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FLEX(1)                     General Commands Manual                    FLEX(1)

       flex, lex - fast lexical analyzer generator

       flex [-bcdfhilnpstvwBFILTV78+? -C[aefFmr] -ooutput -Pprefix -Sskeleton]
       [--help --version] [filename ...]

       This manual describes flex, a tool for generating programs that perform
       pattern-matching on text.  The manual includes both tutorial and
       reference sections:

               a brief overview of the tool

           Some Simple Examples

           Format Of The Input File

               the extended regular expressions used by flex

           How The Input Is Matched
               the rules for determining what has been matched

               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

               flex command-line options, and the "%option"

           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

               those error messages produced by flex (or scanners
               it generates) whose meanings might not be apparent

               files used by flex

           Deficiencies / Bugs
               known problems with flex

           See Also
               other documentation, related tools

               includes contact information

       flex is a tool for generating scanners: programs which recognized
       lexical patterns in text.  flex reads the given input files, or its
       standard input if no file names are given, for a description of a
       scanner to generate.  The description is in the form of pairs of
       regular expressions and C code, called rules.  flex generates as output
       a C source file, lex.yy.c, which defines a routine yylex().  This file
       is compiled and linked with the -lfl library to produce an executable.
       When the executable is run, it analyzes its input for occurrences of
       the regular expressions.  Whenever it finds one, it executes the
       corresponding C code.

       First some simple examples to get the flavor of how one uses flex.  The
       following flex input specifies a scanner which whenever it encounters
       the string "username" will replace it with the user's login name:

           username    printf( "%s", getlogin() );

       By default, any text not matched by a flex scanner is copied to the
       output, so the net effect of this scanner is to copy its input file to
       its output with each occurrence of "username" expanded.  In this input,
       there is just one rule.  "username" is the pattern and the "printf" is
       the action.  The "%%" marks the beginning of the rules.

       Here's another simple example:

                   int num_lines = 0, num_chars = 0;

           \n      ++num_lines; ++num_chars;
           .       ++num_chars;

                   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" );
                       yyin = stdin;


       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

       The details of this example will be explained in the following

       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-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


       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

           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

       In the definitions section (but not in the rules section), an
       unindented comment (i.e., a line beginning with "/*") is also copied
       verbatim to the output up to the next "*/".

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

           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)
                      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)
                      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
                      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,


       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 r's.  To match "foo" or zero-or-more "bar"'s, use:


       and to match zero-or-more "foo"'s-or-"bar"'s:


       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:


       If your scanner is case-insensitive (the -i flag), then [:upper:] and
       [:lower:] are equivalent to [:alpha:].

       Some notes on patterns:

       -      A negated character class such as the example "[^A-Z]" above
              will match a newline unless "\n" (or an equivalent escape
              sequence) is one of the characters explicitly present in the
              negated character class (e.g., "[^A-Z\n]").  This is unlike how
              many other regular expression tools treat negated character
              classes, but unfortunately the inconsistency is historically
              entrenched.  Matching newlines means that a pattern like [^"]*
              can match the entire input unless there's another quote in the

       -      A rule can have at most one instance of trailing context (the
              '/' operator or the '$' operator).  The start condition, '^',
              and "<<EOF>>" patterns can only occur at the beginning of a
              pattern, and, as well as with '/' and '$', cannot be grouped
              inside parentheses.  A '^' which does not occur at the beginning
              of a rule or a '$' which does not occur at the end of a rule
              loses its special properties and is treated as a normal

              The following are illegal:


              Note that the first of these, can be written "foo/bar\n".

              The following will result in '$' or '^' being treated as a
              normal character:


              If what's wanted is a "foo" or a bar-followed-by-a-newline, the
              following could be used (the special '|' action is explained

                  foo      |
                  bar$     /* action goes here */

              A similar trick will work for matching a foo or a bar-at-the-

       When the generated scanner is run, it analyzes its input looking for
       strings which match any of its patterns.  If it finds more than one
       match, it takes the one matching the most text (for trailing context
       rules, this includes the length of the trailing part, even though it
       will then be returned to the input).  If it finds two or more matches
       of the same length, the rule listed first in the flex input file is

       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

       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 defines yytext to be an array of YYLMAX characters, which
       defaults to a fairly large value.  You can change the size by simply
       #define'ing YYLMAX to a different value in the first section of your
       flex input.  As mentioned above, with %pointer yytext grows dynamically
       to accommodate large tokens.  While this means your %pointer scanner
       can accommodate very large tokens (such as matching entire blocks of
       comments), bear in mind that each time the scanner must resize yytext
       it also must rescan the entire token from the beginning, so matching
       such tokens can prove slow.  yytext presently does not dynamically grow
       if a call to unput() results in too much text being pushed back;
       instead, a run-time error results.

       Also note that you cannot use %array with C++ scanner classes (the c++
       option; see below).

       Each pattern in a rule has a corresponding action, which can be any
       arbitrary C statement.  The pattern ends at the first non-escaped
       whitespace character; the remainder of the line is its action.  If the
       action is empty, then when the pattern is matched the input token is
       simply discarded.  For example, here is the specification for a program
       which deletes all occurrences of "zap me" from its input:

           "zap me"

       (It will copy all other characters in the input to the output since
       they will be matched by the default rule.)

       Here is a program which compresses multiple blanks and tabs down to a
       single blank, and throws away whitespace found at the end of a line:

           [ \t]+        putchar( ' ' );
           [ \t]+$       /* ignore this token */

       If the action contains a '{', then the action spans till the balancing
       '}' is found, and the action may cross multiple lines.  flex knows
       about C strings and comments and won't be fooled by braces found within
       them, but also allows actions to begin with %{ and will consider the
       action to be all the text up to the next %} (regardless of ordinary
       braces inside the action).

       An action consisting solely of a vertical bar ('|') means "same as the
       action for the next rule."  See below for an illustration.

       Actions can include arbitrary C code, including return statements to
       return a value to whatever routine called yylex().  Each time yylex()
       is called it continues processing tokens from where it last left off
       until it either reaches the end of the file or executes a return.

       Actions are free to modify yytext except for lengthening it (adding
       characters to its end--these will overwrite later characters in the
       input stream).  This however does not apply when using %array (see
       above); in that case, yytext may be freely modified in any way.

       Actions are free to modify yyleng except they should not do so if the
       action also includes use of yymore() (see below).

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

       -      ECHO copies yytext to the scanner's output.

