yacc
lex
and POSIX
Copyright (C) 1990 The Regents of the University of California. All rights reserved.
This code is derived from software contributed to Berkeley by Vern Paxson.
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flex - fast lexical analyzer generator
flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix -Sskeleton] [--help --version] [filename ...]
This manual describes flex
, a tool for generating programs that
perform pattern-matching on text. The manual includes both tutorial and
reference sections:
flex
is a tool for generating scanners: programs which
recognized lexical patterns in text. flex
reads the given input
files, or its standard input if no file names are given, for a description of a
scanner to generate. The description is in the form of pairs of regular
expressions and C code, called rules. flex
generates as
output a C source file, `lex.yy.c', which defines a routine
`yylex()'. This file is compiled and linked with the
`-lfl' library to produce an executable. When the executable is
run, it analyzes its input for occurrences of the regular expressions. Whenever
it finds one, it executes the corresponding C code.
First some simple examples to get the flavor of how one uses
flex
. The following flex
input specifies a scanner
which whenever it encounters the string "username" will replace it with the
user's login name:
%% username printf( "%s", getlogin() );
By default, any text not matched by a flex
scanner is copied to
the output, so the net effect of this scanner is to copy its input file to its
output with each occurrence of "username" expanded. In this input, there is just
one rule. "username" is the pattern and the "printf" is the
action. The "%%" marks the beginning of the rules.
Here's another simple example:
int num_lines = 0, num_chars = 0; %% \n ++num_lines; ++num_chars; . ++num_chars; %% main() { yylex(); printf( "# of lines = %d, # of chars = %d\n", num_lines, num_chars ); }
This scanner counts the number of characters and the number of lines in its input (it produces no output other than the final report on the counts). The first line declares two globals, "num_lines" and "num_chars", which are accessible both inside `yylex()' and in the `main()' routine declared after the second "%%". There are two rules, one which matches a newline ("\n") and increments both the line count and the character count, and one which matches any character other than a newline (indicated by the "." regular expression).
A somewhat more complicated example:
/* scanner for a toy Pascal-like language */ %{ /* need this for the call to atof() below */ #include <math.h> %} DIGIT [0-9] ID [a-z][a-z0-9]* %% {DIGIT}+ { printf( "An integer: %s (%d)\n", yytext, atoi( yytext ) ); } {DIGIT}+"."{DIGIT}* { printf( "A float: %s (%g)\n", yytext, atof( yytext ) ); } if|then|begin|end|procedure|function { printf( "A keyword: %s\n", yytext ); } {ID} printf( "An identifier: %s\n", yytext ); "+"|"-"|"*"|"/" printf( "An operator: %s\n", yytext ); "{"[^}\n]*"}" /* eat up one-line comments */ [ \t\n]+ /* eat up whitespace */ . printf( "Unrecognized character: %s\n", yytext ); %% main( argc, argv ) int argc; char **argv; { ++argv, --argc; /* skip over program name */ if ( argc > 0 ) yyin = fopen( argv[0], "r" ); else yyin = stdin; yylex(); }
This is the beginnings of a simple scanner for a language like Pascal. It identifies different types of tokens and reports on what it has seen.
The details of this example will be explained in the following sections.
The flex
input file consists of three sections, separated by a
line with just `%%' in it:
definitions %% rules %% user code
The definitions section contains declarations of simple name definitions to simplify the scanner specification, and declarations of start conditions, which are explained in a later section. Name definitions have the form:
name definition
The "name" is a word beginning with a letter or an underscore ('_') followed by zero or more letters, digits, '_', or '-' (dash). The definition is taken to begin at the first non-white-space character following the name and continuing to the end of the line. The definition can subsequently be referred to using "{name}", which will expand to "(definition)". For example,
DIGIT [0-9] ID [a-z][a-z0-9]*
defines "DIGIT" to be a regular expression which matches a single digit, and "ID" to be a regular expression which matches a letter followed by zero-or-more letters-or-digits. A subsequent reference to
{DIGIT}+"."{DIGIT}*
is identical to
([0-9])+"."([0-9])*
and matches one-or-more digits followed by a '.' followed by zero-or-more digits.
The rules section of the flex
input contains a series
of rules of the form:
pattern action
where the pattern must be unindented and the action must begin on the same line.
See below for a further description of patterns and actions.
Finally, the user code section is simply copied to `lex.yy.c' verbatim. It is used for companion routines which call or are called by the scanner. The presence of this section is optional; if it is missing, the second `%%' in the input file may be skipped, too.
In the definitions and rules sections, any indented text or text enclosed in `%{' and `%}' is copied verbatim to the output (with the `%{}''s removed). The `%{}''s must appear unindented on lines by themselves.
In the rules section, any indented or %{} text appearing before the first
rule may be used to declare variables which are local to the scanning routine
and (after the declarations) code which is to be executed whenever the scanning
routine is entered. Other indented or %{} text in the rule section is still
copied to the output, but its meaning is not well-defined and it may well cause
compile-time errors (this feature is present for POSIX
compliance;
see below for other such features).
In the definitions section (but not in the rules section), an unindented comment (i.e., a line beginning with "/*") is also copied verbatim to the output up to the next "*/".
The patterns in the input are written using an extended set of regular expressions. These are:
2a
flex
cannot
match correctly; see notes in the Deficiencies / Bugs section below regarding
"dangerous trailing context".)
Note that inside of a character class, all regular expression operators lose their special meaning except escape ('\') and the character class operators, '-', ']', and, at the beginning of the class, '^'.
The regular expressions listed above are grouped according to precedence, from highest precedence at the top to lowest at the bottom. Those grouped together have equal precedence. For example,
foo|bar*
is the same as
(foo)|(ba(r*))
since the '*' operator has higher precedence than concatenation, and concatenation higher than alternation ('|'). This pattern therefore matches either the string "foo" or the string "ba" followed by zero-or-more r's. To match "foo" or zero-or-more "bar"'s, use:
foo|(bar)*
and to match zero-or-more "foo"'s-or-"bar"'s:
(foo|bar)*
In addition to characters and ranges of characters, character classes can also contain character class expressions. These are expressions enclosed inside `[': and `:'] delimiters (which themselves must appear between the '[' and ']' of the character class; other elements may occur inside the character class, too). The valid expressions are:
[:alnum:] [:alpha:] [:blank:] [:cntrl:] [:digit:] [:graph:] [:lower:] [:print:] [:punct:] [:space:] [:upper:] [:xdigit:]
These expressions all designate a set of characters equivalent to the corresponding standard C `isXXX' function. For example, `[:alnum:]' designates those characters for which `isalnum()' returns true - i.e., any alphabetic or numeric. Some systems don't provide `isblank()', so flex defines `[:blank:]' as a blank or a tab.
For example, the following character classes are all equivalent:
[[:alnum:]] [[:alpha:][:digit:] [[:alpha:]0-9] [a-zA-Z0-9]
If your scanner is case-insensitive (the `-i' flag), then `[:upper:]' and `[:lower:]' are equivalent to `[:alpha:]'.
Some notes on patterns:
foo/bar$ <sc1>foo<sc2>barNote that the first of these, can be written "foo/bar\n". The following will result in '$' or '^' being treated as a normal character:
foo|(bar$) foo|^barIf what's wanted is a "foo" or a bar-followed-by-a-newline, the following could be used (the special '|' action is explained below):
foo | bar$ /* action goes here */A similar trick will work for matching a foo or a bar-at-the-beginning-of-a-line.
