Each pattern in a rule has a corresponding action, which can be any arbitrary C statement. The pattern ends at the first non-escaped whitespace character; the remainder of the line is its action. If the action is empty, then when the pattern is matched the input token is simply discarded. For example, here is the specification for a program which deletes all occurrences of "zap me" from its input:
%% "zap me"
(It will copy all other characters in the input to the output since they will be matched by the default rule.)
Here is a program which compresses multiple blanks and tabs down to a single blank, and throws away whitespace found at the end of a line:
%% [ \t]+ putchar( ' ' ); [ \t]+$ /* ignore this token */
If the action contains a '{', then the action spans till
the balancing '}' is found, and the action may cross
multiple lines. flex
knows about C strings and comments and
won't be fooled by braces found within them, but also
allows actions to begin with `%{' and will consider the
action to be all the text up to the next `%}' (regardless of
ordinary braces inside the action).
An action consisting solely of a vertical bar ('|') means "same as the action for the next rule." See below for an illustration.
Actions can include arbitrary C code, including return
statements to return a value to whatever routine called
`yylex()'. Each time `yylex()' is called it continues
processing tokens from where it last left off until it either
reaches the end of the file or executes a return.
Actions are free to modify yytext
except for lengthening
it (adding characters to its end--these will overwrite
later characters in the input stream). This however does
not apply when using `%array' (see above); in that case,
yytext
may be freely modified in any way.
Actions are free to modify yyleng
except they should not
do so if the action also includes use of `yymore()' (see
below).
There are a number of special directives which can be included within an action:
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.
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".
Two notes regarding use of `yymore()'. First, `yymore()'
depends on the value of yyleng
correctly reflecting the
size of the current token, so you must not modify yyleng
if you are using `yymore()'. Second, the presence of
`yymore()' in the scanner's action entails a minor
performance penalty in the scanner's matching speed.
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.
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.
%% "/*" { 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_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.
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