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

       -      REJECT directs the scanner to proceed on to the "second best"
              rule which matched the input (or a prefix of the input).  The
              rule is chosen as described above in "How the Input is Matched",
              and yytext and yyleng set up appropriately.  It may either be
              one which matched as much text as the originally chosen rule but
              came later in the flex input file, or one which matched less
              text.  For example, the following will both count the words in
              the input and call the routine special() whenever "frob" is

                          int word_count = 0;

                  frob        special(); REJECT;
                  [^ \t\n]+   ++word_count;

              Without the REJECT, any "frob"'s in the input would not be
              counted as words, since the scanner normally executes only one
              action per token.  Multiple REJECT's are allowed, each one
              finding the next best choice to the currently active rule.  For
              example, when the following scanner scans the token "abcd", it
              will write "abcdabcaba" to the output:

                  a        |
                  ab       |
                  abc      |
                  abcd     ECHO; REJECT;
                  .|\n     /* eat up any unmatched character */

              (The first three rules share the fourth's action since they use
              the special '|' action.)  REJECT is a particularly expensive
              feature in terms of scanner performance; if it is used in any of
              the scanner's actions it will slow down all of the scanner's
              matching.  Furthermore, REJECT cannot be used with the -Cf or
              -CF options (see below).

              Note also that unlike the other special actions, REJECT is a
              branch; code immediately following it in the action will not be

       -      yymore() tells the scanner that the next time it matches a rule,
              the corresponding token should be appended onto the current
              value of yytext rather than replacing it.  For example, given
              the input "mega-kludge" the following will write "mega-mega-
              kludge" to the output:

                  mega-    ECHO; yymore();
                  kludge   ECHO;

              First "mega-" is matched and echoed to the output.  Then
              "kludge" is matched, but the previous "mega-" is still hanging
              around at the beginning of yytext so the ECHO for the "kludge"
              rule will actually write "mega-kludge".

       Two notes regarding use of yymore().  First, yymore() depends on the
       value of yyleng correctly reflecting the size of the current token, so
       you must not modify yyleng if you are using yymore().  Second, the
       presence of yymore() in the scanner's action entails a minor
       performance penalty in the scanner's matching speed.

       -      yyless(n) returns all but the first n characters of the current
              token back to the input stream, where they will be rescanned
              when the scanner looks for the next match.  yytext and yyleng
              are adjusted appropriately (e.g., yyleng will now be equal to n
              ).  For example, on the input "foobar" the following will write
              out "foobarbar":

                  foobar    ECHO; yyless(3);
                  [a-z]+    ECHO;

              An argument of 0 to yyless will cause the entire current input
              string to be scanned again.  Unless you've changed how the
              scanner will subsequently process its input (using BEGIN, for
              example), this will result in an endless loop.

       Note that yyless is a macro and can only be used in the flex input
       file, not from other source files.

       -      unput(c) puts the character c back onto the input stream.  It
              will be the next character scanned.  The following action will
              take the current token and cause it to be rescanned enclosed in

                  int i;
                  /* Copy yytext because unput() trashes yytext */
                  char *yycopy = strdup( yytext );
                  unput( ')' );
                  for ( i = yyleng - 1; i >= 0; --i )
                      unput( yycopy[i] );
                  unput( '(' );
                  free( yycopy );

              Note that since each unput() puts the given character back at
              the beginning of the input stream, pushing back strings must be
              done back-to-front.

       An important potential problem when using unput() is that if you are
       using %pointer (the default), a call to unput() destroys the contents
       of yytext, starting with its rightmost character and devouring one
       character to the left with each call.  If you need the value of yytext
       preserved after a call to unput() (as in the above example), you must
       either first copy it elsewhere, or build your scanner using %array
       instead (see How The Input Is Matched).

       Finally, note that you cannot put back EOF to attempt to mark the input
       stream with an end-of-file.

       -      input() reads the next character from the input stream.  For
              example, the following is one way to eat up C comments:

                  "/*"        {
                              register int c;

                              for ( ; ; )
                                  while ( (c = input()) != '*' &&
                                          c != EOF )
                                      ;    /* eat up text of comment */

                                  if ( c == '*' )
                                      while ( (c = input()) == '*' )
                                      if ( c == '/' )
                                          break;    /* found the end */

                                  if ( c == EOF )
                                      error( "EOF in comment" );

              (Note that if the scanner is compiled using C++, then input() is
              instead referred to as yyinput(), in order to avoid a name clash
              with the C++ stream by the name of input.)

       -      YY_FLUSH_BUFFER flushes the scanner's internal buffer so that
              the next time the scanner attempts to match a token, it will
              first refill the buffer using YY_INPUT (see The Generated
              Scanner, below).  This action is a special case of the more
              general yy_flush_buffer() function, described below in the
              section Multiple Input Buffers.

       -      yyterminate() can be used in lieu of a return statement in an
              action.  It terminates the scanner and returns a 0 to the
              scanner's caller, indicating "all done".  By default,
              yyterminate() is also called when an end-of-file is encountered.
              It is a macro and may be redefined.

       The output of flex is the file lex.yy.c, which contains the scanning
       routine yylex(), a number of tables used by it for matching tokens, and
       a number of auxiliary routines and macros.  By default, yylex() is
       declared as follows:

           int yylex()
               ... various definitions and the actions in here ...

       (If your environment supports function prototypes, then it will be "int
       yylex( void )".)  This definition may be changed by defining the
       "YY_DECL" macro.  For example, you could use:

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

       to give the scanning routine the name lexscan, returning a float, and
       taking two floats as arguments.  Note that if you give arguments to the
       scanning routine using a K&R-style/non-prototyped function declaration,
       you must terminate the definition with a semi-colon (;).

       Whenever yylex() is called, it scans tokens from the global input file
       yyin (which defaults to stdin).  It continues until it either reaches
       an end-of-file (at which point it returns the value 0) or one of its
       actions executes a return statement.

       If the scanner reaches an end-of-file, subsequent calls are undefined
       unless either yyin is pointed at a new input file (in which case
       scanning continues from that file), or yyrestart() is called.
       yyrestart() takes one argument, a FILE * pointer (which can be nil, if
       you've set up YY_INPUT to scan from a source other than yyin), and
       initializes yyin for scanning from that file.  Essentially there is no
       difference between just assigning yyin to a new input file or using
       yyrestart() to do so; the latter is available for compatibility with
       previous versions of flex, and because it can be used to switch input
       files in the middle of scanning.  It can also be used to throw away the
       current input buffer, by calling it with an argument of yyin; but
       better is to use YY_FLUSH_BUFFER (see above).  Note that yyrestart()
       does not reset the start condition to INITIAL (see Start Conditions,

       If yylex() stops scanning due to executing a return statement in one of
       the actions, the scanner may then be called again and it will resume
       scanning where it left off.

       By default (and for purposes of efficiency), the scanner uses block-
       reads rather than simple getc() calls to read characters from yyin.
       The nature of how it gets its input can be controlled by defining the
       YY_INPUT macro.  YY_INPUT's calling sequence is
       "YY_INPUT(buf,result,max_size)".  Its action is to place up to max_size
       characters in the character array buf and return in the integer
       variable result either the number of characters read or the constant
       YY_NULL (0 on Unix systems) to indicate EOF.  The default YY_INPUT
       reads from the global file-pointer "yyin".

       A sample definition of YY_INPUT (in the definitions section of the
       input file):

           #define YY_INPUT(buf,result,max_size) \
               { \
               int c = getchar(); \
               result = (c == EOF) ? YY_NULL : (buf[0] = c, 1); \

       This definition will change the input processing to occur one character
       at a time.

