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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 section Patterns, 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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