Access the NEW Basecamp Support Portal

Lexical analysis

« Back to Glossary Index

Lexical Analysis and Components
– Rule-based programs perform lexical tokenization.
– Lexical analysis involves the use of lexers and parsers in compilers.
– Lexing consists of scanning and evaluating stages.
– Lexers can be generated by a lexer generator or written by hand.
– The scanner is the first stage of lexical analysis and is based on a finite-state machine.
– Lexical tokenization converts raw text into meaningful lexical tokens.
– Lexical tokenization is a sub-task of parsing input.
– Lexical token consists of a token name and an optional token value.
– Examples of common tokens include identifier, keyword, separator/punctuator, operator, and literal.
– The lexical grammar defines the lexical syntax of a programming language.
– Lexical syntax is usually a regular language defined using regular expressions.
– The lexer handles different types of tokens, and lexemes are sequences of characters within tokens.

Lexical Analysis and Lexeme Disambiguation
– The concept of lexeme in rule-based natural language processing is different from linguistics.
– Lexeme in rule-based natural language processing is similar to a word in linguistics.
– Lexeme can be similar to a morpheme in rule-based natural language processing.
– Lexeme in rule-based natural language processing is equal to the linguistic equivalent in analytic languages.
– Lexeme in rule-based natural language processing is not equal to the linguistic equivalent in highly synthetic languages.

Challenges and Techniques in Lexical Analysis
– Tokenization often occurs at the word level, but defining what constitutes a word can be challenging.
– Simple heuristics are often used in tokenization, such as including punctuation and whitespace in tokens.
– Edge cases like contractions, hyphenated words, and URIs can complicate tokenization.
– Languages without word boundaries or with agglutinative structures pose additional challenges.
– Addressing difficult tokenization problems may require complex heuristics, special-case tables, or language models.
– Lexer generators like lex and flex offer fast development and advanced features for generating lexers.
– Hand-written lexers may be used, but modern lexer generators often produce faster lexers.

Advanced Concepts in Lexical Analysis
– Lexical analysis primarily segments the input stream into tokens, but lexers may omit or insert tokens.
– Line continuation is a feature in some languages where a newline is a statement terminator.
– Semicolon insertion is a feature that automatically inserts semicolons in certain contexts.
– Semicolon insertion is mainly done at the lexer level and is a feature of BCPL, Go, and JavaScript.
– The off-side rule can be implemented in the lexer, as seen in Python, where indenting affects token emission.
– Context-sensitive lexing is required in some cases, such as semicolon insertion in Go or concatenation of string literals in Python.

Additional Resources and References
– Lexicalization and lexical semantics are related concepts to lexical analysis.
– There is a list of parser generators that can be referenced.
– The off-side rule is further explained in the Off-side rule topic.
– Additional resources on lexical analysis include books like ‘Anatomy of a Compiler and The Tokenizer’ and ‘Structure and Interpretation of Computer Programs’.
– References such as ‘Compilers Principles, Techniques, & Tools’ and ‘RE2C: A more versatile scanner generator’ provide further information on tokens and lexemes.

Lexical analysis (Wikipedia)

Lexical tokenization is conversion of a text into (semantically or syntactically) meaningful lexical tokens belonging to categories defined by a "lexer" program. In case of a natural language, those categories include nouns, verbs, adjectives, punctuations etc. In case of a programming language, the categories include identifiers, operators, grouping symbols and data types. Lexical tokenization is not the same process as the probabilistic tokenization, used for a large language model's data preprocessing, that encodes text into numerical tokens, using byte pair encoding.

« Back to Glossary Index

Request an article

Please let us know what you were looking for and our team will not only create the article but we'll also email you to let you know as soon as it's been published.
Most articles take 1-2 business days to research, write, and publish.
Content/Article Request Form

Submit your RFP

We can't wait to read about your project. Use the form below to submit your RFP!
Request for Proposal

Contact and Business Information

Provide details about how we can contact you and your business.

Quote Request Details

Provide some information about why you'd like a quote.