Tribblix: manual page: lex.1

LEX(1) User Commands LEX(1)

NAME

lex - generate programs for lexical tasks

SYNOPSIS

lex [-cntv] [-e | -w] [-V -Q [y | n]] [-Y directory]] [file]...

DESCRIPTION

The lex utility generates C programs to be used in lexical processing
of character input, and that can be used as an interface to yacc. The
C programs are generated from lex source code and conform to the ISO
C standard. Usually, the lex utility writes the program it generates
to the file lex.yy.c. The state of this file is unspecified if lex
exits with a non-zero exit status. See EXTENDED DESCRIPTION for a
complete description of the lex input language.

OPTIONS

The following options are supported:

-c
Indicates C-language action (default option).

-e
Generates a program that can handle EUC characters
(cannot be used with the -w option). yytext[] is of type
unsigned char[].

-n
Suppresses the summary of statistics usually written with
the -v option. If no table sizes are specified in the
lex source code and the -v option is not specified, then
-n is implied.

-t
Writes the resulting program to standard output instead
of lex.yy.c.

-v
Writes a summary of lex statistics to the standard error.
(See the discussion of lex table sizes under the heading
Definitions in lex.) If table sizes are specified in the
lex source code, and if the -n option is not specified,
the -v option may be enabled.

-w
Generates a program that can handle EUC characters
(cannot be used with the -e option). Unlike the -e
option, yytext[] is of type wchar_t[].

-V
Prints out version information on standard error.

-Q[y|n]
Prints 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.

-Y directory
Designates an alternate directory that contains the
driver files used by lex.

OPERANDS

The following operand is supported:

file
A pathname of an input file. If more than one such file is
specified, all files will be concatenated to produce a
single lex program. If no file operands are specified, or if
a file operand is -, the standard input will be used.

OUTPUT

The lex output files are described below.

Stdout

If the -t option is specified, the text file of C source code output
of lex will be written to standard output.

Stderr

If the -t option is specified informational, error and warning
messages concerning the contents of lex source code input will be
written to the standard error.

If the -t option is not specified:

1. Informational error and warning messages concerning the
contents of lex source code input will be written to
either the standard output or standard error.

2. If the -v option is specified and the -n option is not
specified, lex statistics will also be written to standard
error. These statistics may also be generated if table
sizes are specified with a % operator in the Definitions
in lex section (see EXTENDED DESCRIPTION), as long as the
-n option is not specified.

Output Files

A text file containing C source code will be written to lex.yy.c, or
to the standard output if the -t option is present.

EXTENDED DESCRIPTION

Each input file contains lex source code, which is a table of regular
expressions with corresponding actions in the form of C program
fragments.

When lex.yy.c is compiled and linked with the lex library (using the
-l l operand with c89 or cc), the resulting program reads character
input from the standard input and partitions it into strings that
match the given expressions.

When an expression is matched, these actions will occur:

o The input string that was matched is left in yytext as a
null-terminated string; yytext is either an external
character array or a pointer to a character string. As
explained in Definitions in lex, the type can be
explicitly selected using the %array or %pointer
declarations, but the default is %array.

o The external int yyleng is set to the length of the
matching string.

o The expression's corresponding program fragment, or
action, is executed.

During pattern matching, lex searches the set of patterns for the
single longest possible match. Among rules that match the same number
of characters, the rule given first will be chosen.

The general format of lex source is:

Definitions
%%
Rules
%%
User Subroutines

The first %% is required to mark the beginning of the rules (regular
expressions and actions); the second %% is required only if user
subroutines follow.

Any line in the Definitions in lex section beginning with a blank
character will be assumed to be a C program fragment and will be
copied to the external definition area of the lex.yy.c file.
Similarly, anything in the Definitions in lex section included
between delimiter lines containing only %{ and %} will also be copied
unchanged to the external definition area of the lex.yy.c file.

