[VC MFC C++ flex bison] flex 特别action

`ECHO' copies yytext to the scanner's output.

BEGIN

followed by the name of a start condition places the scanner in the corresponding start condition (see below).

REJECT

directs the scanner to proceed on to the "second best" rule which matched the input (or a prefix of the input). The rule is chosen as described above in "How the Input is Matched", and

yytext

and

yyleng

set up appropriately. It may either be one which matched as much text as the originally chosen rule but came later in the

flex

input file, or one which matched less text. For example, the following will both count the words in the input and call the routine special() whenever "frob" is seen:

int word_count = 0;
%%

frob        special(); REJECT;
[^ /t/n]+   ++word_count;

Without the

REJECT

, any "frob"'s in the input would not be counted as words, since the scanner normally executes only one action per token. Multiple

REJECT's

are allowed, each one finding the next best choice to the currently active rule. For example, when the following scanner scans the token "abcd", it will write "abcdabcaba" to the output:

%%
a        |
ab       |
abc      |
abcd     ECHO; REJECT;
.|/n     /* eat up any unmatched character */

(The first three rules share the fourth's action since they use the special '|' action.)

REJECT

is a particularly expensive feature in terms of scanner performance; if it is used in any of the scanner's actions it will slow down all of the scanner's matching. Furthermore,

REJECT

cannot be used with the `-Cf' or `-CF' options (see below). Note also that unlike the other special actions,

REJECT

is a branch; code immediately following it in the action will not be executed.

`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. For example, given the input "mega-kludge" the following will write "mega-mega-kludge" to the output:

%%
mega-    ECHO; yymore();
kludge   ECHO;

First "mega-" is matched and echoed to the output. Then "kludge" is matched, but the previous "mega-" is still hanging around at the beginning of

yytext

so the `ECHO' for the "kludge" rule will actually write "mega-kludge".

`yyless(n)' returns all but the first n characters of the current token back to the input stream, where they will be rescanned when the scanner looks for the next match.

yytext

and

yyleng

are adjusted appropriately (e.g.,

yyleng

will now be equal to n ). For example, on the input "foobar" the following will write out "foobarbar":

%%
foobar    ECHO; yyless(3);
[a-z]+    ECHO;

An argument of 0 to

yyless

will cause the entire current input string to be scanned again. Unless you've changed how the scanner will subsequently process its input (using

BEGIN

, for example), this will result in an endless loop. Note that

yyless

is a macro and can only be used in the flex input file, not from other source files.

`unput(c)' puts the character

c

back onto the input stream. It will be the next character scanned. The following action will take the current token and cause it to be rescanned enclosed in parentheses.

{
int i;
/* Copy yytext because unput() trashes yytext */
char *yycopy = strdup( yytext );
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
unput( yycopy[i] );
unput( '(' );
free( yycopy );
}

Note that since each `unput()' puts the given character back at the beginning of the input stream, pushing back strings must be done back-to-front. An important potential problem when using `unput()' is that if you are using `%pointer' (the default), a call to `unput()' destroys the contents of

yytext

, starting with its rightmost character and devouring one character to the left with each call. If you need the value of yytext preserved after a call to `unput()' (as in the above example), you must either first copy it elsewhere, or build your scanner using `%array' instead (see How The Input Is Matched). Finally, note that you cannot put back

EOF

to attempt to mark the input stream with an end-of-file.

`input()' reads the next character from the input stream. For example, the following is one way to eat up C comments:

%%
"/*"        {
register int c;

for ( ; ; )
{
while ( (c = input()) != '*' &&
c != EOF )
;    /* eat up text of comment */

if ( c == '*' )
{
while ( (c = input()) == '*' )
;
if ( c == '/' )
break;    /* found the end */
}

if ( c == EOF )
{
error( "EOF in comment" );
break;
}
}
}

(Note that if the scanner is compiled using `C++', then `input()' is instead referred to as `yyinput()', in order to avoid a name clash with the `C++' stream by the name of

input

.)

YY_FLUSH_BUFFER flushes the scanner's internal buffer so that the next time the scanner attempts to match a token, it will first refill the buffer using

YY_INPUT

(see The Generated Scanner, below). This action is a special case of the more general `yy_flush_buffer()' function, described below in the section Multiple Input Buffers.

`yyterminate()' can be used in lieu of a return statement in an action. It terminates the scanner and returns a 0 to the scanner's caller, indicating "all done". By default, `yyterminate()' is also called when an end-of-file is encountered. It is a macro and may be redefined.