When the generated scanner is run, it analyzes its input looking for strings
which match any of its patterns. If it finds more than one match, it takes the
one matching the most text (for trailing context rules, this includes the length
of the trailing part, even though it will then be returned to the input). If it
finds two or more matches of the same length, the rule listed first in the
flex
input file is chosen.
Once the match is determined, the text corresponding to the match (called the
token) is made available in the global character pointer
yytext
, and its length in the global integer yyleng
.
The action corresponding to the matched pattern is then executed (a
more detailed description of actions follows), and then the remaining input is
scanned for another match.
If no match is found, then the default rule is executed: the next
character in the input is considered matched and copied to the standard output.
Thus, the simplest legal flex
input is:
%%
which generates a scanner that simply copies its input (one character at a time) to its output.
Note that yytext
can be defined in two different ways: either as
a character pointer or as a character array. You can control
which definition flex
uses by including one of the special
directives `%pointer' or `%array' in the first
(definitions) section of your flex input. The default is
`%pointer', unless you use the `-l' lex compatibility
option, in which case yytext
will be an array. The advantage of
using `%pointer' is substantially faster scanning and no buffer
overflow when matching very large tokens (unless you run out of dynamic memory).
The disadvantage is that you are restricted in how your actions can modify
yytext
(see the next section), and calls to the
`unput()' function destroys the present contents of
yytext
, which can be a considerable porting headache when moving
between different lex
versions.
The advantage of `%array' is that you can then modify
yytext
to your heart's content, and calls to `unput()'
do not destroy yytext
(see below). Furthermore, existing
lex
programs sometimes access yytext
externally using
declarations of the form:
extern char yytext[];
This definition is erroneous when used with `%pointer', but correct for `%array'.
`%array' defines yytext
to be an array of
YYLMAX
characters, which defaults to a fairly large value. You can
change the size by simply #define'ing YYLMAX
to a different value
in the first section of your flex
input. As mentioned above, with
`%pointer' yytext grows dynamically to accommodate large tokens.
While this means your `%pointer' scanner can accommodate very large
tokens (such as matching entire blocks of comments), bear in mind that each time
the scanner must resize yytext
it also must rescan the entire token
from the beginning, so matching such tokens can prove slow. yytext
presently does not dynamically grow if a call to `unput()'
results in too much text being pushed back; instead, a run-time error results.
Also note that you cannot use `%array' with C++ scanner classes
(the c++
option; see below).
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.
The output of flex
is the file `lex.yy.c', which
contains the scanning routine `yylex()', a number of tables used by
it for matching tokens, and a number of auxiliary routines and macros. By
default, `yylex()' is declared as follows:
int yylex() { ... various definitions and the actions in here ... }
(If your environment supports function prototypes, then it will be "int yylex( void )".) This definition may be changed by defining the "YY_DECL" macro. For example, you could use:
#define YY_DECL float lexscan( a, b ) float a, b;
to give the scanning routine the name lexscan
, returning a
float, and taking two floats as arguments. Note that if you give arguments to
the scanning routine using a K&R-style/non-prototyped function declaration,
you must terminate the definition with a semi-colon (`;').
Whenever `yylex()' is called, it scans tokens from the global
input file yyin
(which defaults to stdin). It continues until it
either reaches an end-of-file (at which point it returns the value 0) or one of
its actions executes a return
statement.
If the scanner reaches an end-of-file, subsequent calls are undefined unless
either yyin
is pointed at a new input file (in which case scanning
continues from that file), or `yyrestart()' is called.
`yyrestart()' takes one argument, a `FILE *' pointer
(which can be nil, if you've set up YY_INPUT
to scan from a source
other than yyin
), and initializes yyin
for scanning
from that file. Essentially there is no difference between just assigning
yyin
to a new input file or using `yyrestart()' to do
so; the latter is available for compatibility with previous versions of
flex
, and because it can be used to switch input files in the
middle of scanning. It can also be used to throw away the current input buffer,
by calling it with an argument of yyin
; but better is to use
YY_FLUSH_BUFFER
(see above). Note that `yyrestart()'
does not reset the start condition to INITIAL
(see Start
Conditions, below).
If `yylex()' stops scanning due to executing a
return
statement in one of the actions, the scanner may then be
called again and it will resume scanning where it left off.
By default (and for purposes of efficiency), the scanner uses block-reads
rather than simple `getc()' calls to read characters from
yyin
. The nature of how it gets its input can be controlled by
defining the YY_INPUT
macro. YY_INPUT's calling sequence is
"YY_INPUT(buf,result,max_size)". Its action is to place up to
max_size characters in the character array buf and return
in the integer variable result either the number of characters read
or the constant YY_NULL (0 on Unix systems) to indicate EOF. The default
YY_INPUT reads from the global file-pointer "yyin".
A sample definition of YY_INPUT (in the definitions section of the input file):
%{ #define YY_INPUT(buf,result,max_size) \ { \ int c = getchar(); \ result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \ } %}
This definition will change the input processing to occur one character at a time.
When the scanner receives an end-of-file indication from YY_INPUT, it then
checks the `yywrap()' function. If `yywrap()' returns
false (zero), then it is assumed that the function has gone ahead and set up
yyin
to point to another input file, and scanning continues. If it
returns true (non-zero), then the scanner terminates, returning 0 to its caller.
Note that in either case, the start condition remains unchanged; it does
not revert to INITIAL
.
If you do not supply your own version of `yywrap()', then you must either use `%option noyywrap' (in which case the scanner behaves as though `yywrap()' returned 1), or you must link with `-lfl' to obtain the default version of the routine, which always returns 1.
Three routines are available for scanning from in-memory buffers rather than files: `yy_scan_string()', `yy_scan_bytes()', and `yy_scan_buffer()'. See the discussion of them below in the section Multiple Input Buffers.
The scanner writes its `ECHO' output to the yyout
global (default, stdout), which may be redefined by the user simply by assigning
it to some other FILE
pointer.
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' starting 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 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]+"."[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]+ { Version 2.5 December 1994 18 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
).
Note that start conditions do not have their own name-space; %s's and %x's declare names in the same fashion as #define's.
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.
Three routines are available for manipulating stacks of start conditions:
BEGIN
.
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 Options below).
Some scanners (such as those which support "include" files) require reading
from several input streams. As flex
scanners do a large amount of
buffering, one cannot control where the next input will be read from by simply
writing a YY_INPUT
which is sensitive to the scanning context.
YY_INPUT
is only called when the scanner reaches the end of its
buffer, which may be a long time after scanning a statement such as an "include"
which requires switching the input source.
To negotiate these sorts of problems, flex
provides a mechanism
for creating and switching between multiple input buffers. An input buffer is
created by using:
YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )
which takes a FILE
pointer and a size and creates a buffer
associated with the given file and large enough to hold size
characters (when in doubt, use YY_BUF_SIZE
for the size). It
returns a YY_BUFFER_STATE
handle, which may then be passed to other
routines (see below). The YY_BUFFER_STATE
type is a pointer to an
opaque struct
yy_buffer_state
structure, so you may
safely initialize YY_BUFFER_STATE variables to `((YY_BUFFER_STATE)
0)' if you wish, and also refer to the opaque structure in order to
correctly declare input buffers in source files other than that of your scanner.
