lex(1) — USER COMMANDS

NAME

lex − generate programs for simple lexical tasks

SYNOPSIS

lex [−ctvn −V −Q[y|n]] [file]

DESCRIPTION

The lex command 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 these strings are found.

lex generates a file named lex.yy.c. When lex.yy.c is compiled and linked with the lex library, it copies the input to the output except when a string specified in the file is found. When a specified string 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 patterns in the file. The patterns 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 non-negative number of, any positive number of, and either zero or one occurrence of, the previous character or character class. Thus, [a−zA−Z]+ matches a string of letters. The character . is the class of all ASCII characters except new-line. 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 new-line, and the character $ at the end of an expression requires a trailing new-line. 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 \.

Three 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 lex library contains a main() that calls it. The macros input and output read from and write to stdin and stdout, respectively.

The function yymore accumulates additional characters into the same yytext. The function yyless(n) pushes back yyleng −n characters into the input stream. (yyleng is an external int variable giving the length in bytes of yytext.) The function yywrap is called whenever the scanner reaches end of file and indicates whether normal wrapup should continue. The action REJECT on the right side of the rule causes the match to be rejected and the next suitable match executed. The action ECHO on the right side of the rule is equivalent to printf("%s", yytext).

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 %% that begin with a non-blank character define the string on the left to be the remainder of the line; it can be called out later by surrounding it with {}. In this section, C code (and preprocessor statements) can also be included between %{ and %}. Note that curly brackets do not imply parentheses; only string substitution is done.

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

The flags must appear before any files.

−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 two-line summary of statistics.

−n Will not print out the −v summary.

−V Print out version information on standard error.

−Q[y|n] Print out version information to output file lex.yy.c by using −Qy. The −Qn option does not print out version information and is the default.

Multiple files are treated as a single file. If no files are specified, standard input is used.

Certain default table sizes are too small for some users. The table sizes for the resulting finite state machine can be set in the definitions section:

%p n number of positions is n (default 2500)

%n n number of states is n (500)

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

%a n number of transitions is n (2000)

%k n number of packed character classes is n (2500)

%o n size of output array is n (3000)

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

EXAMPLE

D[0−9]
%{
void
skipcommnts(void)
{
for(;;)
{
while(input()!=’∗’)
;
if(input()==’/’)
return;
else

unput(yytext[yyleng-1]);
}
}
%}
%%
ifprintf("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");
"\n";/∗no action ∗/
"/∗" skipcommnts();
%%

INTERNATIONAL FUNCTIONS

lex can process characters from supplementary code sets as well as ASCII characters.

Characters from supplementary code sets can be specified in comments which exist in definitions, rules, and user subroutines.

Characters from supplementary code sets can be specified in strings which exist in actions in rules and in user subroutines.

Character strings from supplementary code sets can be defined as tokens.

input(), unput(c) and output(c) functions are performed in byte. The value of yyleng is in bytes, not in characters.

Characters from supplementary code sets which define token are restricted to single-byte characters.

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