Any such input (beginning with a blank character or within %{ and %}
delimiter lines) appearing at the beginning of the Rules section
before any rules are specified will be written to lex.yy.c after the
declarations of variables for the yylex function and before the first
line of code in yylex. Thus, user variables local to yylex can be
declared here, as well as application code to execute upon entry to
yylex.

The action taken by lex when encountering any input beginning with a
blank character or within %{ and %} delimiter lines appearing in the
Rules section but coming after one or more rules is undefined. The
presence of such input may result in an erroneous definition of the
yylex function.

Definitions in lex

Definitions in lex appear before the first %% delimiter. Any line in
this section not contained between %{ and %} lines and not beginning
with a blank character is assumed to define a lex substitution
string. The format of these lines is:

name substitute

If a name does not meet the requirements for identifiers in the ISO C
standard, the result is undefined. The string substitute will replace
the string { name } when it is used in a rule. The name string is
recognized in this context only when the braces are provided and when
it does not appear within a bracket expression or within double-
quotes.

In the Definitions in lex section, any line beginning with a %
(percent sign) character and followed by an alphanumeric word
beginning with either s or S defines a set of start conditions. Any
line beginning with a % followed by a word beginning with either x or
X defines a set of exclusive start conditions. When the generated
scanner is in a %s state, patterns with no state specified will be
also active; in a %x state, such patterns will not be active. The
rest of the line, after the first word, is considered to be one or
more blank-character-separated names of start conditions. Start
condition names are constructed in the same way as definition names.
Start conditions can be used to restrict the matching of regular
expressions to one or more states as described in Regular expressions
in lex.

Implementations accept either of the following two mutually exclusive
declarations in the Definitions in lex section:

%array
Declare the type of yytext to be a null-terminated
character array.

%pointer
Declare the type of yytext to be a pointer to a null-
terminated character string.

Note: When using the %pointer option, you may not also use the yyless
function to alter yytext.

%array is the default. If %array is specified (or neither %array nor
%pointer is specified), then the correct way to make an external
reference to yyext is with a declaration of the form:

extern char yytext[]

If %pointer is specified, then the correct external reference is of
the form:

extern char *yytext;

lex will accept declarations in the Definitions in lex section for
setting certain internal table sizes. The declarations are shown in
the following table.

Table Size Declaration in lex

+-----------------------------------------------------------+
|Declaration Description Default |
+-----------------------------------------------------------+
|%pn Number of positions 2500 |
|%nn Number of states 500 |
|%a n Number of transitions 2000 |
|%en Number of parse tree nodes 1000 |
|%kn Number of packed character classes 10000 |
|%on Size of the output array 3000 |
+-----------------------------------------------------------+

Programs generated by lex need either the -e or -w option to handle
input that contains EUC characters from supplementary codesets. If
neither of these options is specified, yytext is of the type char[],
and the generated program can handle only ASCII characters.

When the -e option is used, yytext is of the type unsigned char[] and
yyleng gives the total number of bytes in the matched string. With
this option, the macros input(), unput(c), and output(c) should do a
byte-based I/O in the same way as with the regular ASCII lex. Two
more variables are available with the -e option, yywtext and yywleng,
which behave the same as yytext and yyleng would under the -w option.

When the -w option is used, yytext is of the type wchar_t[] and
yyleng gives the total number of characters in the matched string.
If you supply your own input(), unput(c), or output(c) macros with
this option, they must return or accept EUC characters in the form of
wide character (wchar_t). This allows a different interface between
your program and the lex internals, to expedite some programs.

Rules in lex

The Rules in lex source files are a table in which the left column
contains regular expressions and the right column contains actions (C
program fragments) to be executed when the expressions are
recognized.

ERE action
ERE action
...

The extended regular expression (ERE) portion of a row will be
separated from action by one or more blank characters. A regular
expression containing blank characters is recognized under one of the
following conditions:

o The entire expression appears within double-quotes.

o The blank characters appear within double-quotes or square
brackets.

o Each blank character is preceded by a backslash character.

User Subroutines in lex

Anything in the user subroutines section will be copied to lex.yy.c
following yylex.