Note that the FILE
pointer in the call to
yy_create_buffer
is only used as the value of yyin
seen by YY_INPUT
; if you redefine YY_INPUT
so it no
longer uses yyin
, then you can safely pass a nil FILE
pointer to yy_create_buffer
. You select a particular buffer to scan
from using:
void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )
switches the scanner's input buffer so subsequent tokens will come from
new_buffer. Note that `yy_switch_to_buffer()' may be
used by `yywrap()' to set things up for continued scanning, instead
of opening a new file and pointing yyin
at it. Note also that
switching input sources via either `yy_switch_to_buffer()' or
`yywrap()' does not change the start condition.
void yy_delete_buffer( YY_BUFFER_STATE buffer )
is used to reclaim the storage associated with a buffer. You can also clear the current contents of a buffer using:
void yy_flush_buffer( YY_BUFFER_STATE buffer )
This function discards the buffer's contents, so the next time the scanner
attempts to match a token from the buffer, it will first fill the buffer anew
using YY_INPUT
.
`yy_new_buffer()' is an alias for
`yy_create_buffer()', provided for compatibility with the C++ use
of new
and delete
for creating and destroying dynamic
objects.
Finally, the YY_CURRENT_BUFFER
macro returns a
YY_BUFFER_STATE
handle to the current buffer.
Here is an example of using these features for writing a scanner which expands include files (the `<<EOF>>' feature is discussed below):
/* the "incl" state is used for picking up the name * of an include file */ %x incl %{ #define MAX_INCLUDE_DEPTH 10 YY_BUFFER_STATE include_stack[MAX_INCLUDE_DEPTH]; int include_stack_ptr = 0; %} %% include BEGIN(incl); [a-z]+ ECHO; [^a-z\n]*\n? ECHO; <incl>[ \t]* /* eat the whitespace */ <incl>[^ \t\n]+ { /* got the include file name */ if ( include_stack_ptr >= MAX_INCLUDE_DEPTH ) { fprintf( stderr, "Includes nested too deeply" ); exit( 1 ); } include_stack[include_stack_ptr++] = YY_CURRENT_BUFFER; yyin = fopen( yytext, "r" ); if ( ! yyin ) error( ... ); yy_switch_to_buffer( yy_create_buffer( yyin, YY_BUF_SIZE ) ); BEGIN(INITIAL); } <<EOF>> { if ( --include_stack_ptr < 0 ) { yyterminate(); } else { yy_delete_buffer( YY_CURRENT_BUFFER ); yy_switch_to_buffer( include_stack[include_stack_ptr] ); } }
Three routines are available for setting up input buffers for scanning
in-memory strings instead of files. All of them create a new input buffer for
scanning the string, and return a corresponding YY_BUFFER_STATE
handle (which you should delete with `yy_delete_buffer()' when done
with it). They also switch to the new buffer using
`yy_switch_to_buffer()', so the next call to `yylex()'
will start scanning the string.
len
bytes (including possibly NUL's) starting at
location bytes. Note that both of these functions create and scan a copy of the string or bytes. (This may be desirable, since `yylex()' modifies the contents of the buffer it is scanning.) You can avoid the copy by using:
YY_END_OF_BUFFER_CHAR
(ASCII NUL). These last two bytes are not
scanned; thus, scanning consists of `base[0]' through
`base[size-2]', inclusive. If you fail to set up base
in this manner (i.e., forget the final two YY_END_OF_BUFFER_CHAR
bytes), then `yy_scan_buffer()' returns a nil pointer instead of
creating a new input buffer. The type yy_size_t
is an integral
type to which you can cast an integer expression reflecting the size of the
buffer. The special rule "<<EOF>>" indicates actions which are to be taken when an end-of-file is encountered and yywrap() returns non-zero (i.e., indicates no further files to process). The action must finish by doing one of four things:
yyin
to a new input file (in previous versions of
flex, after doing the assignment you had to call the special action
YY_NEW_FILE
; this is no longer necessary);
return
statement;
<<EOF>> rules may not be used with other patterns; they may only be qualified with a list of start conditions. If an unqualified <<EOF>> rule is given, it applies to all start conditions which do not already have <<EOF>> actions. To specify an <<EOF>> rule for only the initial start condition, use
<INITIAL><<EOF>>
These rules are useful for catching things like unclosed comments. An example:
%x quote %% ...other rules for dealing with quotes... <quote><<EOF>> { error( "unterminated quote" ); yyterminate(); } <<EOF>> { if ( *++filelist ) yyin = fopen( *filelist, "r" ); else yyterminate(); }
The macro YY_USER_ACTION
can be defined to provide an action
which is always executed prior to the matched rule's action. For example, it
could be #define'd to call a routine to convert yytext to lower-case. When
YY_USER_ACTION
is invoked, the variable yy_act
gives
the number of the matched rule (rules are numbered starting with 1). Suppose you
want to profile how often each of your rules is matched. The following would do
the trick:
#define YY_USER_ACTION ++ctr[yy_act]
where ctr
is an array to hold the counts for the different
rules. Note that the macro YY_NUM_RULES
gives the total number of
rules (including the default rule, even if you use `-s', so a
correct declaration for ctr
is:
int ctr[YY_NUM_RULES];
The macro YY_USER_INIT
may be defined to provide an action which
is always executed before the first scan (and before the scanner's internal
initializations are done). For example, it could be used to call a routine to
read in a data table or open a logging file.
The macro `yy_set_interactive(is_interactive)' can be used to control whether the current buffer is considered interactive. An interactive buffer is processed more slowly, but must be used when the scanner's input source is indeed interactive to avoid problems due to waiting to fill buffers (see the discussion of the `-I' flag below). A non-zero value in the macro invocation marks the buffer as interactive, a zero value as non-interactive. Note that use of this macro overrides `%option always-interactive' or `%option never-interactive' (see Options below). `yy_set_interactive()' must be invoked prior to beginning to scan the buffer that is (or is not) to be considered interactive.
The macro `yy_set_bol(at_bol)' can be used to control whether the current buffer's scanning context for the next token match is done as though at the beginning of a line. A non-zero macro argument makes rules anchored with
The macro `YY_AT_BOL()' returns true if the next token scanned from the current buffer will have '^' rules active, false otherwise.
In the generated scanner, the actions are all gathered in one large switch
statement and separated using YY_BREAK
, which may be redefined. By
default, it is simply a "break", to separate each rule's action from the
following rule's. Redefining YY_BREAK
allows, for example, C++
users to #define YY_BREAK to do nothing (while being very careful that every
rule ends with a "break" or a "return"!) to avoid suffering from unreachable
statement warnings where because a rule's action ends with "return", the
YY_BREAK
is inaccessible.
