| FLEX(1) | General Commands Manual | FLEX(1) | 
    Description
        a brief overview of the tool
    Some Simple Examples
    Format Of The Input File
    Patterns
        the extended regular expressions used by flex
    How The Input Is Matched
        the rules for determining what has been matched
    Actions
        how to specify what to do when a pattern is matched
    The Generated Scanner
        details regarding the scanner that flex produces;
        how to control the input source
    Start Conditions
        introducing context into your scanners, and
        managing "mini-scanners"
    Multiple Input Buffers
        how to manipulate multiple input sources; how to
        scan from strings instead of files
    End-of-file Rules
        special rules for matching the end of the input
    Miscellaneous Macros
        a summary of macros available to the actions
    Values Available To The User
        a summary of values available to the actions
    Interfacing With Yacc
        connecting flex scanners together with yacc parsers
    Options
        flex command-line options, and the "%option"
        directive
    Performance Considerations
        how to make your scanner go as fast as possible
    Generating C++ Scanners
        the (experimental) facility for generating C++
        scanner classes
    Incompatibilities With Lex And POSIX
        how flex differs from AT&T lex and the POSIX lex
        standard
    Diagnostics
        those error messages produced by flex (or scanners
        it generates) whose meanings might not be apparent
    Files
        files used by flex
    Deficiencies / Bugs
        known problems with flex
    See Also
        other documentation, related tools
    Author
        includes contact information
    %%
    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.
    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 "*/".
    x          match the character 'x'
    .          any character (byte) except newline
    [xyz]      a "character class"; in this case, the pattern
                 matches either an 'x', a 'y', or a 'z'
    [abj-oZ]   a "character class" with a range in it; matches
                 an 'a', a 'b', any letter from 'j' through 'o',
                 or a 'Z'
    [^A-Z]     a "negated character class", i.e., any character
                 but those in the class.  In this case, any
                 character EXCEPT an uppercase letter.
    [^A-Z\n]   any character EXCEPT an uppercase letter or
                 a newline
    r*         zero or more r's, where r is any regular expression
    r+         one or more r's
    r?         zero or one r's (that is, "an optional r")
    r{2,5}     anywhere from two to five r's
    r{2,}      two or more r's
    r{4}       exactly 4 r's
    {name}     the expansion of the "name" definition
               (see above)
    "[xyz]\"foo"
               the literal string: [xyz]"foo
    \X         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 '*')
    \0         a NUL character (ASCII code 0)
    \123       the character with octal value 123
    \x2a       the character with hexadecimal value 2a
    (r)        match an r; parentheses are used to override
                 precedence (see below)
    rs         the regular expression r followed by the
                 regular expression s; called "concatenation"
    r|s        either an r or an s
    r/s        an r but only if it is followed by an s.  The
                 text matched by s is included when determining
                 whether this rule is the "longest match",
                 but is then returned to the input before
                 the action is executed.  So the action only
                 sees the text matched by r.  This type
                 of pattern is called trailing context".
                 (There are some combinations of r/s that flex
                 cannot match correctly; see notes in the
                 Deficiencies / Bugs section below regarding
                 "dangerous trailing context".)
    ^r         an r, but only at the beginning of a line (i.e.,
                 which just starting to scan, or right after a
                 newline has been scanned).
    r$         an r, but only at the end of a line (i.e., just
                 before a newline).  Equivalent to "r/\n".
               Note that flex's notion of "newline" is exactly
               whatever the C compiler used to compile flex
               interprets '\n' as; in particular, on some DOS
               systems you must either filter out \r's in the
               input yourself, or explicitly use r/\r\n for "r$".
    <s>r       an r, but only in start condition s (see
                 below for discussion of start conditions)
    <s1,s2,s3>r
               same, but in any of start conditions s1,
                 s2, or s3
    <*>r       an r in any start condition, even an exclusive one.
    <<EOF>>    an end-of-file
    <s1,s2><<EOF>>
               an end-of-file when in start condition s1 or s2
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>bar
    