       When the scanner receives an end-of-file indication from YY_INPUT, it
       then checks the yywrap() function.  If yywrap() returns false (zero),
       then it is assumed that the function has gone ahead and set up yyin to
       point to another input file, and scanning continues.  If it returns
       true (non-zero), then the scanner terminates, returning 0 to its
       caller.  Note that in either case, the start condition remains
       unchanged; it does not revert to INITIAL.

       If you do not supply your own version of yywrap(), then you must either
       use %option noyywrap (in which case the scanner behaves as though
       yywrap() returned 1), or you must link with -lfl to obtain the default
       version of the routine, which always returns 1.

       Three routines are available for scanning from in-memory buffers rather
       than files: yy_scan_string(), yy_scan_bytes(), and yy_scan_buffer().
       See the discussion of them below in the section Multiple Input Buffers.

       The scanner writes its ECHO output to the yyout global (default,
       stdout), which may be redefined by the user simply by assigning it to
       some other FILE pointer.

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

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

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

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

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

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

       If the distinction between inclusive and exclusive start conditions is
       still a little vague, here's a simple example illustrating the
       connection between the two.  The set of rules:

           %s example

           <example>foo   do_something();

           bar            something_else();

       is equivalent to

           %x example

           <example>foo   do_something();

           <INITIAL,example>bar    something_else();

       Without the <INITIAL,example> qualifier, the bar pattern in the second
       example wouldn't be active (i.e., couldn't match) when in start
       condition example.  If we just used <example> to qualify bar, though,
       then it would only be active in example and not in INITIAL, while in
       the first example it's active in both, because in the first example the
       example starting condition is an inclusive (%s) start condition.

       Also note that the special start-condition specifier <*> matches every
       start condition.  Thus, the above example could also have been written;

           %x example

           <example>foo   do_something();

           <*>bar    something_else();

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

           <*>.|\n     ECHO;

       BEGIN(0) returns to the original state where only the rules with no
       start conditions are active.  This state can also be referred to as the
       start-condition "INITIAL", so BEGIN(INITIAL) is equivalent to BEGIN(0).
       (The parentheses around the start condition name are not required but
       are considered good style.)

       BEGIN actions can also be given as indented code at the beginning of
       the rules section.  For example, the following will cause the scanner
       to enter the "SPECIAL" start condition whenever yylex() is called and
       the global variable enter_special is true:

                   int enter_special;

           %x SPECIAL
                   if ( enter_special )

           ...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

           [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

           %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);

       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 */
                   *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:


       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,

               "\\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

       void yy_push_state(int new_state)
              pushes the current start condition onto the top of the start
              condition stack and switches to new_state as though you had used
              BEGIN new_state (recall that start condition names are also

       void yy_pop_state()
              pops the top of the stack and switches to it via BEGIN.

       int yy_top_state()
              returns the top of the stack without altering the stack's

       The start condition stack grows dynamically and so has no built-in size
       limitation.  If memory is exhausted, program execution aborts.

       To use start condition stacks, your scanner must include a %option
       stack directive (see Options below).

       Some scanners (such as those which support "include" files) require
       reading from several input streams.  As flex scanners do a large amount
       of buffering, one cannot control where the next input will be read from
       by simply writing a YY_INPUT which is sensitive to the scanning
       context.  YY_INPUT is only called when the scanner reaches the end of
       its buffer, which may be a long time after scanning a statement such as
       an "include" which requires switching the input source.

       To negotiate these sorts of problems, flex provides a mechanism for
       creating and switching between multiple input buffers.  An input buffer
       is created by using:

           YY_BUFFER_STATE yy_create_buffer( FILE *file, int size )

       which takes a FILE pointer and a size and creates a buffer associated
       with the given file and large enough to hold size characters (when in
       doubt, use YY_BUF_SIZE for the size).  It returns a YY_BUFFER_STATE
       handle, which may then be passed to other routines (see below).  The
       YY_BUFFER_STATE type is a pointer to an opaque struct yy_buffer_state
       structure, so you may safely initialize YY_BUFFER_STATE variables to
       ((YY_BUFFER_STATE) 0) if you wish, and also refer to the opaque
       structure in order to correctly declare input buffers in source files
       other than that of your scanner.  Note that the FILE pointer in the
       call to yy_create_buffer is only used as the value of yyin seen by
       YY_INPUT; if you redefine YY_INPUT so it no longer uses yyin, then you
       can safely pass a nil FILE pointer to yy_create_buffer.  You select a
       particular buffer to scan from using:

           void yy_switch_to_buffer( YY_BUFFER_STATE new_buffer )

       switches the scanner's input buffer so subsequent tokens will come from
       new_buffer.  Note that yy_switch_to_buffer() may be used by yywrap() to
       set things up for continued scanning, instead of opening a new file and
       pointing yyin at it.  Note also that switching input sources via either
       yy_switch_to_buffer() or yywrap() does not change the start condition.

           void yy_delete_buffer( YY_BUFFER_STATE buffer )

       is used to reclaim the storage associated with a buffer.  ( 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++] =

                   yyin = fopen( yytext, "r" );

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

                       yy_create_buffer( yyin, YY_BUF_SIZE ) );


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

                       yy_delete_buffer( YY_CURRENT_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.

       yy_scan_string(const char *str)
              scans a NUL-terminated string.

       yy_scan_bytes(const char *bytes, int len)
              scans len bytes (including possibly NUL's) starting at location

       Note that both of these functions create and scan a copy of the string
       or bytes.  (This may be desirable, since yylex() modifies the contents
       of the buffer it is scanning.)  You can avoid the copy by using:

       yy_scan_buffer(char *base, yy_size_t size)
              which scans in place the buffer starting at base, consisting of
              size bytes, the last two bytes of which must be
              YY_END_OF_BUFFER_CHAR (ASCII NUL).  These last two bytes are not
              scanned; thus, scanning consists of base[0] through
              base[size-2], inclusive.

              If you fail to set up base in this manner (i.e., forget the
              final two YY_END_OF_BUFFER_CHAR bytes), then yy_scan_buffer()
              returns a nil pointer instead of creating a new input buffer.

              The type yy_size_t is an integral type to which you can cast an
              integer expression reflecting the size of the buffer.

       The special rule "<<EOF>>" indicates actions which are to be taken when
       an end-of-file is encountered and yywrap() returns non-zero (i.e.,
       indicates no further files to process).  The action must finish by
       doing one of four things:

       -      assigning yyin to a new input file (in previous versions of
              flex, after doing the assignment you had to call the special
              action YY_NEW_FILE; this is no longer necessary);

       -      executing a return statement;

       -      executing the special yyterminate() action;

       -      or, switching to a new buffer using yy_switch_to_buffer() as
              shown in the example above.

       <<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

           %x quote

           ...other rules for dealing with quotes...