This section summarizes the various values available to the user in the rule actions.
yytext
is instead declared `char
yytext[YYLMAX]', where YYLMAX
is a macro definition that
you can redefine in the first section if you don't like the default value
(generally 8KB). Using `%array' results in somewhat slower
scanners, but the value of yytext
becomes immune to calls to
`input()' and `unput()', which potentially destroy
its value when yytext
is a character pointer. The opposite of
`%array' is `%pointer', which is the default. You
cannot use `%array' when generating C++ scanner classes (the
`-+' flag).
flex
reads from. It may be redefined but doing so only makes sense before scanning
begins or after an EOF has been encountered. Changing it in the midst of
scanning will have unexpected results since flex
buffers its
input; use `yyrestart()' instead. Once scanning terminates
because an end-of-file has been seen, you can assign yyin
at the
new input file and then call the scanner again to continue scanning.
yyin
at the new input file. The switch-over to the new file is
immediate (any previously buffered-up input is lost). Note that calling
`yyrestart()' with yyin
as an argument thus throws
away the current input buffer and continues scanning the same input file.
YY_CURRENT_BUFFER
returns a YY_BUFFER_STATE
handle to the current buffer.
YY_START
returns an integer value corresponding to the
current start condition. You can subsequently use this value with
BEGIN
to return to that start condition. yacc
One of the main uses of flex
is as a companion to the
yacc
parser-generator. yacc
parsers expect to call a
routine named `yylex()' to find the next input token. The routine
is supposed to return the type of the next token as well as putting any
associated value in the global yylval
. To use flex
with yacc
, one specifies the `-d' option to
yacc
to instruct it to generate the file `y.tab.h'
containing definitions of all the `%tokens' appearing in the
yacc
input. This file is then included in the flex
scanner. For example, if one of the tokens is "TOK_NUMBER", part of the scanner
might look like:
%{ #include "y.tab.h" %} %% [0-9]+ yylval = atoi( yytext ); return TOK_NUMBER;
flex
has the following options:
yy_flex_debug
is non-zero (which is
the default), the scanner will write to stderr
a line of the
form: --accepting rule at line 53 ("the matched text")The line number refers to the location of the rule in the file defining the scanner (i.e., the file that was fed to flex). Messages are also generated when the scanner backs up, accepts the default rule, reaches the end of its input buffer (or encounters a NUL; at this point, the two look the same as far as the scanner's concerned), or reaches an end-of-file.
flex's
options to
stdout
and then exits. `-?' and
`--help' are synonyms for `-h'.
flex
to generate a case-insensitive
scanner. The case of letters given in the flex
input patterns
will be ignored, and tokens in the input will be matched regardless of case.
The matched text given in yytext
will have the preserved case
(i.e., it will not be folded).
lex
implementation. Note that this does not mean full compatibility. Use
of this option costs a considerable amount of performance, and it cannot be
used with the `-+, -f, -F, -Cf', or `-CF' options.
For details on the compatibilities it provides, see the section
"Incompatibilities With Lex And POSIX" below. This option also results in the
name YY_FLEX_LEX_COMPAT
being #define'd in the generated scanner.
flex
input file which will cause a
serious loss of performance in the resulting scanner. If you give the flag
twice, you will also get comments regarding features that lead to minor
performance losses. Note that the use of REJECT
, `%option
yylineno' and variable trailing context (see the Deficiencies / Bugs
section below) entails a substantial performance penalty; use of
`yymore()', the `^' operator, and the
`-I' flag entail minor performance penalties.
stdout
) to be suppressed. If the scanner encounters input that
does not match any of its rules, it aborts with an error. This option is
useful for finding holes in a scanner's rule set.
flex
to write the scanner it generates to standard
output instead of `lex.yy.c'.
flex
should write to stderr
a
summary of statistics regarding the scanner it generates. Most of the
statistics are meaningless to the casual flex
user, but the first
line identifies the version of flex
(same as reported by
`-V'), and the next line the flags used when generating the
scanner, including those that are on by default.
flex
to generate a batch scanner, the
opposite of interactive scanners generated by `-I' (see
below). In general, you use `-B' when you are certain
that your scanner will never be used interactively, and you want to squeeze a
little more performance out of it. If your goal is instead to squeeze
out a lot more performance, you should be using the
`-Cf' or `-CF' options (discussed below), which turn
on `-B' automatically anyway.
"case" return TOK_CASE; "switch" return TOK_SWITCH; ... "default" return TOK_DEFAULT; [a-z]+ return TOK_ID;then you're better off using the full table representation. If only the "identifier" rule is present and you then use a hash table or some such to detect the keywords, you're better off using `-F'. This option is equivalent to `-CFr' (see below). It cannot be used with `-+'.
flex
to generate an interactive scanner.
An interactive scanner is one that only looks ahead to decide what token has
been matched if it absolutely must. It turns out that always looking one extra
character ahead, even if the scanner has already seen enough text to
disambiguate the current token, is a bit faster than only looking ahead when
necessary. But scanners that always look ahead give dreadful interactive
performance; for example, when a user types a newline, it is not recognized as
a newline token until they enter another token, which often means
typing in another whole line. Flex
scanners default to
interactive unless you use the `-Cf' or
`-CF' table-compression options (see below). That's because if
you're looking for high-performance you should be using one of these options,
so if you didn't, flex
assumes you'd rather trade off a bit of
run-time performance for intuitive interactive behavior. Note also that you
cannot use `-I' in conjunction with `-Cf'
or `-CF'. Thus, this option is not really needed; it is on by
default for all those cases in which it is allowed. You can force a scanner to
not be interactive by using `-B' (see above).
flex
not to generate `#line'
directives. Without this option, flex
peppers the generated
scanner with #line directives so error messages in the actions will be
correctly located with respect to either the original flex
input
file (if the errors are due to code in the input file), or `lex.yy.c'
(if the errors are flex's
fault -- you should report these sorts
of errors to the email address given below).
flex
run in trace
mode. It will generate a
lot of messages to stderr
concerning the form of the input and
the resultant non-deterministic and deterministic finite automata. This option
is mostly for use in maintaining flex
.
stdout
and exits.
`--version' is a synonym for `-V'.
flex
to generate a 7-bit scanner, i.e., one which
can only recognized 7-bit characters in its input. The advantage of using
`-7' is that the scanner's tables can be up to half the size of
those generated using the `-8' option (see below). The
disadvantage is that such scanners often hang or crash if their input contains
an 8-bit character. Note, however, that unless you generate your scanner using
the `-Cf' or `-CF' table compression options, use of
`-7' will save only a small amount of table space, and make your
scanner considerably less portable. Flex's
default behavior is to
generate an 8-bit scanner unless you use the `-Cf' or
`-CF', in which case flex
defaults to generating
7-bit scanners unless your site was always configured to generate 8-bit
scanners (as will often be the case with non-USA sites). You can tell whether
flex generated a 7-bit or an 8-bit scanner by inspecting the flag summary in
the `-v' output as described above. Note that if you use
`-Cfe' or `-CFe' (those table compression options,
but also using equivalence classes as discussed see below), flex still
defaults to generating an 8-bit scanner, since usually with these compression
options full 8-bit tables are not much more expensive than 7-bit tables.
flex
to generate an 8-bit scanner, i.e., one which
can recognize 8-bit characters. This flag is only needed for scanners
generated using `-Cf' or `-CF', as otherwise flex
defaults to generating an 8-bit scanner anyway. See the discussion of
`-7' above for flex's default behavior and the tradeoffs between
7-bit and 8-bit scanners.
flex
to construct equivalence
classes, i.e., sets of characters which have identical lexical properties
(for example, if the only appearance of digits in the flex
input
is in the character class "[0-9]" then the digits '0', '1', ..., '9' will all
be put in the same equivalence class). Equivalence classes usually give
dramatic reductions in the final table/object file sizes (typically a factor
of 2-5) and are pretty cheap performance-wise (one array look-up per character
scanned). `-Cf' specifies that the full scanner tables
should be generated - flex
should not compress the tables by
taking advantages of similar transition functions for different states.
