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What is Tokenization?

Tokenization breaks text into smaller parts for easier machine analysis, helping machines understand human language.
Sep 2023  · 9 min read

Tokenization, in the realm of Natural Language Processing (NLP) and machine learning, refers to the process of converting a sequence of text into smaller parts, known as tokens. These tokens can be as small as characters or as long as words. The primary reason this process matters is that it helps machines understand human language by breaking it down into bite-sized pieces, which are easier to analyze.

Tokenization Explained

Imagine you're trying to teach a child to read. Instead of diving straight into complex paragraphs, you'd start by introducing them to individual letters, then syllables, and finally, whole words. In a similar vein, tokenization breaks down vast stretches of text into more digestible and understandable units for machines.

The primary goal of tokenization is to represent text in a manner that's meaningful for machines without losing its context. By converting text into tokens, algorithms can more easily identify patterns. This pattern recognition is crucial because it makes it possible for machines to understand and respond to human input. For instance, when a machine encounters the word "running", it doesn't see it as a singular entity but rather as a combination of tokens that it can analyze and derive meaning from.

To delve deeper into the mechanics, consider the sentence, "Chatbots are helpful." When we tokenize this sentence by words, it transforms into an array of individual words:

["Chatbots", "are", "helpful"].

This is a straightforward approach where spaces typically dictate the boundaries of tokens. However, if we were to tokenize by characters, the sentence would fragment into:

["C", "h", "a", "t", "b", "o", "t", "s", " ", "a", "r", "e", " ", "h", "e", "l", "p", "f", "u", "l"].

This character-level breakdown is more granular and can be especially useful for certain languages or specific NLP tasks.

In essence, tokenization is akin to dissecting a sentence to understand its anatomy. Just as doctors study individual cells to understand an organ, NLP practitioners use tokenization to dissect and understand the structure and meaning of text.

It's worth noting that while our discussion centers on tokenization in the context of language processing, the term "tokenization" is also used in the realms of security and privacy, particularly in data protection practices like credit card tokenization. In such scenarios, sensitive data elements are replaced with non-sensitive equivalents, called tokens. This distinction is crucial to prevent any confusion between the two contexts.

Types of Tokenization

Tokenization methods vary based on the granularity of the text breakdown and the specific requirements of the task at hand. These methods can range from dissecting text into individual words to breaking them down into characters or even smaller units. Here's a closer look at the different types:

  • Word tokenization. This method breaks text down into individual words. It's the most common approach and is particularly effective for languages with clear word boundaries like English.
  • Character tokenization. Here, the text is segmented into individual characters. This method is beneficial for languages that lack clear word boundaries or for tasks that require a granular analysis, such as spelling correction.
  • Subword tokenization. Striking a balance between word and character tokenization, this method breaks text into units that might be larger than a single character but smaller than a full word. For instance, "Chatbots" could be tokenized into "Chat" and "bots". This approach is especially useful for languages that form meaning by combining smaller units or when dealing with out-of-vocabulary words in NLP tasks.

Tokenization Use Cases

Tokenization serves as the backbone for a myriad of applications in the digital realm, enabling machines to process and understand vast amounts of text data. By breaking down text into manageable chunks, tokenization facilitates more efficient and accurate data analysis. Here are some prominent use cases where tokenization plays a pivotal role:

  • Search engines. When you type a query into a search engine like Google, it employs tokenization to dissect your input. This breakdown helps the engine sift through billions of documents to present you with the most relevant results.
  • Machine translation. Tools such as Google Translate utilize tokenization to segment sentences in the source language. Once tokenized, these segments can be translated and then reconstructed in the target language, ensuring the translation retains the original context.
  • Speech recognition. Voice-activated assistants like Siri or Alexa rely heavily on tokenization. When you pose a question or command, your spoken words are first converted into text. This text is then tokenized, allowing the system to process and act upon your request.

Tokenization Challenges

Navigating the intricacies of human language, with its nuances and ambiguities, presents a set of unique challenges for tokenization. Here's a deeper dive into some of these obstacles:

  • Ambiguity. Language is inherently ambiguous. Consider the sentence "Flying planes can be dangerous." Depending on how it's tokenized and interpreted, it could mean that the act of piloting planes is risky or that planes in flight pose a danger. Such ambiguities can lead to vastly different interpretations.
  • Languages without clear boundaries. Some languages, like Chinese or Japanese, don't have clear spaces between words, making tokenization a more complex task. Determining where one word ends and another begins can be a significant challenge in such languages.
  • Handling special characters. Texts often contain more than just words. Email addresses, URLs, or special symbols can be tricky to tokenize. For instance, should "[email protected]" be treated as a single token or split at the period or the "@" symbol?

