NLP Fundamentals & Text Processing
Understand the NLP pipeline from raw text to machine-ready features — tokenization, stemming, embeddings, and the evolution from rule-based to deep learning approaches.
The NLP Pipeline
Natural Language Processing (NLP) is the branch of AI that enables machines to understand, interpret, and generate human language. Modern NLP powers search engines, virtual assistants, machine translation, chatbots, and content generation. The standard NLP pipeline processes text through several stages: Text Collection (gathering raw data), Preprocessing (cleaning, normalizing), Tokenization (splitting into words/subwords), Feature Extraction (converting text to numbers), Modeling (applying ML/DL algorithms), and Post-processing (formatting output). The field has undergone three major paradigm shifts: Rule-based systems (1950s-1990s) using handcrafted grammar rules, Statistical methods (1990s-2013) using probabilistic models like n-grams and HMMs, and Deep Learning (2013-present) using neural networks that learn representations directly from data. The transformer architecture (2017) revolutionized NLP by enabling models to process entire sequences in parallel with attention mechanisms.
Text Preprocessing Techniques
- Tokenization: Splitting text into tokens — word-level ('I love NLP' → ['I', 'love', 'NLP']), subword-level (BPE, WordPiece — handles rare words), character-level
- Lowercasing: Converting all text to lowercase — simple but effective for reducing vocabulary size. Exception: named entities (Apple vs apple)
- Stop Word Removal: Removing common words (the, is, and) that add little meaning — useful for bag-of-words models, less useful for deep learning
- Stemming: Reducing words to root form — 'running', 'runs', 'ran' → 'run'. Porter Stemmer is most common. Fast but sometimes inaccurate
- Lemmatization: Dictionary-based root form — 'better' → 'good', 'mice' → 'mouse'. More accurate than stemming but slower. Uses WordNet
- Text Normalization: Expanding contractions (don't → do not), handling numbers, removing special characters, fixing encoding issues
- Sentence Segmentation: Splitting text into sentences — non-trivial due to abbreviations (Dr., U.S.A.), decimal numbers, and URLs
Text Representation Methods
- Bag of Words (BoW): Counts word occurrences — simple, interpretable, but loses word order and creates sparse, high-dimensional vectors
- TF-IDF: Term Frequency × Inverse Document Frequency — weights words by importance within a document corpus. Better than raw counts for information retrieval
- Word2Vec (2013): Dense word vectors trained to predict surrounding words (Skip-gram) or be predicted from context (CBOW). king - man + woman ≈ queen
- GloVe (2014): Global Vectors — combines word co-occurrence statistics with neural training. Often better than Word2Vec for many tasks
- FastText (2016): Extends Word2Vec with subword information — can handle out-of-vocabulary words by composing character n-grams
- Contextual Embeddings (2018+): ELMo, BERT, GPT — word meaning changes based on context. 'bank' gets different embeddings in 'river bank' vs 'savings bank'