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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LEX(CP) XENIX System V LEX(CP)
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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LEX(CP) XENIX System V LEX(CP)
%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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