Regular Expressions in lex

The lex utility supports the set of Extended Regular Expressions
(EREs) described on regex(7) with the following additions and
exceptions to the syntax:

...
Any string enclosed in double-quotes will represent the
characters within the double-quotes as themselves,
except that backslash escapes (which appear in the
following table) are recognized. Any backslash-escape
sequence is terminated by the closing quote. For
example, "\01""1" represents a single string: the octal
value 1 followed by the character 1.

<state>r

<state1, state2, ...>r

The regular expression r will be matched only when the program is
in one of the start conditions indicated by state, state1, and so
forth. For more information, see Actions in lex. As an exception
to the typographical conventions of the rest of this document, in
this case <state> does not represent a metavariable, but the
literal angle-bracket characters surrounding a symbol. The start
condition is recognized as such only at the beginning of a
regular expression.

r/x

The regular expression r will be matched only if it is followed
by an occurrence of regular expression x. The token returned in
yytext will only match r. If the trailing portion of r matches
the beginning of x, the result is unspecified. The r expression
cannot include further trailing context or the $ (match-end-of-
line) operator; x cannot include the ^ (match-beginning-of-line)
operator, nor trailing context, nor the $ operator. That is, only
one occurrence of trailing context is allowed in a lex regular
expression, and the ^ operator only can be used at the beginning
of such an expression. A further restriction is that the
trailing-context operator / (slash) cannot be grouped within
parentheses.

{name}

When name is one of the substitution symbols from the Definitions
section, the string, including the enclosing braces, will be
replaced by the substitute value. The substitute value will be
treated in the extended regular expression as if it were enclosed
in parentheses. No substitution will occur if {name} occurs
within a bracket expression or within double-quotes.

Within an ERE, a backslash character (\\, \a, \b, \f, \n, \r, \t, \v)
is considered to begin an escape sequence. In addition, the escape
sequences in the following table will be recognized.

A literal newline character cannot occur within an ERE; the escape
sequence \n can be used to represent a newline character. A newline
character cannot be matched by a period operator.

Escape Sequences in lex

+--------------------------------------------------------------------------------+
|Escape Sequences in lex |
+--------------------------------------------------------------------------------+
| Escape Sequence Description Meaning |
+--------------------------------------------------------------------------------+
| \digits A backslash character The character whose |
| followed by the longest encoding is represented by |
| sequence of one, two or the one-, two- or |
| three octal-digit three-digit octal integer. |
| characters (01234567). Multi-byte characters |
| Ifall of the digits are require multiple, |
| 0, (that is, concatenated escape |
| representation of the sequences of this type, |
| NUL character), the including the leading \ for |
| behavior is undefined. each byte. |
+--------------------------------------------------------------------------------+
| \xdigits A backslash character The character whose |
| followed by the longest encoding is represented by |
| sequence of the hexadecimal integer. |
| hexadecimal-digit |
| characters |
| (01234567abcdefABCDEF). |
| If all of the digits |
| are 0, (that is, |
| representation of the |
| NUL character), the |
| behavior is undefined. |
+--------------------------------------------------------------------------------+
| \c A backslash character The character c, unchanged. |
| followed by any |
| character not described |
| in this table. (\\, |
| \a, \b, \f, \en, \r, |
| \t, \v). |
+--------------------------------------------------------------------------------+

The order of precedence given to extended regular expressions for lex
is as shown in the following table, from high to low.

Note:
The escaped characters entry is not meant to imply that
these are operators, but they are included in the table to
show their relationships to the true operators. The start
condition, trailing context and anchoring notations have
been omitted from the table because of the placement
restrictions described in this section; they can only
appear at the beginning or ending of an ERE.

