Start Conditions

Start conditions provide greater sensitivity to prior context than is afforded by the ^ operator alone. You might want to apply different rules to an expression depending on a prior context that is more complex than the end of a line or the start of a file.

In this situation you could set a flag to mark the change in context that is the condition for the application of a rule, then write code to test the flag. Alternatively, you could define for lex the different ``start conditions'' under which it is to apply each rule.

Consider this problem:

• Copy the input to the output, except change the word magic to the word first on every line that begins with the letter a.

• Change magic to second on every line that begins with b.

• Change magic to third on every line that begins with c. Here is how the problem might be handled with a flag.

  Recall that ECHO is a lex macro equivalent to printf("%s", yytext):

  int flag 
  %% 
  ^a {flag = 'a'; ECHO;} 
  ^b {flag = 'b'; ECHO;} 
  ^c {flag = 'c'; ECHO;} 
  \n {flag = 0; ECHO;} 
  magic { 
  switch (flag) 
       	{ 
            		case 'a': printf("first"); break; 
            		case 'b': printf("second"); break; 
            		case 'c': printf("third"); break; 
            		default: ECHO; break; 
       	} 
  } 

To handle the same problem with start conditions, each start condition must be introduced to lex in the definitions section with a line, such as the following one, where the conditions can be named in any order:

%Start name1 name2 ...  

The word Start can be abbreviated to S or s. The conditions are referenced at the head of a rule with <> brackets. So the following is a rule that is recognized only when the scanner is in start condition name1:

<name1>expression 

To enter a start condition, execute the action following statement:

BEGIN name1; 

The above statement changes the start condition to name1. To resume the normal state, use the following:

BEGIN 0;

This resets the initial condition of the scanner.

A rule can be active in several start conditions. For example, the following is a legal prefix:

<name1,name2,name3> 

Any rule not beginning with the <> prefix operators is always active.

The example can be written with start conditions as follows:

%Start AA BB CC 
%% 
^a			{ECHO; BEGIN AA;} 
^b			{ECHO; BEGIN BB;} 
^c			{ECHO; BEGIN CC;} 
\n			{ECHO; BEGIN 0;} 
<AA>magic			printf("first"); 
<BB>magic			printf("second"); 
<CC>magic			printf("third");