`-CF' specifies that the alternate fast scanner representation
(described above under the `-F' flag) should be used. This option
cannot be used with `-+'. `-Cm' directs
flex
to construct meta-equivalence classes, which are
sets of equivalence classes (or characters, if equivalence classes are not
being used) that are commonly used together. Meta-equivalence classes are
often a big win when using compressed tables, but they have a moderate
performance impact (one or two "if" tests and one array look-up per character
scanned). `-Cr' causes the generated scanner to bypass
use of the standard I/O library (stdio) for input. Instead of calling
`fread()' or `getc()', the scanner will use the
`read()' system call, resulting in a performance gain which
varies from system to system, but in general is probably negligible unless you
are also using `-Cf' or `-CF'. Using
`-Cr' can cause strange behavior if, for example, you read from
yyin
using stdio prior to calling the scanner (because the
scanner will miss whatever text your previous reads left in the stdio input
buffer). `-Cr' has no effect if you define YY_INPUT
(see The Generated Scanner above). A lone `-C' specifies that the
scanner tables should be compressed but neither equivalence classes nor
meta-equivalence classes should be used. The options `-Cf' or
`-CF' and `-Cm' do not make sense together - there
is no opportunity for meta-equivalence classes if the table is not being
compressed. Otherwise the options may be freely mixed, and are cumulative. The
default setting is `-Cem', which specifies that flex
should generate equivalence classes and meta-equivalence classes. This setting
provides the highest degree of table compression. You can trade off
faster-executing scanners at the cost of larger tables with the following
generally being true: slowest & smallest -Cem -Cm -Ce -C -C{f,F}e -C{f,F} -C{f,F}a fastest & largestNote that scanners with the smallest tables are usually generated and compiled the quickest, so during development you will usually want to use the default, maximal compression. `-Cfe' is often a good compromise between speed and size for production scanners.
put
instead of `lex.yy.c'. If you combine
`-o' with the `-t' option, then the scanner is
written to stdout
but its `#line' directives (see
the `-L' option above) refer to the file output
.
flex
for
all globally-visible variable and function names to instead be
prefix. For example, `-Pfoo' changes the name of
yytext
to `footext'. It also changes the name of the
default output file from `lex.yy.c' to `lex.foo.c'. Here are
all of the names affected: yy_create_buffer yy_delete_buffer yy_flex_debug yy_init_buffer yy_flush_buffer yy_load_buffer_state yy_switch_to_buffer yyin yyleng yylex yylineno yyout yyrestart yytext yywrap(If you are using a C++ scanner, then only
yywrap
and
yyFlexLexer
are affected.) Within your scanner itself, you can
still refer to the global variables and functions using either version of
their name; but externally, they have the modified name. This option lets you
easily link together multiple flex
programs into the same
executable. Note, though, that using this option also renames
`yywrap()', so you now must either provide your own
(appropriately-named) version of the routine for your scanner, or use
`%option noyywrap', as linking with `-lfl' no longer
provides one for you by default.
flex
constructs its scanners. You'll never need this option unless you are doing
flex
maintenance or development. flex
also provides a mechanism for controlling options within
the scanner specification itself, rather than from the flex command-line. This
is done by including `%option' directives in the first section of
the scanner specification. You can specify multiple options with a single
`%option' directive, and multiple directives in the first section
of your flex input file. Most options are given simply as names, optionally
preceded by the word "no" (with no intervening whitespace) to negate their
meaning. A number are equivalent to flex flags or their negation:
7bit -7 option 8bit -8 option align -Ca option backup -b option batch -B option c++ -+ option caseful or case-sensitive opposite of -i (default) case-insensitive or caseless -i option debug -d option default opposite of -s option ecs -Ce option fast -F option full -f option interactive -I option lex-compat -l option meta-ecs -Cm option perf-report -p option read -Cr option stdout -t option verbose -v option warn opposite of -w option (use "%option nowarn" for -w) array equivalent to "%array" pointer equivalent to "%pointer" (default)
Some `%option's' provide features otherwise not available:
noyywrap
(see below).
yyin
and yyout
to nil FILE
pointers,
instead of stdin
and stdout
.
flex
to generate a scanner that maintains the number
of the current line read from its input in the global variable
yylineno
. This option is implied by `%option
lex-compat'.
yyin
at a new
file and calls `yylex()' again). flex
scans your rule actions to determine whether you use the
REJECT
or `yymore()' features. The reject
and yymore
options are available to override its decision as to
whether you use the options, either by setting them (e.g., `%option
reject') to indicate the feature is indeed used, or unsetting them to
indicate it actually is not used (e.g., `%option noyymore').
Three options take string-delimited values, offset with '=':
%option outfile="ABC"
is equivalent to `-oABC', and
%option prefix="XYZ"
is equivalent to `-PXYZ'.
Finally,
%option yyclass="foo"
only applies when generating a C++ scanner (`-+' option). It
informs flex
that you have derived `foo' as a subclass
of yyFlexLexer
so flex
will place your actions in the
member function `foo::yylex()' instead of
`yyFlexLexer::yylex()'. It also generates a
`yyFlexLexer::yylex()' member function that emits a run-time error
(by invoking `yyFlexLexer::LexerError()') if called. See Generating
C++ Scanners, below, for additional information.
A number of options are available for lint purists who want to suppress the appearance of unneeded routines in the generated scanner. Each of the following, if unset, results in the corresponding routine not appearing in the generated scanner:
input, unput yy_push_state, yy_pop_state, yy_top_state yy_scan_buffer, yy_scan_bytes, yy_scan_string
(though `yy_push_state()' and friends won't appear anyway unless you use `%option stack').
The main design goal of flex
is that it generate
high-performance scanners. It has been optimized for dealing well with large
sets of rules. Aside from the effects on scanner speed of the table compression
`-C' options outlined above, there are a number of options/actions
which degrade performance. These are, from most expensive to least:
REJECT %option yylineno arbitrary trailing context pattern sets that require backing up %array %option interactive %option always-interactive '^' beginning-of-line operator yymore()
with the first three all being quite expensive and the last two being quite cheap. Note also that `unput()' is implemented as a routine call that potentially does quite a bit of work, while `yyless()' is a quite-cheap macro; so if just putting back some excess text you scanned, use `yyless()'.
REJECT
should be avoided at all costs when performance is
important. It is a particularly expensive option.
Getting rid of backing up is messy and often may be an enormous amount of work for a complicated scanner. In principal, one begins by using the `-b' flag to generate a `lex.backup' file. For example, on the input
%% foo return TOK_KEYWORD; foobar return TOK_KEYWORD;
the file looks like:
State #6 is non-accepting - associated rule line numbers: 2 3 out-transitions: [ o ] jam-transitions: EOF [ \001-n p-\177 ] State #8 is non-accepting - associated rule line numbers: 3 out-transitions: [ a ] jam-transitions: EOF [ \001-` b-\177 ] State #9 is non-accepting - associated rule line numbers: 3 out-transitions: [ r ] jam-transitions: EOF [ \001-q s-\177 ] Compressed tables always back up.