+---------------------------------------------------------+
| ERE Precedence in lex |
+---------------------------------------------------------+
|collation-related bracket symbols [= =] [: :] [. .] |
|escaped characters \<special character> |
|bracket expression [ ] |
|quoting "..." |
|grouping () |
|definition {name} |
|single-character RE duplication * + ? |
|concatenation |
|interval expression {m,n} |
|alternation | |
+---------------------------------------------------------+

The ERE anchoring operators (^ and $) do not appear in the table.
With lex regular expressions, these operators are restricted in their
use: the ^ operator can only be used at the beginning of an entire
regular expression, and the $ operator only at the end. The operators
apply to the entire regular expression. Thus, for example, the
pattern (^abc)|(def$) is undefined; it can instead be written as two
separate rules, one with the regular expression ^abc and one with
def$, which share a common action via the special | action (see
below). If the pattern were written ^abc|def$, it would match either
of abc or def on a line by itself.

Unlike the general ERE rules, embedded anchoring is not allowed by
most historical lex implementations. An example of embedded anchoring
would be for patterns such as (^)foo($) to match foo when it exists
as a complete word. This functionality can be obtained using existing
lex features:

^foo/[ \n]|
" foo"/[ \n] /* found foo as a separate word */

Notice also that $ is a form of trailing context (it is equivalent to
/\n and as such cannot be used with regular expressions containing
another instance of the operator (see the preceding discussion of
trailing context).

The additional regular expressions trailing-context operator /
(slash) can be used as an ordinary character if presented within
double-quotes, "/"; preceded by a backslash, \/; or within a bracket
expression, [/]. The start-condition < and > operators are special
only in a start condition at the beginning of a regular expression;
elsewhere in the regular expression they are treated as ordinary
characters.

The following examples clarify the differences between lex regular
expressions and regular expressions appearing elsewhere in this
document. For regular expressions of the form r/x, the string
matching r is always returned; confusion may arise when the beginning
of x matches the trailing portion of r. For example, given the
regular expression a*b/cc and the input aaabcc, yytext would contain
the string aaab on this match. But given the regular expression x*/xy
and the input xxxy, the token xxx, not xx, is returned by some
implementations because xxx matches x*.

In the rule ab*/bc, the b* at the end of r will extend r's match into
the beginning of the trailing context, so the result is unspecified.
If this rule were ab/bc, however, the rule matches the text ab when
it is followed by the text bc. In this latter case, the matching of r
cannot extend into the beginning of x, so the result is specified.

Actions in lex

The action to be taken when an ERE is matched can be a C program
fragment or the special actions described below; the program fragment
can contain one or more C statements, and can also include special
actions. The empty C statement ; is a valid action; any string in the
lex.yy.c input that matches the pattern portion of such a rule is
effectively ignored or skipped. However, the absence of an action is
not valid, and the action lex takes in such a condition is undefined.

The specification for an action, including C statements and special
actions, can extend across several lines if enclosed in braces:

ERE <one or more blanks> { program statement
program statement }

The default action when a string in the input to a lex.yy.c program
is not matched by any expression is to copy the string to the output.
Because the default behavior of a program generated by lex is to read
the input and copy it to the output, a minimal lex source program
that has just %% generates a C program that simply copies the input
to the output unchanged.

Four special actions are available:

| ECHO; REJECT; BEGIN

|
The action | means that the action for the next rule is
the action for this rule. Unlike the other three actions,
| cannot be enclosed in braces or be semicolon-
terminated. It must be specified alone, with no other
actions.

ECHO;
Writes the contents of the string yytext on the output.

REJECT;
Usually only a single expression is matched by a given
string in the input. REJECT means "continue to the next
expression that matches the current input," and causes
whatever rule was the second choice after the current
rule to be executed for the same input. Thus, multiple
rules can be matched and executed for one input string or
overlapping input strings. For example, given the regular
expressions xyz and xy and the input xyz, usually only
the regular expression xyz would match. The next
attempted match would start after z. If the last action
in the xyz rule is REJECT, both this rule and the xy rule
would be executed. The REJECT action may be implemented
in such a fashion that flow of control does not continue
after it, as if it were equivalent to a goto to another
part of yylex. The use of REJECT may result in somewhat
larger and slower scanners.

