How the input is matched

BASIC PRINCIPLES.

(1)

When the generated scanner is run, it analyses its input string looking for strings which match any of its patterns.

(2)

If the current input can be matched by several expressions, then the ambiguity is resolved by the following rules.

(2.1)

The longest match is preferred.

(2.2)

The rule given first is preferred.

(3)

Once the match is determined,

(3.1)

the text corresponding to it is available in the global character pointer yytext its length is yyleng and the current line number is yylineno,

(3.2)

and the action corresponding to the matched pattern is then executed,

(3.3)

and then the remaining input is scanned for another match.

LEX ROUTINES AND MACROS. There are a number of special directives which can be included within an action.

ECHO

copies yytext to the scanner's output.

REJECT

directs the scanner to proceed on to the "second best" rule which matched the input (or a prefix of the input). For example, the following will both count the words in the input and call the routine special() whenever "foo" is seen:

int word_count = 0;
%%
foo     special(); REJECT;
[^ \t\n]+   ++word_count;

Multiple REJECT's are allowed, each one finding the next best choice to the currently active rule.

yymore()

tells the scanner that the next time it matches a rule, the corresponding token should be appended onto the current value of yytext rather than replacing it. The following

%%
very[ ] {ECHO; yymore();}
good ECHO;

will write "very very good" to the output.

See also yyless(), yyterminate(), input(), output(), unput() in the documentation.

START CONDITIONS (or start conditions) are used to limit the scope of certain rules or to change the way the lexer treats part of the file.

• Start conditions are declared in the definition section. For instance

  %s SC
  

  creates a start condition called SC.
• In the transition rules section, a rule of the form

  <SC>[aeiouy]* ECHO;
  

  will only apply in the state SC.
• A state is changed with a statement

  BEGIN SC;
  

  in an action.
• LEX starts in state zero (0), also called initial state.

  BEGIN 0;
  

  will change back to this initial state.
• Rules that do not have start states can be applied in any state.