LEX(1P) POSIX Programmer's Manual LEX(1P)PROLOG
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implementation of this interface may differ (consult the corresponding
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NAMElex - generate programs for lexical tasks (DEVELOPMENT)
SYNOPSISlex [-t][-n|-v][file ...]
DESCRIPTION
The lex utility shall generate C programs to be used in lexical pro‐
cessing of character input, and that can be used as an interface to
yacc. The C programs shall be generated from lex source code and con‐
form to the ISO C standard. Usually, the lex utility shall write 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 the EXTENDED
DESCRIPTION section for a complete description of the lex input lan‐
guage.
OPTIONS
The lex utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines.
The following options shall be supported:
-n Suppress 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 Write the resulting program to standard output instead of
lex.yy.c.
-v Write a summary of lex statistics to the standard output. (See
the discussion of lex table sizes in Definitions in lex .) If
the -t option is specified and -n is not specified, this report
shall be written to standard error. If table sizes are specified
in the lex source code, and if the -n option is not specified,
the -v option may be enabled.
OPERANDS
The following operand shall be supported:
file A pathname of an input file. If more than one such file is spec‐
ified, all files shall be concatenated to produce a single lex
program. If no file operands are specified, or if a file operand
is '-', the standard input shall be used.
STDIN
The standard input shall be used if no file operands are specified, or
if a file operand is '-' . See INPUT FILES.
INPUT FILES
The input files shall be text files containing lex source code, as
described in the EXTENDED DESCRIPTION section.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of lex:
LANG Provide a default value for the internationalization variables
that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
ables for the precedence of internationalization variables used
to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all
the other internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges, equivalence
classes, and multi-character collating elements within regular
expressions. If this variable is not set to the POSIX locale,
the results are unspecified.
LC_CTYPE
Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments and input files),
and the behavior of character classes within regular expres‐
sions. If this variable is not set to the POSIX locale, the
results are unspecified.
LC_MESSAGES
Determine the locale that should be used to affect the format
and contents of diagnostic messages written to standard error.
NLSPATH
Determine the location of message catalogs for the processing of
LC_MESSAGES .
ASYNCHRONOUS EVENTS
Default.
STDOUT
If the -t option is specified, the text file of C source code output of
lex shall be written to standard output.
If the -t option is not specified:
* Implementation-defined informational, error, and warning messages
concerning the contents of lex source code input shall be written to
either the standard output or standard error.
* If the -v option is specified and the -n option is not specified,
lex statistics shall also be written to either the standard output
or standard error, in an implementation-defined format. These sta‐
tistics may also be generated if table sizes are specified with a
'%' operator in the Definitions section, as long as the -n option is
not specified.
STDERR
If the -t option is specified, implementation-defined informational,
error, and warning messages concerning the contents of lex source code
input shall be written to the standard error.
If the -t option is not specified:
1. Implementation-defined informational, error, and warning messages
concerning the contents of lex source code input shall 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 shall also be written to either the standard output
or standard error, in an implementation-defined format. These sta‐
tistics may also be generated if table sizes are specified with a
'%' operator in the Definitions section, as long as the -n option
is not specified.
OUTPUT FILES
A text file containing C source code shall be written to lex.yy.c, or
to the standard output if the -t option is present.
EXTENDED DESCRIPTION
Each input file shall contain lex source code, which is a table of reg‐
ular 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 c99), the resulting program shall read character
input from the standard input and shall partition it into strings that
match the given expressions.
When an expression is matched, these actions shall occur:
* The input string that was matched shall be left in yytext as a null-
terminated string; yytext shall either be an external character
array or a pointer to a character string. As explained in Defini‐
tions in lex, the type can be explicitly selected using the %array
or %pointer declarations, but the default is implementation-defined.
* The external int yyleng shall be set to the length of the matching
string.
* The expression's corresponding program fragment, or action, shall be
executed.
During pattern matching, lex shall search the set of patterns for the
single longest possible match. Among rules that match the same number
of characters, the rule given first shall be chosen.
The general format of lex source shall be:
Definitions
%%
Rules
%%
UserSubroutines
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 section beginning with a <blank> shall be
assumed to be a C program fragment and shall be copied to the external
definition area of the lex.yy.c file. Similarly, anything in the Defi‐
nitions section included between delimiter lines containing only "%{"
and "%}" shall also be copied unchanged to the external definition area
of the lex.yy.c file.
Any such input (beginning with a <blank> or within "%{" and "%}" delim‐
iter lines) appearing at the beginning of the Rules section before any
rules are specified shall 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> 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 appear before the first "%%" delimiter. Any line in this
section not contained between "%{" and "%}" lines and not beginning
with a <blank> shall be assumed to define a lex substitution string.
