the first iteration, WordPiece might merge the most frequent pair ‘le’ into a new symbol, say ‘X’, resulting in “unbeXievable” and “incredXible.” Subsequent iterations could merge ‘Xie’ into ‘Y’, yielding “unbeYvable” and “incredYble.” The algorithm continues, creating subword units like ‘Yvable’ that effectively represent the linguistic nuances in the data.

WordPiece’s adaptability makes it suitable for various natural language processing tasks. It allows the model to represent words as combinations of subword units, offering a flexible and data-driven approach to tokenization. WordPiece merges frequent pairs of individual characters but often emphasizes preserving complete words during merging. Tends to produce subword units that align with complete words.

3. SentencePiece:

SentencePiece is a versatile subword tokenization algorithm that can operate at either the character or word level. Using unsupervised learning and a unigram language model, SentencePiece determines probable subword units based on the training data. In a sentence like “language models are powerful,” SentencePiece could generate subword units like ‘lang,’ ‘uage,’ ‘mod,’ ‘els,’ offering a flexible vocabulary that effectively represents the language’s intricacies. SentencePiece’s adaptability makes it suitable for various linguistic structures in NLP tasks, providing a comprehensive representation of text.

Byte Pair Encoding (BPE) and WordPiece tokenization techniques are used in the training process of BERT (Bidirectional Encoder Representations from Transformers), which is a popular pre-trained language model. SentencePiece, on the other hand, is a separate tokenization method and is not used in BERT by default.

Tokenization Tools & Library

Various tools and libraries contribute to this process, each offering unique features and suitability for specific applications. Here, we delve into some prominent tokenization tools and their distinctive characteristics:

1. NLTK (Natural Language Toolkit)

NLTK stands as a versatile Python library for NLP tasks, offering diverse tokenization methods, including word and sentence tokenization.NLTK is widely known for its versatility, making it an excellent choice for educational purposes, quick prototyping, and a broad range of NLP applications.

2. spaCy

spaCy, an efficient NLP library, provides tokenization along with part-of-speech tagging and entity recognition. It’s designed for production-level applications and large-scale text processing Renowned for its speed and accuracy, spaCy is commonly used in industry applications, especially where precision in tasks like named entity recognition is crucial.

In the spaCy Tokenizer example, the spaCy library is used to load an English language model (en_core_web_sm), and the nlp object is employed to process the text. The resulting doc object contains tokens, and these tokens are extracted for further use.

3. Stanford NLP

Stanford NLP offers a comprehensive suite of NLP tools, including a robust tokenization module. Developed by the Stanford Natural Language Processing Group, it excels in linguistic analysis.Stanford NLP is esteemed for its high-quality tokenization and is often employed in research and applications requiring in-depth linguistic insights.

4. Transformers Tokenizers (Hugging Face)

Transformers Tokenizers, part of the Hugging Face Transformers library, specialize in handling subword tokenization for transformer-based models like BERT and GPT.These tokenizers are indispensable for tasks involving state-of-the-art transformer architectures, offering flexibility for various downstream applications such as sentiment analysis and question answering.

5. Keras Tokenizer

Keras, a high-level neural networks API, includes a straightforward tokenizer for text data preprocessing, commonly used in conjunction with Keras neural network models.User-friendly and suitable for basic text tokenization tasks, Keras Tokenizer is often applied in machine learning projects utilizing Keras.

In the Keras Tokenizer example, the Tokenizer class is used to fit on the text data and convert the text into sequences of numbers. It also provides information about the word index.

Reference – Utility for Tokenizers

Understanding the strengths of each tool empowers practitioners to make informed choices based on the specific requirements of their NLP projects. Whether it’s versatility, speed, linguistic analysis, or transformer compatibility, these tools cater to a diverse array of needs in the ever-evolving field of natural language processing.

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