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

       The macro YY_USER_ACTION can be defined to provide an action which is
       always executed prior to the matched rule's action.  For example, it
       could be #define'd to call a routine to convert yytext to lower-case.
       When YY_USER_ACTION is invoked, the variable yy_act gives the number of
       the matched rule (rules are numbered starting with 1).  Suppose you
       want to profile how often each of your rules is matched.  The following
       would do the trick:

           #define YY_USER_ACTION ++ctr[yy_act]

       where ctr is an array to hold the counts for the different rules.  Note
       that the macro YY_NUM_RULES gives the total number of rules (including
       the default rule, even if you use -s), so a correct declaration for ctr

           int ctr[YY_NUM_RULES];

       The macro YY_USER_INIT may be defined to provide an action which is
       always executed before the first scan (and before the scanner's
       internal initializations are done).  For example, it could be used to
       call a routine to read in a data table or open a logging file.

       The macro yy_set_interactive(is_interactive) can be used to control
       whether the current buffer is considered interactive.  An interactive
       buffer is processed more slowly, but must be used when the scanner's
       input source is indeed interactive to avoid problems due to waiting to
       fill buffers (see the discussion of the -I flag below).  A non-zero
       value in the macro invocation marks the buffer as interactive, a zero
       value as non-interactive.  Note that use of this macro overrides
       %option always-interactive or %option never-interactive (see Options
       below).  yy_set_interactive() must be invoked prior to beginning to
       scan the buffer that is (or is not) to be considered interactive.

       The macro yy_set_bol(at_bol) can be used to control whether the current
       buffer's scanning context for the next token match is done as though at
       the beginning of a line.  A non-zero macro argument makes rules
       anchored with

       The macro YY_AT_BOL() returns true if the next token scanned from the
       current buffer will have '^' rules active, false otherwise.

       In the generated scanner, the actions are all gathered in one large
       switch statement and separated using YY_BREAK, which may be redefined.
       By default, it is simply a "break", to separate each rule's action from
       the following rule's.  Redefining YY_BREAK allows, for example, C++
       users to #define YY_BREAK to do nothing (while being very careful that
       every rule ends with a "break" or a "return"!) to avoid suffering from
       unreachable statement warnings where because a rule's action ends with
       "return", the YY_BREAK is inaccessible.

       This section summarizes the various values available to the user in the
       rule actions.

       -      char *yytext holds the text of the current token.  It may be
              modified but not lengthened (you cannot append characters to the

              If the special directive %array appears in the first section of
              the scanner description, then yytext is instead declared char
              yytext[YYLMAX], where YYLMAX is a macro definition that you can
              redefine in the first section if you don't like the default
              value (generally 8KB).  Using %array results in somewhat slower
              scanners, but the value of yytext becomes immune to calls to
              input() and unput(), which potentially destroy its value when
              yytext is a character pointer.  The opposite of %array is
              %pointer, which is the default.

              You cannot use %array when generating C++ scanner classes (the
              -+ flag).

       -      int yyleng holds the length of the current token.

       -      FILE *yyin is the file which by default flex reads from.  It may
              be redefined but doing so only makes sense before scanning
              begins or after an EOF has been encountered.  Changing it in the
              midst of scanning will have unexpected results since flex
              buffers its input; use yyrestart() instead.  Once scanning
              terminates because an end-of-file has been seen, you can assign
              yyin at the new input file and then call the scanner again to
              continue scanning.

       -      void yyrestart( FILE *new_file ) may be called to point yyin at
              the new input file.  The switch-over to the new file is
              immediate (any previously buffered-up input is lost).  Note that
              calling yyrestart() with yyin as an argument thus throws away
              the current input buffer and continues scanning the same input

       -      FILE *yyout is the file to which ECHO actions are done.  It can
              be reassigned by the user.

       -      YY_CURRENT_BUFFER returns a YY_BUFFER_STATE handle to the
              current buffer.

       -      YY_START returns an integer value corresponding to the current
              start condition.  You can subsequently use this value with BEGIN
              to return to that start condition.

       One of the main uses of flex is as a companion to the yacc parser-
       generator.  yacc parsers expect to call a routine named yylex() to find
       the next input token.  The routine is supposed to return the type of
       the next token as well as putting any associated value in the global
       yylval.  To use flex with yacc, one specifies the -d option to yacc to
       instruct it to generate the file y.tab.h containing definitions of all
       the %tokens appearing in the yacc input.  This file is then included in
       the flex scanner.  For example, if one of the tokens is "TOK_NUMBER",
       part of the scanner might look like:

           #include "y.tab.h"


           [0-9]+        yylval = atoi( yytext ); return TOK_NUMBER;

       flex has the following options:

       -b, --backup
              Generate backing-up information to lex.backup.  This is a list
              of scanner states which require backing up and the input
              characters on which they do so.  By adding rules one can remove
              backing-up states.  If all backing-up states are eliminated and
              -Cf or -CF is used, the generated scanner will run faster (see
              the -p flag).  Only users who wish to squeeze every last cycle
              out of their scanners need worry about this option.  (See the
              section on Performance Considerations below.)

       -c     is a do-nothing, deprecated option included for POSIX

       -d, --debug
              makes the generated scanner run in debug mode.  Whenever a
              pattern is recognized and the global yy_flex_debug is non-zero
              (which is the default), the scanner will write to stderr a line
              of the form:

                  --accepting rule at line 53 ("the matched text")

              The line number refers to the location of the rule in the file
              defining the scanner (i.e., the file that was fed to flex).
              Messages are also generated when the scanner backs up, accepts
              the default rule, reaches the end of its input buffer (or
              encounters a NUL; at this point, the two look the same as far as
              the scanner's concerned), or reaches an end-of-file.

       -f, --full
              specifies fast scanner.  No table compression is done and stdio
              is bypassed.  The result is large but fast.  This option is
              equivalent to -Cfr (see below).

       -h, --help
              generates a "help" summary of flex's options to stdout and then
              exits.  -?  and --help are synonyms for -h.

       -i, --case-insensitive
              instructs flex to generate a case-insensitive scanner.  The case
              of letters given in the flex input patterns will be ignored, and
              tokens in the input will be matched regardless of case.  The
              matched text given in yytext will have the preserved case (i.e.,
              it will not be folded).

       -l, --lex-compat
              turns on maximum compatibility with the original AT&T lex
              implementation.  Note that this does not mean full
              compatibility.  Use of this option costs a considerable amount
              of performance, and it cannot be used with the -+, -f, -F, -Cf,
              or -CF options.  For details on the compatibilities it provides,
              see the section "Incompatibilities With Lex And POSIX" below.
              This option also results in the name YY_FLEX_LEX_COMPAT being
              #define'd in the generated scanner.

       -n     is another do-nothing, deprecated option included only for POSIX

       -p, --perf-report
              generates a performance report to stderr.  The report consists
              of comments regarding features of the flex input file which will
              cause a serious loss of performance in the resulting scanner.
              If you give the flag twice, you will also get comments regarding
              features that lead to minor performance losses.

