LEX, also known as Flex (Fast Lexical Analyzer), is not a traditional compiler in the sense that it doesn't translate a complete program written in one language to another language. Instead, LEX is a tool used to generate lexical analyzers, which are the first stage of a compiler.
What is a lexical analyzer?
A lexical analyzer, also known as a scanner or tokenizer, breaks down the source code into meaningful units called tokens. These tokens can be keywords (like "if", "else", "for"), identifiers (variable names), operators (+, -, *, /), constants (numbers, strings), or punctuation symbols.
The lexical analyzer essentially reads the source code character by character and groups these characters into tokens based on a set of rules defined using regular expressions.
How does LEX work?
LEX takes a file containing these rules (usually named .l or .ll) as input and generates a C program file (usually named lex.yy.c) as output. This C program, when compiled, creates the actual lexical analyzer that can be used by the compiler to tokenize the source code.
Here's a simplified overview of the process:
- Write LEX rules: You define patterns (using regular expressions) to match different types of tokens in the
.lor.llfile. - Compile with LEX: You run the LEX program on the
.lor.llfile, which generates the C codelex.yy.c. - Compile with C compiler: You compile the
lex.yy.cfile using a C compiler to create the executable lexical analyzer. - Use the lexical analyzer: The generated lexical analyzer can then be used by the compiler to tokenize the source code.
LEX is often used in conjunction with another tool called YACC (Yet Another Compiler Compiler), which generates parsers. The parser takes the stream of tokens generated by the lexical analyzer and verifies the syntax of the code based on the grammar of the programming language.
Benefits of using LEX:
- Simplifies lexical analysis: LEX provides a high-level and easy-to-use way to define lexical analysis rules.
- Portable: LEX-generated code is portable across different platforms.
- Efficient: LEX generates efficient lexical analyzers.