    Note that the first of these, can be written "foo/bar\n".
    foo|(bar$)
    foo|^bar
    
    If 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.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).
    %%
    "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:
            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).
    %%
    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.
    %%
    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.
    {
    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.)
    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.
    <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 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]+      {
                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:
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).
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. ( buffer can be
  nil, in which case the routine does nothing.) 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.
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:
<<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();
             }
    #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.
    %{
    #include "y.tab.h"
    %}
    %%
    [0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;
    --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.
    "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.
    slowest & smallest
          -Cem
          -Cm
          -Ce
          -C
          -C{f,F}e
          -C{f,F}
          -C{f,F}a
    fastest & largest
    
    Note 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.
    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.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:
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 (e.g., %option nounput ), 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).
    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 headscratching
  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.
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:
Also provided are member functions equivalent to yy_switch_to_buffer(), yy_create_buffer() (though the first argument is an std::istream* object pointer and not a FILE*), yy_flush_buffer(), yy_delete_buffer(), and yyrestart() (again, the first argument is a std::istream* object pointer).
The second class defined in FlexLexer.h is yyFlexLexer, which is derived from FlexLexer. It defines the following additional member functions:
In addition, yyFlexLexer defines the following protected virtual functions which you can redefine in derived classes to tailor the scanner:
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';
    %%
    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.
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:
    fatal flex scanner internal error--end of buffer missed
    
    To 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.
    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.
    %%
    foo|bar<space here>
      { foobar_action(); }
    
    flex does not support this feature.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 semi-colons, 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.
warning, -s option given but default rule can be matched means that it is possible (perhaps only in a particular start condition) that the default rule (match any single character) is the only one that will match a particular input. Since -s was given, presumably this is not intended.
reject_used_but_not_detected undefined or yymore_used_but_not_detected undefined - These errors can occur at compile time. They indicate that the scanner uses 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.
flex scanner jammed - a scanner compiled with -s has encountered an input string which wasn't matched by any of its rules. This error can also occur due to internal problems.
token too large, exceeds YYLMAX - your scanner uses %array and one of its rules matched a string longer than the YYLMAX constant (8K bytes by default). You can increase the value by #define'ing YYLMAX in the definitions section of your flex input.
scanner requires -8 flag to use the character 'x' - Your scanner specification includes recognizing the 8-bit character 'x' and you did not specify the -8 flag, and your scanner defaulted to 7-bit because you used the -Cf or -CF table compression options. See the discussion of the -7 flag for details.
flex scanner push-back overflow - you used unput() to push back so much text that the scanner's buffer could not hold both the pushed-back text and the current token in yytext. Ideally the scanner should dynamically resize the buffer in this case, but at present it does not.
input buffer overflow, can't enlarge buffer because scanner uses REJECT - the scanner was working on matching an extremely large token and needed to expand the input buffer. This doesn't work with scanners that use REJECT.
fatal flex scanner internal error--end of buffer missed - This can occur in an scanner which is reentered after a long-jump has jumped out (or over) the scanner's activation frame. Before reentering the scanner, use:
    yyrestart( yyin );
or, as noted above, switch to using the C++ scanner class.
too many start conditions in <> construct! - you listed more start conditions in a <> construct than exist (so you must have listed at least one of them twice).
For some trailing context rules, parts which are actually fixed-length are not recognized as such, leading to the above mentioned 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.
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).
Thanks to the many flex beta-testers, feedbackers, and contributors, especially Francois Pinard, Casey Leedom, Robert Abramovitz, Stan Adermann, Terry Allen, David Barker-Plummer, John Basrai, Neal Becker, 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 de Raadt, Mike Donahue, Chuck Doucette, Tom Epperly, Leo Eskin, Chris Faylor, Chris Flatters, Jon Forrest, Jeffrey Friedl, Joe Gayda, Kaveh R. Ghazi, Wolfgang Glunz, 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, David Loffredo, 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, Larry Schwimmer, 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.
| December 2021 | Version 2.5 |