User Commands lex(1)
NAME
lex - generate programs for lexical tasks
SYNOPSIS
lex [-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 charac-
ters (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 stan-
dard 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 charac-
ters (cannot be used with the -e option). Unlike
the -e option, yytext[] is of type wchart[].
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-V Prints out version information on standard
error.
-Q[yn] 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.
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 warn-
ing 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 writ-
ten 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.
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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 pro-
gram reads character input from the standard input and par-
titions 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 charac-
ter 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
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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 defini-
tion 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 begin-
ning 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 sub-
stitute 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
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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 refer-
ence is of the form:
extern char *yytext;
lex will accept declarations in the Definitions in lex sec-
tion for setting certain internal table sizes. The declara-
tions are shown in the following table.
Table Size Declaration in lex
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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 ASCI 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 ASCI 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 wchart[]
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
(wchart). 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
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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 Expres-
sions (EREs) described on regex(5) with the following addi-
tions 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 exam-
ple, "\01""1" represents a single string: the
octal value 1 followed by the character 1.
r
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
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
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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 $ opera-
tor. 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 char-
acter. A newline character cannot be matched by a period
operator.
Escape Sequences in lex
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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 expres-
sions 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.
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ERE Precedence in lex
collation-related bracket symbols [= =] [: :] [. .]
escaped characters \
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 ^abcdef$,
it would match either of abc or def on a line by itself.
Unlike the general ERE rules, embedded anchoring is not
allowed by most historical lex implementations. An example
of embedded anchoring would be for patterns such as
(^)foo($) 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
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> 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 else-
where 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 unde-
fined.
The specification for an action, including C statements and
special actions, can extend across several lines if enclosed
in braces:
ERE { 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 pro-
gram 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
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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 "con-
tinue to the next expression that matches the
current input," and causes whatever rule was the
second choice after the current rule to be exe-
cuted 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 con-
trol 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 new-
state. 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.
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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 accord-
ingly.
intyyless(int n) Retains n initial characters in yytext,
NUL-terminated, and treats the remain-
ing 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 inter-
mediate 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 charac-
ters than have been input is unspeci-
fied.
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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 rede-
fined by a portable application.
Except for input, unput and main, all external and static
names generated by lex begin with the prefix yy or Y.
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 treat-
ment of regular expressions in the lex source code or gen-
erated 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
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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
/* need this for printf(), fopen() and stdin below */
#include
%}
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));
}
ifthenbeginendprocedurefunction {
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[])
{
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]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, LCAL,
LCOLATE, LCTYPE, LCMESAGES, and NLSPATH.
EXIT STATUS
The following exit values are returned:
0 Successful completion.
>0 An error occurred.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWbtool
Interface Stability Standard
SEE ALSO
yacc(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] program must define the EXTERNC macro.
For example:
example% C -DEXTERNC ... file
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