The primary function of lexical analysis is to bridge the gap between the raw, unstructured source code and the more structured input required by the subsequent parsing phase. Imagine source code as a continuous stream of characters. The lexical analyzer acts like a meticulous reader, scanning this stream from left to right, character by character, and grouping them into meaningful units called "tokens." Consider the analogy of language: before you can understand the grammar and meaning of a sentence, you first need to identify individual words, punctuation, and other symbols. Lexical analysis performs this "word recognition" for the compiler.

While it might seem simpler to combine lexical analysis with parsing, separating these two phases offers several significant advantages:
1. Simplicity of Design:
- Lexical Simplicity: Regular expressions are powerful enough to describe token patterns. Using them keeps the lexical analyzer relatively simple and efficient. It doesn't need to understand complex grammatical structures.
- Parser Simplicity: The parser can then deal with a clean stream of tokens, rather than individual characters. This significantly simplifies the parser's design, as it can focus solely on the grammatical structure of the language.
2. Efficiency:
- Optimized Scanners: Lexical analyzers can be highly optimized for speed. They often use techniques like finite automata, which are very efficient for pattern matching.
- Reduced Lookahead: Parsers often require "lookahead" (peeking ahead at future tokens) to make parsing decisions. By providing tokens, the lexical analyzer reduces the amount of lookahead the parser needs, simplifying its task.
3. Portability and Flexibility:
- Language-Specific Tokenization: The lexical rules for a language can be defined independently of its grammatical rules. This makes it easier to modify or adapt a compiler for different dialects or new versions of a language without overhauling the entire compiler.
4. Error Handling Improvement:
- Lexical errors (e.g., illegal characters, malformed numbers) can be detected and reported early in the compilation process, often before the parser even begins its work. This leads to clearer and more precise error messages for the programmer.

The lexical analyzer has specific responsibilities such as:
- Reads Input Stream: Continuously consumes characters from the source code file.
- Identifies Lexemes: Groups sequences of characters that form valid patterns for tokens.
- Generates Tokens: For each recognized lexeme, it constructs a token (token name, optional attribute value) and passes it to the next phase (parser).
- Removes Non-Essential Information: Discards whitespace (spaces, tabs, newlines) and comments, as these elements are crucial for human readability but do not contribute to the program's executable logic. This significantly cleans up the input for the parser.
- Handles Simple Error Detection: Detects and reports errors like illegal characters that don't belong to any valid token pattern. More complex errors are typically left for the parser or semantic analyzer.

When identifiers (variable names, function names) are encountered, the lexical analyzer often interacts with the symbol table. It checks if the identifier already exists; if not, it adds a new entry. The attribute value for an identifier token usually points to its entry in the symbol table, where information like its type, scope, etc., will be stored later by other compiler phases.

In some compilers, the lexical analyzer might also handle macro expansion (though this is sometimes done by a preprocessor before lexical analysis).