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(flex.info)Actions
Actions
*******
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:
- `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".
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.
- `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.
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