5.1 Format of the Definitions Section

The definitions section contains declarations of simple name definitions to simplify the scanner specification, and declarations of start conditions, which are explained in a later section.

Name definitions have the form:

    name definition

The ‘name’ is a word beginning with a letter or an underscore (‘_’) followed by zero or more letters, digits, ‘_’, or ‘-’ (dash). The definition is taken to begin at the first non-whitespace character following the name and continuing to the end of the line. The definition can subsequently be referred to using ‘{name}’, which will expand to ‘(definition)’. For example,

    DIGIT    [0-9]
    ID       [a-z][a-z0-9]*

Defines ‘DIGIT’ to be a regular expression which matches a single digit, and ‘ID’ to be a regular expression which matches a letter followed by zero-or-more letters-or-digits. A subsequent reference to

    {DIGIT}+"."{DIGIT}*

is identical to

    ([0-9])+"."([0-9])*

and matches one-or-more digits followed by a ‘.’ followed by zero-or-more digits.

An unindented comment (i.e., a line beginning with ‘/*’) is copied verbatim to the output up to the next ‘*/’.

Any indented text or text enclosed in ‘%{’ and ‘%}’ is also copied verbatim to the output (with the %{ and %} symbols removed). The %{ and %} symbols must appear unindented on lines by themselves.

A %top block is similar to a ‘%{’ ... ‘%}’ block, except that the code in a %top block is relocated to the top of the generated file, before any flex definitions ¹. The %top block is useful when you want certain preprocessor macros to be defined or certain files to be included before the generated code. The single characters, ‘{’ and ‘}’ are used to delimit the %top block, as show in the example below:

    %top{
        /* This code goes at the "top" of the generated file. */
        #include <stdint.h>
        #include <inttypes.h>
    }

Multiple %top blocks are allowed, and their order is preserved.