The first few lines tell us that there's a scanner state in which it can make a transition on an 'o' but not on any other character, and that in that state the currently scanned text does not match any rule. The state occurs when trying to match the rules found at lines 2 and 3 in the input file. If the scanner is in that state and then reads something other than an 'o', it will have to back up to find a rule which is matched. With a bit of head-scratching one can see that this must be the state it's in when it has seen "fo". When this has happened, if anything other than another 'o' is seen, the scanner will have to back up to simply match the 'f' (by the default rule).
The comment regarding State #8 indicates there's a problem when "foob" has been scanned. Indeed, on any character other than an 'a', the scanner will have to back up to accept "foo". Similarly, the comment for State #9 concerns when "fooba" has been scanned and an 'r' does not follow.
The final comment reminds us that there's no point going to all the trouble of removing backing up from the rules unless we're using `-Cf' or `-CF', since there's no performance gain doing so with compressed scanners.
The way to remove the backing up is to add "error" rules:
%% foo return TOK_KEYWORD; foobar return TOK_KEYWORD; fooba | foob | fo { /* false alarm, not really a keyword */ return TOK_ID; }
Eliminating backing up among a list of keywords can also be done using a "catch-all" rule:
%% foo return TOK_KEYWORD; foobar return TOK_KEYWORD; [a-z]+ return TOK_ID;
This is usually the best solution when appropriate.
Backing up messages tend to cascade. With a complicated set of rules it's not
uncommon to get hundreds of messages. If one can decipher them, though, it often
only takes a dozen or so rules to eliminate the backing up (though it's easy to
make a mistake and have an error rule accidentally match a valid token. A
possible future flex
feature will be to automatically add rules to
eliminate backing up).
It's important to keep in mind that you gain the benefits of eliminating backing up only if you eliminate every instance of backing up. Leaving just one means you gain nothing.
Variable trailing context (where both the leading and trailing
parts do not have a fixed length) entails almost the same performance loss as
REJECT
(i.e., substantial). So when possible a rule like:
%% mouse|rat/(cat|dog) run();
is better written:
%% mouse/cat|dog run(); rat/cat|dog run();
or as
%% mouse|rat/cat run(); mouse|rat/dog run();
Note that here the special '|' action does not provide any savings, and can even make things worse (see Deficiencies / Bugs below).
Another area where the user can increase a scanner's performance (and one
that's easier to implement) arises from the fact that the longer the tokens
matched, the faster the scanner will run. This is because with long tokens the
processing of most input characters takes place in the (short) inner scanning
loop, and does not often have to go through the additional work of setting up
the scanning environment (e.g., yytext
) for the action. Recall the
scanner for C comments:
%x comment %% int line_num = 1; "/*" BEGIN(comment); <comment>[^*\n]* <comment>"*"+[^*/\n]* <comment>\n ++line_num; <comment>"*"+"/" BEGIN(INITIAL);
This could be sped up by writing it as:
%x comment %% int line_num = 1; "/*" BEGIN(comment); <comment>[^*\n]* <comment>[^*\n]*\n ++line_num; <comment>"*"+[^*/\n]* <comment>"*"+[^*/\n]*\n ++line_num; <comment>"*"+"/" BEGIN(INITIAL);
Now instead of each newline requiring the processing of another action, recognizing the newlines is "distributed" over the other rules to keep the matched text as long as possible. Note that adding rules does not slow down the scanner! The speed of the scanner is independent of the number of rules or (modulo the considerations given at the beginning of this section) how complicated the rules are with regard to operators such as '*' and '|'.
A final example in speeding up a scanner: suppose you want to scan through a file containing identifiers and keywords, one per line and with no other extraneous characters, and recognize all the keywords. A natural first approach is:
%% asm | auto | break | ... etc ... volatile | while /* it's a keyword */ .|\n /* it's not a keyword */
To eliminate the back-tracking, introduce a catch-all rule:
%% asm | auto | break | ... etc ... volatile | while /* it's a keyword */ [a-z]+ | .|\n /* it's not a keyword */
Now, if it's guaranteed that there's exactly one word per line, then we can reduce the total number of matches by a half by merging in the recognition of newlines with that of the other tokens:
%% asm\n | auto\n | break\n | ... etc ... volatile\n | while\n /* it's a keyword */ [a-z]+\n | .|\n /* it's not a keyword */
One has to be careful here, as we have now reintroduced backing up into the
scanner. In particular, while we know that there will never be any
characters in the input stream other than letters or newlines, flex
can't figure this out, and it will plan for possibly needing to back up when it
has scanned a token like "auto" and then the next character is something other
than a newline or a letter. Previously it would then just match the "auto" rule
and be done, but now it has no "auto" rule, only a "auto\n" rule. To eliminate
the possibility of backing up, we could either duplicate all rules but without
final newlines, or, since we never expect to encounter such an input and
therefore don't how it's classified, we can introduce one more catch-all rule,
this one which doesn't include a newline:
%% asm\n | auto\n | break\n | ... etc ... volatile\n | while\n /* it's a keyword */ [a-z]+\n | [a-z]+ | .|\n /* it's not a keyword */
Compiled with `-Cf', this is about as fast as one can get a
flex
scanner to go for this particular problem.
A final note: flex
is slow when matching NUL's, particularly
when a token contains multiple NUL's. It's best to write rules which match
short amounts of text if it's anticipated that the text will often
include NUL's.
Another final note regarding performance: as mentioned above in the section
How the Input is Matched, dynamically resizing yytext
to
accommodate huge tokens is a slow process because it presently requires that the
(huge) token be rescanned from the beginning. Thus if performance is vital, you
should attempt to match "large" quantities of text but not "huge" quantities,
where the cutoff between the two is at about 8K characters/token.
flex
provides two different ways to generate scanners for use
with C++. The first way is to simply compile a scanner generated by
flex
using a C++ compiler instead of a C compiler. You should not
encounter any compilations errors (please report any you find to the email
address given in the Author section below). You can then use C++ code in your
rule actions instead of C code. Note that the default input source for your
scanner remains yyin
, and default echoing is still done to
yyout
. Both of these remain `FILE *' variables and not
C++ streams
.
You can also use flex
to generate a C++ scanner class, using the
`-+' option, (or, equivalently, `%option c++'), which
is automatically specified if the name of the flex executable ends in a
`+', such as flex++
. When using this option, flex
defaults to generating the scanner to the file `lex.yy.cc' instead of
`lex.yy.c'. The generated scanner includes the header file
`FlexLexer.h', which defines the interface to two C++ classes.
The first class, FlexLexer
, provides an abstract base class
defining the general scanner class interface. It provides the following member
functions:
yytext
.
yyleng
.
yy_flex_debug
(see the Options section above). Note that you must
build the scanner using `%option debug' to include debugging
information in it.
Also provided are member functions equivalent to `yy_switch_to_buffer(), yy_create_buffer()' (though the first argument is an `istream*' object pointer and not a `FILE*', `yy_flush_buffer()', `yy_delete_buffer()', and `yyrestart()' (again, the first argument is a `istream*' object pointer).
