The scanner groups the input stream (of characters) into a stream of tokens (lexeme) and constructs a symbol table which is used later for contextual analysis. The lexemes include

· Key words,

· identifiers,

· operators,

· constants: numeric, character, special, and

· comments.

The lexical phase (scanner) groups characters into lexical units or tokens. The input to the lexical phase is a character stream. The output is a stream of tokens. Regular expressions are used to define the tokens recognized by a scanner (or lexical analyzer). The scanner is implemented as a finite state machine.

Lex and Flex are tools for generating scanners is C. Flex is a faster version of Lex.