How do you clean text data for NLP?

Cleaning text data for NLP involves preparing raw text for analysis by removing noise and standardizing formats. The process typically starts with basic normalization. Convert all text to lowercase to ensure consistency, as “Apple” and “apple” would otherwise be treated as distinct tokens. Remove HTML tags, URLs, and special characters using regular expressions—for example, re.sub(r'<.*?>', '', text) strips HTML. Trim extra whitespace and handle punctuation: either remove it (e.g., commas, quotes) or replace it with spaces, depending on the task. For instance, in sentiment analysis, exclamation marks might be meaningful, but they could be irrelevant in topic modeling.

Next, break the text into manageable units through tokenization. Split sentences into words or subwords using tools like NLTK’s word_tokenize() or spaCy’s language models. Remove stopwords (common words like “the” or “and”) if they add little value to your task, but be cautious—some contexts require them (e.g., “not” in sentiment analysis). Apply stemming or lemmatization to reduce words to their root forms. For example, “running” becomes “run” via lemmatization, while stemming might truncate it to “runn.” Libraries like NLTK’s PorterStemmer or spaCy’s lemmatization features automate this. Address spelling errors or slang using predefined dictionaries or tools like textblob for corrections, though this can be error-prone and may require domain-specific tuning.

Finally, handle advanced issues like numeric data, contractions, and domain-specific noise. Replace numbers with placeholders (e.g., “123” becomes <NUM>) or remove them if they’re irrelevant. Expand contractions like “don’t” to “do not” using libraries like contractions. For social media or informal text, normalize emojis (e.g., convert “😊” to “happy_face”) and hashtags (split “#NLPExample” into “nlp example”). Use custom rules for domain-specific terms—for example, replacing medical abbreviations with full terms. Validate your pipeline by testing intermediate outputs and adjusting steps based on the task. For instance, a chatbot might prioritize preserving emojis, while a legal document analyzer might focus on retaining precise punctuation. Iterate and refine based on how the cleaned data performs in your NLP model.