lex(1) User Commands lex(1)NAMElex - generate programs for lexical tasks
SYNOPSISlex [-cntv] [-e | -w] [-V -Q [y | n]] [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 infor‐
mation and is the default.
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 con‐
tents 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 spec‐
ified, 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 frag‐
ments.
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 charac‐
ter 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 sin‐
gle 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 sub‐
routines follow.
Any line in the Definitions in lex section beginning with a blank char‐
acter 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 con‐
taining 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 declara‐
tions 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 rec‐
ognized 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-sepa‐
rated 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 charac‐
ter array.
%pointer Declare the type of yytext to be a pointer to a null-ter‐
minated 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 refer‐
ence 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 set‐
ting 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 nei‐
ther 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 vari‐
ables 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 con‐
tains regular expressions and the right column contains actions (C pro‐
gram fragments) to be executed when the expressions are recognized.
ERE action
ERE action
...
The extended regular expression (ERE) portion of a row will be sepa‐
rated 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(5) 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 ta‐
ble) 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 expres‐
sion.
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 begin‐
ning of x, the result is unspecified. The r expression cannot
include further trailing context or the $ (match-end-of-line) oper‐
ator; x cannot include the ^ (match-beginning-of-line) operator,
nor trailing context, nor the $ operator. That is, only one occur‐
rence 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 fol‐ The character whose encod‐ │
│ lowed by the longest sequence ing is represented by the │
│ of one, two or three octal- one-, two- or three-digit │
│ digit characters (01234567). octal integer. Multi-byte │
│ Ifall of the digits are 0, characters require multi‐ │
│ (that is, representation of ple, concatenated escape │
│ the NUL character), the sequences of this type, │
│ behavior is undefined. including the leading \ for │
│ each byte. │
├──────────────────────────────────────────────────────────────────────────────────────┤
│ \xdigits A backslash character fol‐ The character whose encod‐ │
│ lowed by the longest sequence ing is represented by the │
│ of hexadecimal-digit charac‐ hexadecimal integer. │
│ ters (01234567abcdefABCDEF). │
│ If all of the digits are 0, │
│ (that is, representation of │
│ the NUL character), the │
│ behavior is undefined. │
├──────────────────────────────────────────────────────────────────────────────────────┤
│ \c A backslash character fol‐ The character c, unchanged. │
│ lowed 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 condi‐
tion, 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 begin‐
ning 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 pat‐
tern 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 com‐
plete word. This functionality can be obtained using existing lex fea‐
tures:
^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 con‐
dition at the beginning of a regular expression; elsewhere in the regu‐
lar expression they are treated as ordinary characters.
The following examples clarify the differences between lex regular
expressions and regular expressions appearing elsewhere in this docu‐
ment. 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 frag‐
ment 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.
intyyless(int n) Retains n initial characters in yytext, NUL-termi‐
nated, 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 intermedi‐
ate 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 char‐
acters 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, call‐
ing 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 gener‐
ated 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 reg‐
ular expressions in the lex source code or generated lexical analyzer.
It would seem desirable to have the lexical analyzer interpret the reg‐
ular expressions given in the lex source according to the environment
specified when the lexical analyzer is executed, but this is not possi‐
ble with the current lex technology. Furthermore, the very nature of
the lexical analyzers produced by lex must be closely tied to the lexi‐
cal requirements of the input language being described, which will fre‐
quently be locale-specific anyway. (For example, writing an analyzer
that is used for French text will not automatically be useful for pro‐
cessing other languages.)
EXAMPLES
Example 1 Using lex
The following is an example of a lex program that implements a rudimen‐
tary 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(5) 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
See attributes(5) for descriptions of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Availability │developer/object-file │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Committed │
├─────────────────────────────┼─────────────────────────────┤
│Standard │See standards(5). │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOyacc(1), attributes(5), environ(5), regex(5), standards(5)NOTES
If routines such as yyback(), yywrap(), and yylock() in .l (ell) files
are to be external C functions, the command line to compile a C++ pro‐
gram must define the __EXTERN_C__ macro. For example:
example% CC -D__EXTERN_C__ ... file
SunOS 5.11 22 Aug 1997 lex(1)