The format of these lines shall be:
name substitute
If a name does not meet the requirements for identifiers in the ISO C
standard, the result is undefined. The string substitute shall replace
the string { name} when it is used in a rule. The name string shall be
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 section, any line beginning with a '%' (percent
sign) character and followed by an alphanumeric word beginning with
either 's' or 'S' shall define a set of start conditions. Any line
beginning with a '%' followed by a word beginning with either 'x' or
'X' shall define a set of exclusive start conditions. When the gener‐
ated scanner is in a %s state, patterns with no state specified shall
be also active; in a %x state, such patterns shall not be active. The
rest of the line, after the first word, shall be considered to be one
or more <blank>-separated names of start conditions. Start condition
names shall be 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 shall accept either of the following two mutually-
exclusive declarations in the Definitions 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.
The default type of yytext is implementation-defined. If an application
refers to yytext outside of the scanner source file (that is, via an
extern), the application shall include the appropriate %array or
%pointer declaration in the scanner source file.
Implementations shall accept declarations in the Definitions section
for setting certain internal table sizes. The declarations are shown in
the following table.
Table: Table Size Declarations in lex
Declaration Description Minimum Value
%p n Number of positions 2500
%n n Number of states 500
%a n Number of transitions 2000
%e n Number of parse tree nodes 1000
%k n Number of packed character classes 1000
%o n Size of the output array 3000
In the table, n represents a positive decimal integer, preceded by one
or more <blank>s. The exact meaning of these table size numbers is
implementation-defined. The implementation shall document how these
numbers affect the lex utility and how they are related to any output
that may be generated by the implementation should limitations be
encountered during the execution of lex. It shall be possible to deter‐
mine from this output which of the table size values needs to be modi‐
fied to permit lex to successfully generate tables for the input lan‐
guage. The values in the column Minimum Value represent the lowest
values conforming implementations shall provide.
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 shall be sepa‐
rated from action by one or more <blank>s. A regular expression con‐
taining <blank>s shall be recognized under one of the following condi‐
tions:
* The entire expression appears within double-quotes.
* The <blank>s appear within double-quotes or square brackets.
* Each <blank> is preceded by a backslash character.
User Subroutines in lex
Anything in the user subroutines section shall be copied to lex.yy.c
following yylex().
Regular Expressions in lex
The lex utility shall support the set of extended regular expressions
(see the Base Definitions volume of IEEE Std 1003.1-2001, Section 9.4,
Extended Regular Expressions), with the following additions and excep‐
tions to the syntax:
"..." Any string enclosed in double-quotes shall represent the charac‐
ters within the double-quotes as themselves, except that back‐
slash escapes (which appear in the following table) shall be
recognized. Any backslash-escape sequence shall be 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 shall be matched only when the program
is in one of the start conditions indicated by state, state1,
and so on; see Actions in lex . (As an exception to the typo‐
graphical conventions of the rest of this volume of
IEEE Std 1003.1-2001, in this case <state> does not represent a
metavariable, but the literal angle-bracket characters surround‐
ing a symbol.) The start condition shall be recognized as such
only at the beginning of a regular expression.
r/x The regular expression r shall be matched only if it is followed
by an occurrence of regular expression x ( x is the instance of
trailing context, further defined below). The token returned in
yytext shall 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.
{name} When name is one of the substitution symbols from the Defini‐
tions section, the string, including the enclosing braces, shall
be replaced by the substitute value. The substitute value shall
be treated in the extended regular expression as if it were
enclosed in parentheses. No substitution shall occur if { name}
occurs within a bracket expression or within double-quotes.
Within an ERE, a backslash character shall be considered to begin an
escape sequence as specified in the table in the Base Definitions vol‐
ume of IEEE Std 1003.1-2001, Chapter 5, File Format Notation ( '\\',
'\a', '\b', '\f', '\n', '\r', '\t', '\v' ). In addition, the escape
sequences in the following table shall be recognized.
A literal <newline> cannot occur within an ERE; the escape sequence
'\n' can be used to represent a <newline>. A <newline> shall not be
matched by a period operator.
Table: Escape Sequences in lex
Escape
Sequence Description Meaning
\digits A backslash character followed The character whose encoding
by the longest sequence of is represented by the one,
one, two, or three octal-digit two, or three-digit octal
characters (01234567). If all integer. If the size of a byte
of the digits are 0 (that is, on the system is greater than
representation of the NUL nine bits, the valid escape
character), the behavior is sequence used to represent a
undefined. byte is implementation-
defined. Multi-byte characters
require multiple, concatenated
escape sequences of this type,
including the leading '\' for
each byte.
