What is lex and yacc

Generally, a compiler is a software program that converts the source code into machine code. It goes through several phases to perform this conversion. First, there is a lexical analysis phase. It involves reading the source program one character at a time and converting it into meaningful lexemes tokens. Next, the output of that phase goes to the syntax analysis. It takes the tokens of the lexical analysis and produces a parse tree as the output. Overall, the lexical analyzer performs the lexical analysis while syntax analyzer performs syntax analysis.

Lex is a lexical analyzer whereas Yacc is a parser. Both work together. For example, Lex takes the string input to create tokens, and Yacc uses those tokenized input. What is Lex -Definition, Functionality 2. What is Yacc — Definition, Functionality 3. Difference Between Lex and Yacc -Comparison of key differences. Find the hierarchical structure of the program Yacc. Lesk and E. Schmidt Lex helps write programs whose control flow is directed by instances of regular expressions in the input stream.

It is well suited for editor-script type transformations and for segmenting input in preparation for a parsing routine. Lex source is a table of regular expressions and corresponding program fragments.

The table is translated to a program which reads an input stream, copying it to an output stream and partitioning the input into strings which match the given expressions. As each such string is recognized the corresponding program fragment is executed. The recognition of the expressions is performed by a deterministic finite automaton generated by Lex. The program fragments written by the user are executed in the order in which the corresponding regular expressions occur in the input stream.

To install simply download and run the setup executable. Under editors install vim. Lately I've been using flex and bison under the Cygwin environment. Figure 1: Compilation Sequence. The patterns in the above diagram is a file you create with a text editor. Lex will read your patterns and generate C code for a lexical analyzer or scanner. The lexical analyzer matches strings in the input, based on your patterns, and converts the strings to tokens. Tokens are numerical representations of strings, and simplify processing.

When the lexical analyzer finds identifiers in the input stream it enters them in a symbol table. The symbol table may also contain other information such as data type integer or real and location of each variable in memory. All subsequent references to identifiers refer to the appropriate symbol table index.

The grammar in the above diagram is a text file you create with a text editor.