Flex - a scanner generator - 1.5 Format of the input file

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## 1.5 Format of the input file

The `flex` input file consists of three sections, separated by a line with just ``%%'` in it:

    definitions
    %%
    rules
    %%
    user code

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-white-space 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.

The rules section of the `flex` input contains a series of rules of the form:

    pattern   action

where the pattern must be unindented and the action must begin on the same line.

See below for a further description of patterns and actions.

Finally, the user code section is simply copied to ``lex.yy.c'` verbatim. It is used for companion routines which call or are called by the scanner. The presence of this section is optional; if it is missing, the second ``%%'` in the input file may be skipped, too.

In the definitions and rules sections, any indented text or text enclosed in ``%{'` and ``%}'` is copied verbatim to the output (with the ``%{}'`'s removed). The ``%{}'`'s must appear unindented on lines by themselves.

In the rules section, any indented or %{} text appearing before the first rule may be used to declare variables which are local to the scanning routine and (after the declarations) code which is to be executed whenever the scanning routine is entered. Other indented or %{} text in the rule section is still copied to the output, but its meaning is not well-defined and it may well cause compile-time errors (this feature is present for `POSIX` compliance; see below for other such features).

In the definitions section (but not in the rules section), an unindented comment (i.e., a line beginning with "/*") is also copied verbatim to the output up to the next "*/".

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