              Note that the use of REJECT, %option yylineno, and variable
              trailing context (see the Deficiencies / Bugs section below)
              entails a substantial performance penalty; use of yymore(), the
              ^ operator, and the -I flag entail minor performance penalties.

       -s, --no-default
              causes the default rule (that unmatched scanner input is echoed
              to stdout) to be suppressed.  If the scanner encounters input
              that does not match any of its rules, it aborts with an error.
              This option is useful for finding holes in a scanner's rule set.

       -t, --stdout
              instructs flex to write the scanner it generates to standard
              output instead of lex.yy.c.

       -v, --verbose
              specifies that flex should write to stderr a summary of
              statistics regarding the scanner it generates.  Most of the
              statistics are meaningless to the casual flex user, but the
              first line identifies the version of flex (same as reported by
              -V), and the next line the flags used when generating the
              scanner, including those that are on by default.

       -w, --nowarn
              suppresses warning messages.

       -B, --batch
              instructs flex to generate a batch scanner, the opposite of
              interactive scanners generated by -I (see below).  In general,
              you use -B when you are certain that your scanner will never be
              used interactively, and you want to squeeze a little more
              performance out of it.  If your goal is instead to squeeze out a
              lot more performance, you should  be using the -Cf or -CF
              options (discussed below), which turn on -B automatically

       -F, --fast
              specifies that the fast scanner table representation should be
              used (and stdio bypassed).  This representation is about as fast
              as the full table representation (-f), and for some sets of
              patterns will be considerably smaller (and for others, larger).
              In general, if the pattern set contains both "keywords" and a
              catch-all, "identifier" rule, such as in the set:

                  "case"    return TOK_CASE;
                  "switch"  return TOK_SWITCH;
                  "default" return TOK_DEFAULT;
                  [a-z]+    return TOK_ID;

              then you're better off using the full table representation.  If
              only the "identifier" rule is present and you then use a hash
              table or some such to detect the keywords, you're better off
              using -F.

              This option is equivalent to -CFr (see below).  It cannot be
              used with -+.

       -I, --interactive
              instructs flex to generate an interactive scanner.  An
              interactive scanner is one that only looks ahead to decide what
              token has been matched if it absolutely must.  It turns out that
              always looking one extra character ahead, even if the scanner
              has already seen enough text to disambiguate the current token,
              is a bit faster than only looking ahead when necessary.  But
              scanners that always look ahead give dreadful interactive
              performance; for example, when a user types a newline, it is not
              recognized as a newline token until they enter another token,
              which often means typing in another whole line.

              Flex scanners default to interactive unless you use the -Cf or
              -CF table-compression options (see below).  That's because if
              you're looking for high-performance you should be using one of
              these options, so if you didn't, flex assumes you'd rather trade
              off a bit of run-time performance for intuitive interactive
              behavior.  Note also that you cannot use -I in conjunction with
              -Cf or -CF.  Thus, this option is not really needed; it is on by
              default for all those cases in which it is allowed.

              You can force a scanner to not be interactive by using -B (see

       -L, --noline
              instructs flex not to generate #line directives.  Without this
              option, flex peppers the generated scanner with #line directives
              so error messages in the actions will be correctly located with
              respect to either the original flex input file (if the errors
              are due to code in the input file), or lex.yy.c (if the errors
              are flex's fault -- you should report these sorts of errors to
              the email address given below).

       -T, --trace
              makes flex run in trace mode.  It will generate a lot of
              messages to stderr concerning the form of the input and the
              resultant non-deterministic and deterministic finite automata.
              This option is mostly for use in maintaining flex.

       -V, --version
              prints the version number to stdout and exits.  --version is a
              synonym for -V.

       -7, --7bit
              instructs flex to generate a 7-bit scanner, i.e., one which can
              only recognized 7-bit characters in its input.  The advantage of
              using -7 is that the scanner's tables can be up to half the size
              of those generated using the -8 option (see below).  The
              disadvantage is that such scanners often hang or crash if their
              input contains an 8-bit character.

              Note, however, that unless you generate your scanner using the
              -Cf or -CF table compression options, use of -7 will save only a
              small amount of table space, and make your scanner considerably
              less portable.  Flex's default behavior is to generate an 8-bit
              scanner unless you use the -Cf or -CF, in which case flex
              defaults to generating 7-bit scanners unless your site was
              always configured to generate 8-bit scanners (as will often be
              the case with non-USA sites).  You can tell whether flex
              generated a 7-bit or an 8-bit scanner by inspecting the flag
              summary in the -v output as described above.

              Note that if you use -Cfe or -CFe (those table compression
              options, but also using equivalence classes as discussed see
              below), flex still defaults to generating an 8-bit scanner,
              since usually with these compression options full 8-bit tables
              are not much more expensive than 7-bit tables.

       -8, --8bit
              instructs flex to generate an 8-bit scanner, i.e., one which can
              recognize 8-bit characters.  This flag is only needed for
              scanners generated using -Cf or -CF, as otherwise flex defaults
              to generating an 8-bit scanner anyway.

              See the discussion of -7 above for flex's default behavior and
              the tradeoffs between 7-bit and 8-bit scanners.

       -+, --c++
              specifies that you want flex to generate a C++ scanner class.
              See the section on Generating C++ Scanners below for details.

              controls the degree of table compression and, more generally,
              trade-offs between small scanners and fast scanners.

              -Ca, --align ("align") instructs flex to trade off larger tables
              in the generated scanner for faster performance because the
              elements of the tables are better aligned for memory access and
              computation.  On some RISC architectures, fetching and
              manipulating longwords is more efficient than with smaller-sized
              units such as shortwords.  This option can double the size of
              the tables used by your scanner.

              -Ce, --ecs directs flex to construct equivalence classes, i.e.,
              sets of characters which have identical lexical properties (for
              example, if the only appearance of digits in the flex input is
              in the character class "[0-9]" then the digits '0', '1', ...,
              '9' will all be put in the same equivalence class).  Equivalence
              classes usually give dramatic reductions in the final
              table/object file sizes (typically a factor of 2-5) and are
              pretty cheap performance-wise (one array look-up per character

              -Cf specifies that the full scanner tables should be generated -
              flex should not compress the tables by taking advantages of
              similar transition functions for different states.

              -CF specifies that the alternative fast scanner representation
              (described above under the -F flag) should be used.  This option
              cannot be used with -+.

              -Cm, --meta-ecs directs flex to construct meta-equivalence
              classes, which are sets of equivalence classes (or characters,
              if equivalence classes are not being used) that are commonly
              used together.  Meta-equivalence classes are often a big win
              when using compressed tables, but they have a moderate
              performance impact (one or two "if" tests and one array look-up
              per character scanned).

              -Cr, --read causes the generated scanner to bypass use of the
              standard I/O library (stdio) for input.  Instead of calling
              fread() or getc(), the scanner will use the read() system call,
              resulting in a performance gain which varies from system to
              system, but in general is probably negligible unless you are
              also using -Cf or -CF.  Using -Cr can cause strange behavior if,
              for example, you read from yyin using stdio prior to calling the
              scanner (because the scanner will miss whatever text your
              previous reads left in the stdio input buffer).

