LEX(CP)                  XENIX System V                   LEX(CP)



     Name
          lex - Generates programs for lexical analysis.

     Syntax
          lex [-ctvn] [ file ] ...

     Description
          lex generates programs to be used in simple lexical analysis
          of text.

          The input files (standard input default) contain strings and
          expressions to be searched for, and C text to be executed
          when strings are found.

          A file lex.yy.c is generated which, when loaded with the
          library, copies the input to the output except when a string
          specified in the file is found; then the corresponding
          program text is executed.  The actual string matched is left
          in yytext, an external character array.  Matching is done in
          order of the strings in the file.  The strings may contain
          square brackets to indicate character classes, as in [abx-z]
          to indicate a, b, x, y, and z; and the operators *, +, and ?
          mean respectively; any nonnegative number of, any positive
          number of, and either zero or one occurrences of, the
          previous character or character class.  The character . is
          the class of all ASCII characters except newline.
          Parentheses for grouping and vertical bar for alternation
          are also supported.  The notation r{d,e} in a rule indicates
          between d and e instances of regular expression r.  It has
          higher precedence than |, but lower than *, ?, +, and
          concatenation.  The character ^ at the beginning of an
          expression permits a successful match only immediately after
          a newline, and the character $ at the end of an expression
          requires a trailing newline.  The character / in an
          expression indicates trailing context; only the part of the
          expression up to the slash is returned in yytext, but the
          remainder of the expression must follow in the input stream.
          An operator character may be used as an ordinary symbol if
          it is within " symbols or preceded by \.  Thus, [a-zA-Z]+
          matches a string of letters.

          Three subroutines defined as macros are expected: input() to
          read a character; unput(c) to replace a character read; and
          output(c) to place an output character.  They are defined in
          terms of the standard streams, but you can override them.
          The program generated is named yylex(), and the library
          contains a main() which calls it.  The action REJECT on the
          right side of the rule causes this match to be rejected and
          the next suitable match executed; the function yymore()
          accumulates additional characters into the same yytext; and
          the function yyless(p) pushes back the portion of the string
          matched beginning at p, which should be between yytext and



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          yytext+yyleng.  The macros input and output use files yyin
          and yyout to read from and write to, defaulted to stdin and
          stdout, respectively.

          Any line beginning with a blank is assumed to contain only C
          text and is copied; if it precedes %% it is copied into the
          external definition area of the lex.yy.c file.  All rules
          should follow a %%, as in YACC.  Lines preceding %% which
          begin with a nonblank character define the string on the
          left to be the remainder of the line; it can be called out
          later by surrounding it with {}.  Note that curly brackets
          do not imply parentheses; only string substitution is done.

     Example
                  D       [0-9]
                  %%
                  if      printf("IF statement\n");
                  [a-z]+  printf("tag, value %s\n",yytext);
                  0{D}+   printf("octal number %s\n",yytext);
                  {D}+    printf("decimal number %s\n",yytext);
                  "++"    printf("unary op\n");
                  "+"     printf("binary op\n");
                  "/*"    {       loop:
                                  while (input() != '*');
                                  switch (input())
                                          {
                                          case '/': break;
                                          case '*': unput('*');
                                          default: go to loop;
                                          }
                                  }

          The external names generated by lex all begin with the
          prefix yy or YY.

          The options must appear before any files.  The option -c
          indicates C actions and is the default, -t causes the
          lex.yy.c program to be written instead to standard output,
          -v provides a one-line summary of statistics of the machine
          generated, -n will not print out the - summary.  Multiple
          files are treated as a single file.  If no files are
          specified, standard input is used.

          Certain table sizes for the resulting finite state machine
          can be set in the definitions section:

               %p n number of positions is n (default 2000)

               %n n number of states is n (500)

               %t n number of parse tree nodes is n (1000)




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               %a n number of transitions is n (3000)

          The use of one or more of the above automatically implies
          the -v option, unless the -n option is used.

     See Also
          yacc(CP)
          XENIX Programmer's Guide















































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