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Input and Output Files

flex's actions are specified by definitions (which may include embedded C code) in one or more input files. The primary output file is `lex.yy.c'. You can also use some of the command-line options to get diagnostic output (see section Command-line Options). This chapter gives the details of how to structure your input to define the scanner you need.

Format of the Input File

The flex input file consists of three sections, separated by a line with just `%%' in it:

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-whitespace 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


is identical to


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 `%{}' removed). The `%{}' 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, an unindented comment (i.e., a line beginning with `/*') is also copied verbatim to the output up to the next `*/'. Also, any line in the definitions section beginning with `#' is ignored, though this style of comment is deprecated and may go away in the future.

Patterns in the Input

The patterns in the input are written using an extended set of regular expressions. These are:

match the character `x'

any character except newline

a "character class"; in this case, the pattern matches either an `x', a `y', or a `z'

a "character class" with a range in it; matches an `a', a `b', any letter from `j' through `o', or a `Z'

a "negated character class", i.e., any character but those in the class. In this case, any character except an uppercase letter.

any character except an uppercase letter or a newline

zero or more r's, where r is any regular expression

one or more r's

r? zero or one r's (that is, "an optional r")

anywhere from two to five r's

two or more r's

exactly 4 r's

the expansion of the name definition (see above)

the literal string: `[xyz]"foo'

if X is an `a', `b', `f', `n', `r', `t', or `v', then the ANSI C interpretation of `\X'. Otherwise, a literal `X' (used to escape operators such as `*')

the character with octal value 123

the character with hexadecimal value 2a

match an r; parentheses are used to override precedence (see below)

the regular expression r followed by the regular expression s; called "concatenation"

either an r or an s

an r but only if it is followed by an s. The s is not part of the matched text. This type of pattern is called trailing context.

an r, but only at the beginning of a line

an r, but only at the end of a line. Equivalent to `r/\n'.

an r, but only in start condition s (see below for discussion of start conditions)

same, but in any of start conditions s1, s2, or s3

an end-of-file

an end-of-file when in start condition s1 or s2

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,


is the same as


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 instances of `r'. To match `foo' or zero or more instances of `bar', use:


and to match zero or more instances of either `foo' or `bar':


Some notes on patterns:

The following are illegal:


You can write the first of these instead as `foo/bar\n'.

In the following examples, `$' and `^' are treated as normal characters:


If what you want to specify is "either `foo', or `bar' followed by a newline" you can use the following (the special `|' action is explained below):

foo      |
bar$     /* action goes here */

A similar trick will work for matching "either `foo', or `bar' at the beginning of a line."

How the Input is Matched

When the generated scanner runs, 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.


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. Once it reaches an end-of-file, however, then any subsequent call to yylex will simply immediately return, unless yyrestart is first called (see below).

Actions are not allowed to modify `yytext' or `yyleng'.

There are a number of special directives which can be included within an action:

copies yytext to the scanner's output.

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

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 section 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' in the input would not be counted as a word, since the scanner normally executes only one action per token. Multiple REJECT actions 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 `-f' or `-F' 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.

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'. The presence of yymore in the scanner's action entails a minor performance penalty in the scanner's matching speed.

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;

`yyless(0)' 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.

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;
unput( ')' );
for ( i = yyleng - 1; i >= 0; --i )
    unput( yytext[i] );
unput( '(' );

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.

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" );

(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 named input.)

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'. Subsequent calls to the scanner will immediately return unless preceded by a call to yyrestart (see below). By default, yyterminate is also called when an end-of-file is encountered. It is a macro and may be redefined.

The Generated Scanner

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 redefining the YY_DECL macro. For example, you could use:

#undef YY_DECL
#define YY_DECL float lexscan( a, b ) float a, b;

to give the scanning routine the name lexscan, returning a float, and taking two float values 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 semicolon (`;').

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. In the former case, when called again the scanner will immediately return unless yyrestart is called to point `yyin' at the new input file. (yyrestart takes one argument, a `FILE *' pointer.) In the latter case (i.e., when an action executes a return), the scanner may then be called again and it will resume scanning where it left off.

By default (and for efficiency), the scanner uses block-reads rather than simple getc calls to read characters from `yyin'. You can control how it gets input by redefining 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 redefinition of YY_INPUT (in the definitions section of the input file):

#undef YY_INPUT
#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.

You also can add in things like keeping track of the input line number this way; but don't expect your scanner to go very fast.