              -Cr has no effect if you define YY_INPUT (see The Generated
              Scanner above).

              A lone -C specifies that the scanner tables should be compressed
              but neither equivalence classes nor meta-equivalence classes
              should be used.

              The options -Cf or -CF and -Cm do not make sense together -
              there is no opportunity for meta-equivalence classes if the
              table is not being compressed.  Otherwise the options may be
              freely mixed, and are cumulative.

              The default setting is -Cem, which specifies that flex should
              generate equivalence classes and meta-equivalence classes.  This
              setting provides the highest degree of table compression.  You
              can trade off faster-executing scanners at the cost of larger
              tables with the following generally being true:

                  slowest & smallest
                  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.

              -Cfe is often a good compromise between speed and size for
              production scanners.

       -ooutput, --outfile=FILE
              directs flex to write the scanner to the file output instead of
              lex.yy.c.  If you combine -o with the -t option, then the
              scanner is written to stdout but its #line directives (see the
              -L option above) refer to the file output.

       -Pprefix, --prefix=STRING
              changes the default yy prefix used by flex for all globally-
              visible variable and function names to instead be prefix.  For
              example, -Pfoo changes the name of yytext to footext.  It also
              changes the name of the default output file from lex.yy.c to
              lex.foo.c.  Here are all of the names affected:


              (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

              This option lets you easily link together multiple flex programs
              into the same executable.  Note, though, that using this option
              also renames yywrap(), so you now must either provide your own
              (appropriately-named) version of the routine for your scanner,
              or use %option noyywrap, as linking with -lfl no longer provides
              one for you by default.

       -Sskeleton_file, --skel=FILE
              overrides the default skeleton file from which flex constructs
              its scanners.  You'll never need this option unless you are
              doing flex maintenance or development.

       -X, --posix-compat
              maximal compatibility with POSIX lex.

              track line count in yylineno.

              name of C++ class.

              create a C header file in addition to the scanner.

              write tables to FILE.

              #define macro defn (default defn is '1').

       -R, --reentrant
              generate a reentrant C scanner

              scanner for bison pure parser.

              include yylloc support.

              initialize yyin/yyout to stdin/stdout.

       --noansi-definitions old-style function definitions.

              empty parameter list in prototypes.

              do not include <unistd.h>.

              do not generate a particular FUNCTION.

       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:

              instructs flex to generate a scanner which always considers its
              input "interactive".  Normally, on each new input file the
              scanner calls isatty() in an attempt to determine whether the
              scanner's input source is interactive and thus should be read a
              character at a time.  When this option is used, however, then no
              such call is made.

       main   directs flex to provide a default main() program for the
              scanner, which simply calls yylex().  This option implies
              noyywrap (see below).

              instructs flex to generate a scanner which never considers its
              input "interactive" (again, no call made to isatty()).  This is
              the opposite of always-interactive.

       stack  enables the use of start condition stacks (see Start Conditions

              if set (i.e., %option stdinit) initializes yyin and yyout to
              stdin and stdout, instead of the default of nil.  Some existing
              lex programs depend on this behavior, even though it is not
              compliant with ANSI C, which does not require stdin and stdout
              to be compile-time constant.

              directs flex to generate a scanner that maintains the number of
              the current line read from its input in the global variable
              yylineno.  This option is implied by %option lex-compat.

       yywrap if unset (i.e., %option noyywrap), makes the scanner not call
              yywrap() upon an end-of-file, but simply assume that there are
              no more files to scan (until the user points yyin at a new file
              and calls yylex() again).

       flex scans your rule actions to determine whether you use the REJECT or
       yymore() features.  The reject and yymore options are available to
       override its decision as to whether you use the options, either by
       setting them (e.g., %option reject) to indicate the feature is indeed
       used, or unsetting them to indicate it actually is not used (e.g.,
       %option noyymore).

       Three options take string-delimited values, offset with '=':

           %option outfile="ABC"

       is equivalent to -oABC, and

           %option prefix="XYZ"

       is equivalent to -PXYZ.  Finally,

           %option yyclass="foo"

       only applies when generating a C++ scanner ( -+ option).  It informs
       flex that you have derived foo as a subclass of yyFlexLexer, so flex
       will place your actions in the member function foo::yylex() instead of
       yyFlexLexer::yylex().  It also generates a yyFlexLexer::yylex() member
       function that emits a run-time error (by invoking
       yyFlexLexer::LexerError()) if called.  See Generating C++ Scanners,
       below, for additional information.

       A number of options are available for lint purists who want to suppress
       the appearance of unneeded routines in the generated scanner.  Each of
       the following, if unset (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).

       The main design goal of flex is that it generate high-performance
       scanners.  It has been optimized for dealing well with large sets of
       rules.  Aside from the effects on scanner speed of the table
       compression -C options outlined above, there are a number of
       options/actions which degrade performance.  These are, from most
       expensive to least:

           %option yylineno
           arbitrary trailing context

           pattern sets that require backing up
           %option interactive
           %option always-interactive

           '^' beginning-of-line operator

       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

       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:
            out-transitions: [ a ]
            jam-transitions: EOF [ \001-`  b-\177 ]

           State #9 is non-accepting -
            associated rule line numbers:
            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

       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

       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             ++line_num;
           <comment>"*"+"/"        BEGIN(INITIAL);

       This could be sped up by writing it as:

           %x comment
                   int line_num = 1;

           "/*"         BEGIN(comment);

           <comment>[^*\n]*\n      ++line_num;
           <comment>"*"+[^*/\n]*\n ++line_num;
           <comment>"*"+"/"        BEGIN(INITIAL);

       Now instead of each newline requiring the processing of another action,
       recognizing the newlines is "distributed" over the other rules to keep
       the matched text as long as possible.  Note that adding rules does not
       slow down the scanner!  The speed of the scanner is independent of the
       number of rules or (modulo the considerations given at the beginning of
       this section) how complicated the rules are with regard to operators
       such as '*' and '|'.