The second class defined in `FlexLexer.h' is
yyFlexLexer
, which is derived from FlexLexer
. It
defines the following additional member functions:
yyFlexLexer
object using the given streams for
input and output. If not specified, the streams default to cin
and cout
, respectively.
yyFlexLexer
and want to access the member functions and variables
of S inside `yylex()', then you need to use
`%option yyclass="S"' to inform flex
that
you will be using that subclass instead of yyFlexLexer
. In this
case, rather than generating `yyFlexLexer::yylex()',
flex
generates `S::yylex()' (and also
generates a dummy `yyFlexLexer::yylex()' that calls
`yyFlexLexer::LexerError()' if called).
yyin
to new_in
(if non-nil) and
yyout
to new_out
(ditto), deleting the previous
input buffer if yyin
is reassigned.
In addition, yyFlexLexer
defines the following protected virtual
functions which you can redefine in derived classes to tailor the scanner:
YY_INTERACTIVE
. If you
redefine LexerInput()
and need to take different actions
depending on whether or not the scanner might be scanning an interactive input
source, you can test for the presence of this name via `#ifdef'.
cerr
and exits. Note that a yyFlexLexer
object contains its entire
scanning state. Thus you can use such objects to create reentrant scanners. You
can instantiate multiple instances of the same yyFlexLexer
class,
and you can also combine multiple C++ scanner classes together in the same
program using the `-P' option discussed above. Finally, note that
the `%array' feature is not available to C++ scanner classes; you
must use `%pointer' (the default).
Here is an example of a simple C++ scanner:
// An example of using the flex C++ scanner class. %{ int mylineno = 0; %} string \"[^\n"]+\" ws [ \t]+ alpha [A-Za-z] dig [0-9] name ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])* num1 [-+]?{dig}+\.?([eE][-+]?{dig}+)? num2 [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)? number {num1}|{num2} %% {ws} /* skip blanks and tabs */ "/*" { int c; while((c = yyinput()) != 0) { if(c == '\n') ++mylineno; else if(c == '*') { if((c = yyinput()) == '/') break; else unput(c); } } } {number} cout << "number " << YYText() << '\n'; \n mylineno++; {name} cout << "name " << YYText() << '\n'; {string} cout << "string " << YYText() << '\n'; %% Version 2.5 December 1994 44 int main( int /* argc */, char** /* argv */ ) { FlexLexer* lexer = new yyFlexLexer; while(lexer->yylex() != 0) ; return 0; }
If you want to create multiple (different) lexer classes, you use the
`-P' flag (or the `prefix=' option) to rename each
yyFlexLexer
to some other xxFlexLexer
. You then can
include `<FlexLexer.h>' in your other sources once per lexer
class, first renaming yyFlexLexer
as follows:
#undef yyFlexLexer #define yyFlexLexer xxFlexLexer #include <FlexLexer.h> #undef yyFlexLexer #define yyFlexLexer zzFlexLexer #include <FlexLexer.h>
if, for example, you used `%option prefix="xx"' for one of your scanners and `%option prefix="zz"' for the other.
IMPORTANT: the present form of the scanning class is experimental and may change considerably between major releases.
lex
and POSIXflex
is a rewrite of the AT&T Unix lex
tool
(the two implementations do not share any code, though), with some extensions
and incompatibilities, both of which are of concern to those who wish to write
scanners acceptable to either implementation. Flex is fully compliant with the
POSIX lex
specification, except that when using
`%pointer' (the default), a call to `unput()' destroys
the contents of yytext
, which is counter to the POSIX
specification.
In this section we discuss all of the known areas of incompatibility between flex, AT&T lex, and the POSIX specification.
flex's
`-l' option turns on maximum compatibility
with the original AT&T lex
implementation, at the cost of a
major loss in the generated scanner's performance. We note below which
incompatibilities can be overcome using the `-l' option.
flex
is fully compatible with lex
with the
following exceptions:
lex
scanner internal variable
yylineno
is not supported unless `-l' or
`%option yylineno' is used. yylineno
should be
maintained on a per-buffer basis, rather than a per-scanner (single global
variable) basis. yylineno
is not part of the POSIX specification.
EOF
. Input is instead controlled by
defining the YY_INPUT
macro. The flex
restriction
that `input()' cannot be redefined is in accordance with the
POSIX specification, which simply does not specify any way of controlling the
scanner's input other than by making an initial assignment to
yyin
.
flex
scanners are not as reentrant as lex
scanners. In particular, if you have an interactive scanner and an interrupt
handler which long-jumps out of the scanner, and the scanner is subsequently
called again, you may get the following message: fatal flex scanner internal error--end of buffer missedTo reenter the scanner, first use
yyrestart( yyin );Note that this call will throw away any buffered input; usually this isn't a problem with an interactive scanner. Also note that flex C++ scanner classes are reentrant, so if using C++ is an option for you, you should use them instead. See "Generating C++ Scanners" above for details.
yyout
(default stdout
). `output()' is not part of the
POSIX specification.
lex
does not support exclusive start conditions (%x), though
they are in the POSIX specification.
flex
encloses them in
parentheses. With lex, the following: NAME [A-Z][A-Z0-9]* %% foo{NAME}? printf( "Found it\n" ); %%will not match the string "foo" because when the macro is expanded the rule is equivalent to "foo[A-Z][A-Z0-9]*?" and the precedence is such that the '?' is associated with "[A-Z0-9]*". With
flex
, the rule will be
expanded to "foo([A-Z][A-Z0-9]*)?" and so the string "foo" will match. Note
that if the definition begins with `^' or ends with
`$' then it is not expanded with parentheses, to allow
these operators to appear in definitions without losing their special
meanings. But the `<s>, /', and
`<<EOF>>' operators cannot be used in a
flex
definition. Using `-l' results in the
lex
behavior of no parentheses around the definition. The POSIX
specification is that the definition be enclosed in parentheses.
lex
allow a rule's action to begin on
a separate line, if the rule's pattern has trailing whitespace: %% foo|bar<space here> { foobar_action(); }
flex
does not support this feature.
lex
`%r' (generate a Ratfor scanner) option
is not supported. It is not part of the POSIX specification.
yytext
is undefined
until the next token is matched, unless the scanner was built using
`%array'. This is not the case with lex
or the POSIX
specification. The `-l' option does away with this
incompatibility.
lex
interprets "abc{1,3}" as "match one, two, or three
occurrences of 'abc'", whereas flex
interprets it as "match 'ab'
followed by one, two, or three occurrences of 'c'". The latter is in agreement
with the POSIX specification.
lex
interprets "^foo|bar" as "match either 'foo' at the beginning
of a line, or 'bar' anywhere", whereas flex
interprets it as
"match either 'foo' or 'bar' if they come at the beginning of a line". The
latter is in agreement with the POSIX specification.
lex
are not required by flex
scanners;
flex
ignores them.
flex
or lex
. Scanners also include
YY_FLEX_MAJOR_VERSION
and YY_FLEX_MINOR_VERSION
indicating which version of flex
generated the scanner (for
example, for the 2.5 release, these defines would be 2 and 5 respectively).