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.

The default yywrap always returns 1. At present, to redefine it you must first `#undef yywrap', as it is currently implemented as a macro. As indicated by the hedging in the previous sentence, it may be changed to a true function in the near future.

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.

Start Conditions

flex provides a mechanism for conditionally activating rules. Any rule whose pattern is prefixed with `<sc>' will only be active when the scanner is in the start condition named sc. For example,

<STRING>[^"]*        { /* eat up the string body ... */

will be active only when the scanner is in the `STRING' start condition, and

<INITIAL,STRING,QUOTE>\.        { /* handle an escape ... */

will be active only when the current start condition is either `INITIAL', `STRING', or `QUOTE'.

Start conditions are declared in the definitions (first) section of the input using unindented lines beginning with either `%s' or `%x' followed by a list of names. The former declares inclusive start conditions, the latter exclusive start conditions. A start condition is activated using the BEGIN action. Until the next BEGIN action is executed, rules with the given start condition will be active and rules with other start conditions will be inactive. If the start condition is inclusive, then rules with no start conditions at all will also be active. If it is exclusive, then only rules qualified with the start condition will be active. A set of rules contingent on the same exclusive start condition describe a scanner which is independent of any of the other rules in the flex input. Because of this, exclusive start conditions make it easy to specify "miniscanners" 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 */

is equivalent to

%x example
<INITIAL,example>foo   /* do something */

The default rule (to ECHO any unmatched character) remains active in start conditions.

`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;

        if ( enter_special )

... 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 this scanner will treat the string 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

[0-9]+      {
            printf( "found an integer, = %d\n",
                    atoi( yytext ) );

"."         printf( "found a dot\n" );

Here is a scanner which recognizes (and discards) C comments while maintaining a count of the current input line.

%x comment
        int line_num = 1;

"/*"         BEGIN(comment);

<comment>[^*\n]*        /* eat anything that's not a '*' */
<comment>"*"+[^*/\n]*   /* eat up '*'s not followed by '/'s */
<comment>\n             ++line_num;
<comment>"*"+"/"        BEGIN(INITIAL);

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;


<foo>"/*"    {
             comment_caller = foo;

<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);

One can then implement a "stack" of start conditions using an array of integers. (It is likely that such stacks will become a full-fledged flex feature in the future.) Note, though, that start conditions do not have their own namespace; `%s' and `%x' declare names in the same fashion as #define.

Multiple Input Buffers

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:

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 sets things up for continued scanning, instead of opening a new file and pointing `yyin' at it.

void yy_delete_buffer( YY_BUFFER_STATE buffer )

is used to reclaim the storage associated with a buffer.

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

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++] =

        yyin = fopen( yytext, "r" );

        if ( ! yyin )
            error( ... );

            yy_create_buffer( yyin, YY_BUF_SIZE ) );


<<EOF>> {
        if ( --include_stack_ptr < 0 )

                 include_stack[include_stack_ptr] );

End-of-File Rules

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:

`<<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


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" );
<<EOF>>  {
         if ( *++filelist )
             yyin = fopen( *filelist, "r" );

Miscellaneous Macros

The macro YY_USER_ACTION can be redefined to provide an action which is always executed prior to the matched rule's action. For example, it could be #defined to call a routine to convert yytext to lower-case.

The macro YY_USER_INIT may be redefined 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.

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.

Interfacing with Parser Generators

One of the main uses of flex is as a companion to parser generators like yacc. 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, specify 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. Then include this file 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;

Translation Table

In the name of POSIX compliance, flex supports a translation table for mapping input characters into groups. The table is specified in the first section, and its format looks like:

1        abcd
52       0123456789
6        \t\ \n

This example specifies that the characters `a', `b', `c', and `d' are to all be lumped into group #1, upper-case letters in group #2, digits in group #52, tabs, blanks, and newlines into group #6, and no other characters will appear in the patterns. The group numbers are actually disregarded by flex; %t serves, though, to lump characters together. Given the above table, for example, the pattern `a(AA)*5' is equivalent to `d(ZQ)*0'. They both say, "match any character in group #1, followed by zero or more pairs of characters from group #2, followed by a character from group #52." Thus `%t' provides a crude way for introducing equivalence classes into the scanner specification.

Note that the `-i' option (see below) coupled with the equivalence classes which flex automatically generates take care of virtually all the instances when one might consider using `%t'. But what the hell, it's there if you want it.

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