To keep things simple, we use regular expressions to define the grammar of the language. A lexer is generally combined with a parser, which analyzes the syntax of the code, and sometimes its semantics. not defined in the regular grammar of that programming language), it will generate an error. If the lexer identifies that a token is invalid (i.e.
#Finite state automata lexer code
However, in languages such as Python, where indentations are really important, there are special rules which are implemented in the lexer such as using INDENT and DEDENT tokens to determine which block does the code belong to. Omitting tokens such as whitespaces, comments and new lines is really common in the process of lexing. Observe that whitespaces are ignored during tokenization. So after tokenization, the expression can be represented by the following table: Consider this expression in the Scheme/Racket programming language:įor example, lexemes such as 3, 8, and 9, will be classified as the NUM token by the lexer. Lexers attach meaning (semantics) to these sequence of characters by classifying lexemes (strings of symbols from the input) into various types, and entries of this mappings are known as tokens. It is a process of converting a sequence of characters into a sequence of tokens by a program known as a lexer. Lexical analysis, often known as tokenizing, is the first phase of a compiler. The diagram below shows the main phases taken for the front-end of the compiling process. After days of readings, it seems that the first thing to do is to build a query parser which takes in SQL text and produces a parse tree which can be sent to the query optimizer or processor.Ĭonceptually, an interpreter or a compiler operates in phases, each of which transforms the code from one representation to another. I was trying to learn how a database engine is actually implemented, and I feel that the best way to do so is to try to build one myself. Lexical Analysis with ANTLR v4 - February 16, 2017
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