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 Indicate C-language action (default option).
-e Generate a program that can handle EUC characters
(cannot be used with the -w option). yytext[] is
of type unsigned char[].
-n Suppress the summary of statistics usually written
with the -v option. If no table sizes are speci-
fied in the lex source code and the -v option is
not specified, then -n is implied.
-t Write the resulting program to standard output
instead of lex.yy.c.
-v Write 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 Generate 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 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.
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
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 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 DESCRIP-
TION), 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 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 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 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
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
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 supplemen-
tary 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 actionERE 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 char-
acter.
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 table) are recog-
nized. Any backslash-escape sequence is terminated
by the closing quote. For example, "\01""1"
represents a single string: the octal value 1 fol-
lowed 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 condi-
tion 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 expres-
sion 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 substi-
tute 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
_______________________________________________________________________________
| 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.
_________________________________________________________________
| 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 */
Note 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 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
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 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
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; Write 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 over-
lapping input strings. For example, given the reg-
ular 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 pre-
viously 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-
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 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 func-
tion 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 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 pos-
sible 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 imple-
ments 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(5) for descriptions of the following environment
variables that affect the execution of lex : 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 attri-
butes:
____________________________________________________________
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|_____________________________|_____________________________|
| Availability | SUNWbtool |
|_____________________________|_____________________________|
SEE ALSO
yacc(1), attributes(5), environ(5), regex(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 __EXTERN_C__ macro.
For example:
example% CC -D__EXTERN_C__ . . . file
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