BEGIN
The action:

BEGIN newstate;

switches the state (start condition) to newstate. If the
string newstate has not been declared previously as a
start condition in the Definitions in lex section, the
results are unspecified. The initial state is indicated
by the digit 0 or the token INITIAL.

The functions or macros described below are accessible to user code
included in the lex input. It is unspecified whether they appear in
the C code output of lex, or are accessible only through the -l l
operand to c89 or cc (the lex library).

int yylex(void)
Performs lexical analysis on the input; this is
the primary function generated by the lex
utility. The function returns zero when the end
of input is reached; otherwise it returns non-
zero values (tokens) determined by the actions
that are selected.

int yymore(void)
When called, indicates that when the next input
string is recognized, it is to be appended to
the current value of yytext rather than
replacing it; the value in yyleng is adjusted
accordingly.

int yyless(int n)
Retains n initial characters in yytext, NUL-
terminated, and treats the remaining characters
as if they had not been read; the value in
yyleng is adjusted accordingly.

int input(void)
Returns the next character from the input, or
zero on end-of-file. It obtains input from the
stream pointer yyin, although possibly via an
intermediate buffer. Thus, once scanning has
begun, the effect of altering the value of yyin
is undefined. The character read is removed from
the input stream of the scanner without any
processing by the scanner.

int unput(int c)
Returns the character c to the input; yytext and
yyleng are undefined until the next expression
is matched. The result of using unput for more
characters than have been input is unspecified.

The following functions appear only in the lex library accessible
through the -l l operand; they can therefore be redefined by a
portable application:

int yywrap(void)

Called by yylex at end-of-file; the default yywrap always will
return 1. If the application requires yylex to continue
processing with another source of input, then the application can
include a function yywrap, which associates another file with the
external variable FILE *yyin and will return a value of zero.

int main(int argc, char *argv[])

Calls yylex to perform lexical analysis, then exits. The user
code can contain main to perform application-specific operations,
calling yylex as applicable.

The reason for breaking these functions into two lists is that only
those functions in libl.a can be reliably redefined by a portable
application.

Except for input, unput and main, all external and static names
generated by lex begin with the prefix yy or YY.

USAGE

Portable applications are warned that in the Rules in lex section, an
ERE without an action is not acceptable, but need not be detected as
erroneous by lex. This may result in compilation or run-time errors.

The purpose of input is to take characters off the input stream and
discard them as far as the lexical analysis is concerned. A common
use is to discard the body of a comment once the beginning of a
comment is recognized.

The lex utility is not fully internationalized in its treatment of
regular expressions in the lex source code or generated lexical
analyzer. It would seem desirable to have the lexical analyzer
interpret the regular expressions given in the lex source according
to the environment specified when the lexical analyzer is executed,
but this is not possible with the current lex technology.
Furthermore, the very nature of the lexical analyzers produced by lex
must be closely tied to the lexical requirements of the input
language being described, which will frequently be locale-specific
anyway. (For example, writing an analyzer that is used for French
text will not automatically be useful for processing other
languages.)

EXAMPLES

Example 1: Using lex

The following is an example of a lex program that implements a
rudimentary scanner for a Pascal-like syntax:

%{
/* need this for the call to atof() below */
#include <math.h>
/* need this for printf(), fopen() and stdin below */
#include <stdio.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 white space */

. printf("Unrecognized character: %s\n", yytext);

%%

int main(int argc, char *argv[])
{
++argv, --argc; /* skip over program name */
if (argc > 0)
yyin = fopen(argv[0], "r");
else
yyin = stdin;

yylex();
}

ENVIRONMENT VARIABLES

See environ(7) for descriptions of the following environment
variables that affect the execution of lex: LANG, LC_ALL, LC_COLLATE,
LC_CTYPE, LC_MESSAGES, and NLSPATH.

EXIT STATUS

The following exit values are returned:

0
Successful completion.

>0
An error occurred.

ATTRIBUTES