       A final example in speeding up a scanner: suppose you want to scan
       through a file containing identifiers and keywords, one per line and
       with no other extraneous characters, and recognize all the keywords.  A
       natural first approach is:

           asm      |
           auto     |
           break    |
           ... etc ...
           volatile |
           while    /* it's a keyword */

           .|\n     /* it's not a keyword */

       To eliminate the back-tracking, introduce a catch-all rule:

           asm      |
           auto     |
           break    |
           ... etc ...
           volatile |
           while    /* it's a keyword */

           [a-z]+   |
           .|\n     /* it's not a keyword */

       Now, if it's guaranteed that there's exactly one word per line, then we
       can reduce the total number of matches by a half by merging in the
       recognition of newlines with that of the other tokens:

           asm\n    |
           auto\n   |
           break\n  |
           ... etc ...
           volatile\n |
           while\n  /* it's a keyword */

           [a-z]+\n |
           .|\n     /* it's not a keyword */

       One has to be careful here, as we have now reintroduced backing up into
       the scanner.  In particular, while we know that there will never be any
       characters in the input stream other than letters or newlines, flex
       can't figure this out, and it will plan for possibly needing to back up
       when it has scanned a token like "auto" and then the next character is
       something other than a newline or a letter.  Previously it would then
       just match the "auto" rule and be done, but now it has no "auto" rule,
       only a "auto\n" rule.  To eliminate the possibility of backing up, we
       could either duplicate all rules but without final newlines, or, since
       we never expect to encounter such an input and therefore don't how it's
       classified, we can introduce one more catch-all rule, this one which
       doesn't include a newline:

           asm\n    |
           auto\n   |
           break\n  |
           ... etc ...
           volatile\n |
           while\n  /* it's a keyword */

           [a-z]+\n |
           [a-z]+   |
           .|\n     /* it's not a keyword */

       Compiled with -Cf, this is about as fast as one can get a flex scanner
       to go for this particular problem.

       A final note: flex is slow when matching NUL's, particularly when a
       token contains multiple NUL's.  It's best to write rules which match
       short amounts of text if it's anticipated that the text will often
       include NUL's.

       Another final note regarding performance: as mentioned above in the
       section How the Input is Matched, dynamically resizing yytext to
       accommodate huge tokens is a slow process because it presently requires
       that the (huge) token be rescanned from the beginning.  Thus if
       performance is vital, you should attempt to match "large" quantities of
       text but not "huge" quantities, where the cutoff between the two is at
       about 8K characters/token.

       flex provides two different ways to generate scanners for use with C++.
       The first way is to simply compile a scanner generated by flex using a
       C++ compiler instead of a C compiler.  You should not encounter any
       compilations errors (please report any you find to the email address
       given in the Author section below).  You can then use C++ code in your
       rule actions instead of C code.  Note that the default input source for
       your scanner remains yyin, and default echoing is still done to yyout.
       Both of these remain FILE * variables and not C++ streams.

       You can also use flex to generate a C++ scanner class, using the -+
       option (or, equivalently, %option c++), which is automatically
       specified if the name of the flex executable ends in a '+', such as
       flex++.  When using this option, flex defaults to generating the
       scanner to the file lex.yy.cc instead of lex.yy.c.  The generated
       scanner includes the header file FlexLexer.h, which defines the
       interface to two C++ classes.

       The first class, FlexLexer, provides an abstract base class defining
       the general scanner class interface.  It provides the following member

       const char* YYText()
              returns the text of the most recently matched token, the
              equivalent of yytext.

       int YYLeng()
              returns the length of the most recently matched token, the
              equivalent of yyleng.

       int lineno() const
              returns the current input line number (see %option yylineno), or
              1 if %option yylineno was not used.

       void set_debug( int flag )
              sets the debugging flag for the scanner, equivalent to assigning
              to yy_flex_debug (see the Options section above).  Note that you
              must build the scanner using %option debug to include debugging
              information in it.

       int debug() const
              returns the current setting of the debugging flag.

       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

       The second class defined in FlexLexer.h is yyFlexLexer, which is
       derived from FlexLexer.  It defines the following additional member

       yyFlexLexer( std::istream* arg_yyin = 0, std::ostream* arg_yyout = 0 )
              constructs a yyFlexLexer object using the given streams for
              input and output.  If not specified, the streams default to cin
              and cout, respectively.

       virtual int yylex()
              performs the same role is yylex() does for ordinary flex
              scanners: it scans the input stream, consuming tokens, until a
              rule's action returns a value.  If you derive a subclass S from
              yyFlexLexer and want to access the member functions and
              variables of S inside yylex(), then you need to use %option
              yyclass="S" to inform flex that you will be using that subclass
              instead of yyFlexLexer.  In this case, rather than generating
              yyFlexLexer::yylex(), flex generates S::yylex() (and also
              generates a dummy yyFlexLexer::yylex() that calls
              yyFlexLexer::LexerError() if called).

       virtual void switch_streams(std::istream* new_in = 0,
              std::ostream* new_out = 0) reassigns yyin to new_in (if non-nil)
              and yyout to new_out (ditto), deleting the previous input buffer
              if yyin is reassigned.

       int yylex( std::istream* new_in, std::ostream* new_out = 0 )
              first switches the input streams via switch_streams( new_in,
              new_out ) and then returns the value of yylex().

       In addition, yyFlexLexer defines the following protected virtual
       functions which you can redefine in derived classes to tailor the

       virtual int LexerInput( char* buf, int max_size )
              reads up to max_size characters into buf and returns the number
              of characters read.  To indicate end-of-input, return 0
              characters.  Note that "interactive" scanners (see the -B and -I
              flags) define the macro YY_INTERACTIVE.  If you redefine
              LexerInput() and need to take different actions depending on
              whether or not the scanner might be scanning an interactive
              input source, you can test for the presence of this name via

       virtual void LexerOutput( const char* buf, int size )
              writes out size characters from the buffer buf, which, while
              NUL-terminated, may also contain "internal" NUL's if the
              scanner's rules can match text with NUL's in them.

       virtual void LexerError( const char* msg )
              reports a fatal error message.  The default version of this
              function writes the message to the stream cerr and exits.

       Note that a yyFlexLexer object contains its entire scanning state.
       Thus you can use such objects to create reentrant scanners.  You can
       instantiate multiple instances of the same yyFlexLexer class, and you
       can also combine multiple C++ scanner classes together in the same
       program using the -P option discussed above.

       Finally, note that the %array feature is not available to C++ scanner
       classes; you must use %pointer (the default).

       Here is an example of a simple C++ scanner:

               // An example of using the flex C++ scanner class.

           int mylineno = 0;

           string  \"[^\n"]+\"

           ws      [ \t]+

           alpha   [A-Za-z]
           dig     [0-9]
           name    ({alpha}|{dig}|\$)({alpha}|{dig}|[_.\-/$])*
           num1    [-+]?{dig}+\.?([eE][-+]?{dig}+)?
           num2    [-+]?{dig}*\.{dig}+([eE][-+]?{dig}+)?
           number  {num1}|{num2}


           {ws}    /* skip blanks and tabs */

           "/*"    {
                   int c;

                   while((c = yyinput()) != 0)
                       if(c == '\n')

                       else if(c == '*')
                           if((c = yyinput()) == '/')

           {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.

       flex is a rewrite of the AT&T Unix lex tool (the two implementations do
       not share any code, though), with some extensions and
       incompatibilities, both of which are of concern to those who wish to
       write scanners acceptable to either implementation.  Flex is fully
       compliant with the POSIX lex specification, except that when using
       %pointer (the default), a call to unput() destroys the contents of
       yytext, which is counter to the POSIX specification.