The following flex
features are not included in lex
or the POSIX specification:
C++ scanners %option start condition scopes start condition stacks interactive/non-interactive scanners yy_scan_string() and friends yyterminate() yy_set_interactive() yy_set_bol() YY_AT_BOL() <<EOF>> <*> YY_DECL YY_START YY_USER_ACTION YY_USER_INIT #line directives %{}'s around actions multiple actions on a line
plus almost all of the flex flags. The last feature in the list refers to the
fact that with flex
you can put multiple actions on the same line,
separated with semicolons, while with lex
, the following
foo handle_foo(); ++num_foos_seen;
is (rather surprisingly) truncated to
foo handle_foo();
flex
does not truncate the action. Actions that are not enclosed
in braces are simply terminated at the end of the line.
[a-z]+ got_identifier(); foo got_foo();Using
REJECT
in a scanner suppresses this warning.
REJECT
or `yymore()' but that flex
failed to notice the fact, meaning that flex
scanned the first
two sections looking for occurrences of these actions and failed to find any,
but somehow you snuck some in (via a #include file, for example). Use
`%option reject' or `%option yymore' to indicate to
flex that you really do use these features.
MAX
constant (8K bytes
by default). You can increase the value by #define'ing YYLMAX
in
the definitions section of your flex
input.
yytext
. Ideally the scanner should dynamically resize
the buffer in this case, but at present it does not.
REJECT
.
yyrestart( yyin );or, as noted above, switch to using the C++ scanner class.
FlexLexer
,
and its derived class, yyFlexLexer
.
Some trailing context patterns cannot be properly matched and generate warning messages ("dangerous trailing context"). These are patterns where the ending of the first part of the rule matches the beginning of the second part, such as "zx*/xy*", where the 'x*' matches the 'x' at the beginning of the trailing context. (Note that the POSIX draft states that the text matched by such patterns is undefined.)
For some trailing context rules, parts which are actually fixed-length are not recognized as such, leading to the abovementioned performance loss. In particular, parts using '|' or {n} (such as "foo{3}") are always considered variable-length.
Combining trailing context with the special '|' action can result in fixed trailing context being turned into the more expensive variable trailing context. For example, in the following:
%% abc | xyz/def
Use of `unput()' invalidates yytext and yyleng, unless the `%array' directive or the `-l' option has been used.
Pattern-matching of NUL's is substantially slower than matching other characters.
Dynamic resizing of the input buffer is slow, as it entails rescanning all the text matched so far by the current (generally huge) token.
Due to both buffering of input and read-ahead, you cannot intermix calls to
<stdio.h> routines, such as, for example, `getchar()', with
flex
rules and expect it to work. Call `input()'
instead.
The total table entries listed by the `-v' flag excludes the
number of table entries needed to determine what rule has been matched. The
number of entries is equal to the number of DFA states if the scanner does not
use REJECT
, and somewhat greater than the number of states if it
does.
REJECT
cannot be used with the `-f' or
`-F' options.
The flex
internal algorithms need documentation.
lex
(1), yacc
(1), sed
(1),
awk
(1).
John Levine, Tony Mason, and Doug Brown: Lex & Yacc; O'Reilly and Associates. Be sure to get the 2nd edition.
M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator.
Alfred Aho, Ravi Sethi and Jeffrey Ullman: Compilers: Principles, Techniques
and Tools; Addison-Wesley (1986). Describes the pattern-matching techniques used
by flex
(deterministic finite automata).
Vern Paxson, with the help of many ideas and much inspiration from Van Jacobson. Original version by Jef Poskanzer. The fast table representation is a partial implementation of a design done by Van Jacobson. The implementation was done by Kevin Gong and Vern Paxson.
Thanks to the many flex
beta-testers, feedbackers, and
contributors, especially Francois Pinard, Casey Leedom, Stan Adermann, Terry
Allen, David Barker-Plummer, John Basrai, Nelson H.F. Beebe,
`benson@odi.com', Karl Berry, Peter A. Bigot, Simon Blanchard,
Keith Bostic, Frederic Brehm, Ian Brockbank, Kin Cho, Nick Christopher, Brian
Clapper, J.T. Conklin, Jason Coughlin, Bill Cox, Nick Cropper, Dave Curtis,
Scott David Daniels, Chris G. Demetriou, Theo Deraadt, Mike Donahue, Chuck
Doucette, Tom Epperly, Leo Eskin, Chris Faylor, Chris Flatters, Jon Forrest, Joe
Gayda, Kaveh R. Ghazi, Eric Goldman, Christopher M. Gould, Ulrich Grepel, Peer
Griebel, Jan Hajic, Charles Hemphill, NORO Hideo, Jarkko Hietaniemi, Scott
Hofmann, Jeff Honig, Dana Hudes, Eric Hughes, John Interrante, Ceriel Jacobs,
Michal Jaegermann, Sakari Jalovaara, Jeffrey R. Jones, Henry Juengst, Klaus
Kaempf, Jonathan I. Kamens, Terrence O Kane, Amir Katz,
`ken@ken.hilco.com', Kevin B. Kenny, Steve Kirsch, Winfried Koenig,
Marq Kole, Ronald Lamprecht, Greg Lee, Rohan Lenard, Craig Leres, John Levine,
Steve Liddle, Mike Long, Mohamed el Lozy, Brian Madsen, Malte, Joe Marshall,
Bengt Martensson, Chris Metcalf, Luke Mewburn, Jim Meyering, R. Alexander
Milowski, Erik Naggum, G.T. Nicol, Landon Noll, James Nordby, Marc Nozell,
Richard Ohnemus, Karsten Pahnke, Sven Panne, Roland Pesch, Walter Pelissero,
Gaumond Pierre, Esmond Pitt, Jef Poskanzer, Joe Rahmeh, Jarmo Raiha, Frederic
Raimbault, Pat Rankin, Rick Richardson, Kevin Rodgers, Kai Uwe Rommel, Jim
Roskind, Alberto Santini, Andreas Scherer, Darrell Schiebel, Raf Schietekat,
Doug Schmidt, Philippe Schnoebelen, Andreas Schwab, Alex Siegel, Eckehard Stolz,
Jan-Erik Strvmquist, Mike Stump, Paul Stuart, Dave Tallman, Ian Lance Taylor,
Chris Thewalt, Richard M. Timoney, Jodi Tsai, Paul Tuinenga, Gary Weik, Frank
Whaley, Gerhard Wilhelms, Kent Williams, Ken Yap, Ron Zellar, Nathan Zelle,
David Zuhn, and those whose names have slipped my marginal mail-archiving skills
but whose contributions are appreciated all the same.
Thanks to Keith Bostic, Jon Forrest, Noah Friedman, John Gilmore, Craig Leres, John Levine, Bob Mulcahy, G.T. Nicol, Francois Pinard, Rich Salz, and Richard Stallman for help with various distribution headaches.
Thanks to Esmond Pitt and Earle Horton for 8-bit character support; to Benson Margulies and Fred Burke for C++ support; to Kent Williams and Tom Epperly for C++ class support; to Ove Ewerlid for support of NUL's; and to Eric Hughes for support of multiple buffers.
This work was primarily done when I was with the Real Time Systems Group at the Lawrence Berkeley Laboratory in Berkeley, CA. Many thanks to all there for the support I received.
Send comments to `vern@ee.lbl.gov'.
This document was generated on 7 November 1998 using the texi2html translator version 1.52.