Advanced tokenization methods, such as context-aware tokenizers like the BERT tokenizer, have been developed to handle such ambiguities. For languages without clear word boundaries, character or subword tokenization can offer a more effective approach. Additionally, predefined rules and regular expressions can assist in handling special characters and complex strings.

Implementing Tokenization

The landscape of Natural Language Processing offers a plethora of tools, each tailored to specific needs and complexities. Here's a guide to some of the most prominent tools and methodologies available for tokenization:

  • NLTK (Natural Language Toolkit). A stalwart in the NLP community, NLTK is a comprehensive Python library that caters to a wide range of linguistic needs. It offers both word and sentence tokenization functionalities, making it a versatile choice for beginners and seasoned practitioners alike.
  • Spacy. A modern and efficient alternative to NLTK, Spacy is another Python-based NLP library. It boasts speed and supports multiple languages, making it a favorite for large-scale applications.
  • BERT tokenizer. Emerging from the BERT pre-trained model, this tokenizer excels in context-aware tokenization. It's adept at handling the nuances and ambiguities of language, making it a top choice for advanced NLP projects (see this tutorial on NLP with BERT).
  • Advanced techniques.
    • Byte-Pair Encoding (BPE). An adaptive tokenization method, BPE tokenizes based on the most frequent byte pairs in a text. It's particularly effective for languages that form meaning by combining smaller units.
    • SentencePiece. An unsupervised text tokenizer and detokenizer mainly for Neural Network-based text generation tasks. It handles multiple languages with a single model and can tokenize text into subwords, making it versatile for various NLP tasks.

Your choice of tool should align with the specific requirements of your project. For those taking their initial steps in NLP, NLTK or Spacy might offer a more approachable learning curve. However, for projects demanding a deeper understanding of context and nuance, the BERT tokenizer stands out as a robust option.

How I Used Tokenization for a Rating Classifier Project

I gained my initial experience with text tokenization while working on a portfolio project three years ago. The project involved a dataset containing user reviews and ratings, which I used to develop a deep-learning text classification model. I used `word_tokenize` from NLTK to clean up the text and `Tokenizer` from Keras to preprocess it.

Let's explore how I used tokenizers in the project:

  1. When working with NLP data, tokenizers are commonly used to process and clean the text dataset. The aim is to eliminate stop words, punctuation, and other irrelevant information from the text. Tokenizers transform the text into a list of words, which can be cleaned using a text-cleaning function.
  2. Afterward, I used the Keras Tokenizer method to transform the text into an array for analysis and to prepare the tokens for the deep learning model. In this case, I used the Bidirectional LSTM model, which produced the most favorable outcomes.
  3. Next, I converted tokens into a sequence by using the `texts_to_sequences` function.
  4. Before feeding the sequence to the model, I had to add padding to make the sequence of numbers the same length.
  5. Finally, I split the dataset into training and testing sets, trained the model on the training set, and evaluated it on the testing set.

Tokenizer has many benefits in the field of natural language processing where it is used to clean, process, and analyze text data. Focusing on text processing can improve model performance.

I recommend taking the Introduction to Natural Language Processing in Python course to learn more about the preprocessing techniques and dive deep into the world of tokenizers.

Want to learn more about AI and machine learning? Check out these resources:


What's the difference between word and character tokenization?

Word tokenization breaks text into words, while character tokenization breaks it into characters.

Why is tokenization important in NLP?

It helps machines understand and process human language by breaking it down into manageable pieces.

Can I use multiple tokenization methods on the same text?

Yes, depending on the task at hand, combining methods might yield better results.

What are the most common tokenization tools used in NLP?

Some of the most popular tokenization tools used in NLP are NLTK, Spacy, Stanford CoreNLP, GENSIM, and TensorFlow Tokenizer. Each has its own strengths and is suited for different tasks.

How does tokenization work for languages like Chinese or Japanese that don't have spaces?

Tokenization uses techniques like character-level segmentation or finding the most probable word boundaries based on statistical models for languages without explicit word separators.

How does tokenization help search engines return relevant results?

It breaks down queries and documents into indexable units, allowing for efficient lookups and matches. This powers speed and accuracy.

Photo of Abid Ali Awan
Abid Ali Awan

I am a certified data scientist who enjoys building machine learning applications and writing blogs on data science. I am currently focusing on content creation, editing, and working with large language models.


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