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10 Start Conditions
flex provides a mechanism for conditionally activating rules.
Any rule whose pattern is prefixed with ‘<sc>’ will only be active
when the scanner is in the start condition named sc. For
example,
<STRING>[^"]* { /* eat up the string body ... */
...
}
will be active only when the scanner is in the STRING start
condition, and
<INITIAL,STRING,QUOTE>\. { /* handle an escape ... */
...
}
will be active only when the current start condition is either
INITIAL, STRING, or QUOTE.
Start conditions are declared in the definitions (first) section of the
input using unindented lines beginning with either ‘%s’ or
‘%x’ followed by a list of names. The former declares
inclusive start conditions, the latter exclusive start
conditions. A start condition is activated using the BEGIN
action. Until the next BEGIN action is executed, rules with the
given start condition will be active and rules with other start
conditions will be inactive. If the start condition is inclusive, then
rules with no start conditions at all will also be active. If it is
exclusive, then only rules qualified with the start condition
will be active. A set of rules contingent on the same exclusive start
condition describe a scanner which is independent of any of the other
rules in the flex input. Because of this, exclusive start
conditions make it easy to specify “mini-scanners” which scan portions
of the input that are syntactically different from the rest (e.g.,
comments).
If the distinction between inclusive and exclusive start conditions is still a little vague, here’s a simple example illustrating the connection between the two. The set of rules:
%s example
%%
<example>foo do_something();
bar something_else();
is equivalent to
%x example
%%
<example>foo do_something();
<INITIAL,example>bar something_else();
Without the <INITIAL,example> qualifier, the bar pattern in
the second example wouldn’t be active (i.e., couldn’t match) when in
start condition example. If we just used <example> to
qualify bar, though, then it would only be active in
example and not in INITIAL, while in the first example
it’s active in both, because in the first example the example
start condition is an inclusive (%s) start condition.
Also note that the special start-condition specifier
<*>
matches every start condition. Thus, the above example could also
have been written:
%x example
%%
<example>foo do_something();
<*>bar something_else();
The default rule (to ECHO any unmatched character) remains active
in start conditions. It is equivalent to:
<*>.|\n ECHO;
BEGIN(0) returns to the original state where only the rules with
no start conditions are active. This state can also be referred to as
the start-condition INITIAL, so BEGIN(INITIAL) is
equivalent to BEGIN(0). (The parentheses around the start
condition name are not required but are considered good style.)
BEGIN actions can also be given as indented code at the beginning
of the rules section. For example, the following will cause the scanner
to enter the SPECIAL start condition whenever yylex() is
called and the global variable enter_special is true:
int enter_special;
%x SPECIAL
%%
if ( enter_special )
BEGIN(SPECIAL);
<SPECIAL>blahblahblah
...more rules follow...
To illustrate the uses of start conditions, here is a scanner which provides two different interpretations of a string like ‘123.456’. By default it will treat it as three tokens, the integer ‘123’, a dot (‘.’), and the integer ‘456’. But if the string is preceded earlier in the line by the string ‘expect-floats’ it will treat it as a single token, the floating-point number ‘123.456’:
%{
#include <math.h>
%}
%s expect
%%
expect-floats BEGIN(expect);
<expect>[0-9]+@samp{.}[0-9]+ {
printf( "found a float, = %f\n",
atof( yytext ) );
}
<expect>\n {
/* that's the end of the line, so
* we need another "expect-number"
* before we'll recognize any more
* numbers
*/
BEGIN(INITIAL);
}
[0-9]+ {
printf( "found an integer, = %d\n",
atoi( yytext ) );
}
"." printf( "found a dot\n" );
Here is a scanner which recognizes (and discards) C comments while maintaining a count of the current input line.
%x comment
%%
int line_num = 1;
"/*" BEGIN(comment);
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(INITIAL);
This scanner goes to a bit of trouble to match as much text as possible with each rule. In general, when attempting to write a high-speed scanner try to match as much possible in each rule, as it’s a big win.
Note that start-conditions names are really integer values and can be stored as such. Thus, the above could be extended in the following fashion:
%x comment foo
%%
int line_num = 1;
int comment_caller;
"/*" {
comment_caller = INITIAL;
BEGIN(comment);
}
...
<foo>"/*" {
comment_caller = foo;
BEGIN(comment);
}
<comment>[^*\n]* /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]* /* eat up '*'s not followed by '/'s */
<comment>\n ++line_num;
<comment>"*"+"/" BEGIN(comment_caller);
Furthermore, you can access the current start condition using the
integer-valued YY_START macro. For example, the above
assignments to comment_caller could instead be written
comment_caller = YY_START;
Flex provides YYSTATE as an alias for YY_START (since that
is what’s used by AT&T lex).
For historical reasons, start conditions do not have their own name-space within the generated scanner. The start condition names are unmodified in the generated scanner and generated header. See option-header. See option-prefix.
Finally, here’s an example of how to match C-style quoted strings using exclusive start conditions, including expanded escape sequences (but not including checking for a string that’s too long):
%x str
%%
char string_buf[MAX_STR_CONST];
char *string_buf_ptr;
\" string_buf_ptr = string_buf; BEGIN(str);
<str>\" { /* saw closing quote - all done */
BEGIN(INITIAL);
*string_buf_ptr = '\0';
/* return string constant token type and
* value to parser
*/
}
<str>\n {
/* error - unterminated string constant */
/* generate error message */
}
<str>\\[0-7]{1,3} {
/* octal escape sequence */
int result;
(void) sscanf( yytext + 1, "%o", &result );
if ( result > 0xff )
/* error, constant is out-of-bounds */
*string_buf_ptr++ = result;
}
<str>\\[0-9]+ {
/* generate error - bad escape sequence; something
* like '\48' or '\0777777'
*/
}
<str>\\n *string_buf_ptr++ = '\n';
<str>\\t *string_buf_ptr++ = '\t';
<str>\\r *string_buf_ptr++ = '\r';
<str>\\b *string_buf_ptr++ = '\b';
<str>\\f *string_buf_ptr++ = '\f';
<str>\\(.|\n) *string_buf_ptr++ = yytext[1];
<str>[^\\\n\"]+ {
char *yptr = yytext;
while ( *yptr )
*string_buf_ptr++ = *yptr++;
}
Often, such as in some of the examples above, you wind up writing a whole bunch of rules all preceded by the same start condition(s). Flex makes this a little easier and cleaner by introducing a notion of start condition scope. A start condition scope is begun with:
<SCs>{
where SCs is a list of one or more start conditions. Inside the
start condition scope, every rule automatically has the prefix
SCs> applied to it, until a ‘}’ which matches the initial
‘{’. So, for example,
<ESC>{
"\\n" return '\n';
"\\r" return '\r';
"\\f" return '\f';
"\\0" return '\0';
}
is equivalent to:
<ESC>"\\n" return '\n';
<ESC>"\\r" return '\r';
<ESC>"\\f" return '\f';
<ESC>"\\0" return '\0';
Start condition scopes may be nested.
The following routines are available for manipulating stacks of start conditions:
- Function: void yy_push_state ( int
new_state) pushes the current start condition onto the top of the start condition stack and switches to
new_stateas though you had usedBEGIN new_state(recall that start condition names are also integers).
The start condition stack grows dynamically and so has no built-in size limitation. If memory is exhausted, program execution aborts.
To use start condition stacks, your scanner must include a %option
stack directive (see section Scanner Options).
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