lex & yacc, 2nd Edition

Chapter 6. A Reference for Lex Specifications

In this chapter, we discuss the format of the lex specification and describe the features and options available. This chapter summarizes the capabilities demonstrated in previous chapters and covers features that have not been discussed.

After the section on the structure of a lex program, the sections in this chapter are in alphabetical order by feature.

Structure of a Lex Specification

A lex program consists of three parts: the definition section, the rules section, and the user subroutines.

    ...definition section ...
    %%
            
    ... rules section ...
    %%
    ... user subroutines ...

The parts are separated by lines consisting of two percent signs. The first two parts are required, although a part may be empty. The third part and the preceding %% line may be omitted. (This structure is the same as that used by yacc, from which it was copied.)

Definition Section

The definition section can include the literal block, definitions, internal table declarations, start conditions, and translations. (There is a section on each in this reference.) Lines that start with whitespace are copied verbatim to the C file. Typically this is used to include comments enclosed in “/*” and “*/”, preceded by whitespace.

Rules Section

The rules section contains pattern lines and C code. A line that starts with whitespace, or material enclosed in “%{” and “%}” is C code. A line that starts with anything else is a pattern line.

C code lines are copied verbatim to the generated C file. ...