       In this section we discuss all of the known areas of incompatibility
       between flex, AT&T lex, and the POSIX specification.

       flex's -l option turns on maximum compatibility with the original AT&T
       lex implementation, at the cost of a major loss in the generated
       scanner's performance.  We note below which incompatibilities can be
       overcome using the -l option.

       flex is fully compatible with lex with the following exceptions:

       -      The undocumented lex scanner internal variable yylineno is not
              supported unless -l or %option yylineno is used.

              yylineno should be maintained on a per-buffer basis, rather than
              a per-scanner (single global variable) basis.

              yylineno is not part of the POSIX specification.

       -      The input() routine is not redefinable, though it may be called
              to read characters following whatever has been matched by a
              rule.  If input() encounters an end-of-file the normal yywrap()
              processing is done.  A ``real'' end-of-file is returned by
              input() as EOF.

              Input is instead controlled by defining the YY_INPUT macro.

              The flex restriction that input() cannot be redefined is in
              accordance with the POSIX specification, which simply does not
              specify any way of controlling the scanner's input other than by
              making an initial assignment to yyin.

       -      The unput() routine is not redefinable.  This restriction is in
              accordance with POSIX.

       -      flex scanners are not as reentrant as lex scanners.  In
              particular, if you have an interactive scanner and an interrupt
              handler which long-jumps out of the scanner, and the scanner is
              subsequently called again, you may get the following message:

                  fatal flex scanner internal error--end of buffer 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.

              Also note that flex C++ scanner classes are reentrant, so if
              using C++ is an option for you, you should use them instead.
              See "Generating C++ Scanners" above for details.

       -      output() is not supported.  Output from the ECHO macro is done
              to the file-pointer yyout (default stdout).

              output() is not part of the POSIX specification.

       -      lex does not support exclusive start conditions (%x), though
              they are in the POSIX specification.

       -      When definitions are expanded, flex encloses them in
              parentheses.  With lex, the following:

                  NAME    [A-Z][A-Z0-9]*
                  foo{NAME}?      printf( "Found it\n" );

              will not match the string "foo" because when the macro is
              expanded the rule is equivalent to "foo[A-Z][A-Z0-9]*?" and the
              precedence is such that the '?' is associated with "[A-Z0-9]*".
              With flex, the rule will be expanded to "foo([A-Z][A-Z0-9]*)?"
              and so the string "foo" will match.

              Note that if the definition begins with ^ or ends with $ then it
              is not expanded with parentheses, to allow these operators to
              appear in definitions without losing their special meanings.
              But the <s>, /, and <<EOF>> operators cannot be used in a flex

              Using -l results in the lex behavior of no parentheses around
              the definition.

              The POSIX specification is that the definition be enclosed in

       -      Some implementations of lex allow a rule's action to begin on a
              separate line, if the rule's pattern has trailing whitespace:

                  foo|bar<space here>
                    { foobar_action(); }

              flex does not support this feature.

       -      The lex %r (generate a Ratfor scanner) option is not supported.
              It is not part of the POSIX specification.

       -      After a call to unput(), yytext is undefined until the next
              token is matched, unless the scanner was built using %array.
              This is not the case with lex or the POSIX specification.  The
              -l option does away with this incompatibility.

       -      The precedence of the {} (numeric range) operator is different.
              lex interprets "abc{1,3}" as "match one, two, or three
              occurrences of 'abc'", whereas flex interprets it as "match 'ab'
              followed by one, two, or three occurrences of 'c'".  The latter
              is in agreement with the POSIX specification.

       -      The precedence of the ^ operator is different.  lex interprets
              "^foo|bar" as "match either 'foo' at the beginning of a line, or
              'bar' anywhere", whereas flex interprets it as "match either
              'foo' or 'bar' if they come at the beginning of a line".  The
              latter is in agreement with the POSIX specification.

       -      The special table-size declarations such as %a supported by lex
              are not required by flex scanners; flex ignores them.

       -      The name FLEX_SCANNER is #define'd so scanners may be written
              for use with either flex or lex.  Scanners also include
              YY_FLEX_MAJOR_VERSION and YY_FLEX_MINOR_VERSION indicating which
              version of flex generated the scanner (for example, for the 2.5
              release, these defines would be 2 and 5 respectively).

       The following flex features are not included in lex or the POSIX

           C++ scanners
           start condition scopes
           start condition stacks
           interactive/non-interactive scanners
           yy_scan_string() and friends
           #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.

       warning, rule cannot be matched indicates that the given rule cannot be
       matched because it follows other rules that will always match the same
       text as it.  For example, in the following "foo" cannot be matched
       because it comes after an identifier "catch-all" rule:

           [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

       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

       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).

       -lfl   library with which scanners must be linked.

              generated scanner (called lexyy.c on some systems).

              generated C++ scanner class, when using -+.

              header file defining the C++ scanner base class, FlexLexer, and
              its derived class, yyFlexLexer.

              skeleton scanner.  This file is only used when building flex,
              not when flex executes.

              backing-up information for -b flag (called lex.bck on some

       Some trailing context patterns cannot be properly matched and generate
       warning messages ("dangerous trailing context").  These are patterns
       where the ending of the first part of the rule matches the beginning of
       the second part, such as "zx*/xy*", where the 'x*' matches the 'x' at
       the beginning of the trailing context.  (Note that the POSIX draft
       states that the text matched by such patterns is undefined.)

       For some trailing context rules, parts which are actually fixed-length
       are not recognized as such, leading to the 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      |

       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

       Dynamic resizing of the input buffer is slow, as it entails rescanning
       all the text matched so far by the current (generally huge) token.

       Due to both buffering of input and read-ahead, you cannot intermix
       calls to <stdio.h> routines, such as, for example, getchar(), with flex
       rules and expect it to work.  Call input() instead.

       The total table entries listed by the -v flag excludes the number of
       table entries needed to determine what rule has been matched.  The
       number of entries is equal to the number of DFA states if the scanner
       does not use REJECT, and somewhat greater than the number of states if
       it does.

       REJECT cannot be used with the -f or -F options.

       The flex internal algorithms need documentation.

       lex(1), yacc(1), sed(1), awk(1).

       John Levine, Tony Mason, and Doug Brown, Lex & Yacc, O'Reilly and
       Associates.  Be sure to get the 2nd edition.

       M. E. Lesk and E. Schmidt, LEX - Lexical Analyzer Generator

       Alfred Aho, Ravi Sethi and Jeffrey Ullman, Compilers: Principles,
       Techniques and Tools, Addison-Wesley (1986).  Describes the pattern-
       matching techniques used by flex (deterministic finite automata).

       Vern Paxson, with the help of many ideas and much inspiration from Van
       Jacobson.  Original version by Jef Poskanzer.  The fast table
       representation is a partial implementation of a design done by Van
       Jacobson.  The implementation was done by Kevin Gong and Vern Paxson.

       Thanks to the many flex beta-testers, feedbackers, and contributors,
       especially Francois Pinard, Casey Leedom, 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

       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.

Version 2.5                      December 2021                         FLEX(1)