\xdigits A backslash character followed The character whose encoding
by the longest sequence of is represented by the hexadec‐
hexadecimal-digit characters imal integer.
(01234567abcdefABCDEF). If all
of the digits are 0 (that is,
representation of the NUL
character), the behavior is
undefined.
\c A backslash character followed The character 'c', unchanged.
by any character not described
in this table or in the table
in the Base Definitions volume
of IEEE Std 1003.1-2001, Chap‐
ter 5, File Format Notation (
'\\', '\a', '\b', '\f', '\n',
'\r', '\t', '\v' ).
Note: If a '\x' sequence needs to be immediately followed by a hexa‐
decimal digit character, a sequence such as "\x1" "1" can be
used, which represents a character containing the value 1, fol‐
lowed by the character '1' .
The order of precedence given to extended regular expressions for lex
differs from that specified in the Base Definitions volume of
IEEE Std 1003.1-2001, Section 9.4, Extended Regular Expressions. The
order of precedence for lex shall be 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, trail‐
ing 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.
Table: ERE Precedence in lex
Extended Regular Expression Precedence
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$", it would match either "abc" or
"def" on a line by itself.
Unlike the general ERE rules, embedded anchoring is not allowed by most
historical lex implementations. An example of embedded anchoring would
be for patterns such as "(^| )foo( |$)" 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 '/' 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 shall be special only in a
start condition at the beginning of a regular expression; elsewhere in
the regular expression they shall be treated as ordinary characters.
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 ';' shall be 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 shall
not be 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 <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 shall be to copy the string to the out‐
put. 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 "%%" shall generate a C program that simply copies the
input to the output unchanged.
Four special actions shall be 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; the applica‐
tion shall ensure that it is 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 shall cause 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 new‐
state has not been declared previously as a start condition in the Def‐
initions 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 c99 (the lex library).
int yylex(void)
Performs lexical analysis on the input; this is the primary
function generated by the lex utility. The function shall return
zero when the end of input is reached; otherwise, it shall
return non-zero values (tokens) determined by the actions that
are selected.
int yymore(void)
When called, indicates that when the next input string is recog‐
nized, it is to be appended to the current value of yytext
rather than replacing it; the value in yyleng shall be 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 shall be adjusted accordingly.
int input(void)
Returns the next character from the input, or zero on end-of-
file. It shall obtain input from the stream pointer yyin,
although possibly via an intermediate buffer. Thus, once scan‐
ning has begun, the effect of altering the value of yyin is
undefined. The character read shall be 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 shall appear only in the lex library accessible
through the -l l operand; they can therefore be redefined by a conform‐
ing application:
int yywrap(void)
Called by yylex() at end-of-file; the default yywrap() shall
always 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 shall 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 opera‐
tions, calling yylex() as applicable.
Except for input(), unput(), and main(), all external and static names
generated by lex shall begin with the prefix yy or YY.
EXIT STATUS
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
Conforming applications are warned that in the Rules section, an ERE
without an action is not acceptable, but need not be detected as erro‐
neous by lex. This may result in compilation or runtime 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 is fre‐
quently locale-specific anyway. (For example, writing an analyzer that
is used for French text is not automatically useful for processing
other languages.)
EXAMPLES
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();
}
RATIONALE
Even though the -c option and references to the C language are retained
in this description, lex may be generalized to other languages, as was
done at one time for EFL, the Extended FORTRAN Language. Since the lex
input specification is essentially language-independent, versions of
this utility could be written to produce Ada, Modula-2, or Pascal code,
and there are known historical implementations that do so.
The current description of lex bypasses the issue of dealing with
internationalized EREs in the lex source code or generated lexical ana‐
lyzer. If it follows the model used by awk (the source code is assumed
to be presented in the POSIX locale, but input and output are in the
locale specified by the environment variables), then the tables in the
lexical analyzer produced by lex would interpret EREs specified in the
lex source in terms of the environment variables specified when lex was
executed. The desired effect would be to have the lexical analyzer
interpret the EREs 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.
The description of octal and hexadecimal-digit escape sequences agrees
with the ISO C standard use of escape sequences. See the RATIONALE for
ed for a discussion of bytes larger than 9 bits being represented by
octal values. Hexadecimal values can represent larger bytes and multi-
byte characters directly, using as many digits as required.
There is no detailed output format specification. The observed behavior
of lex under four different historical implementations was that none of
these implementations consistently reported the line numbers for error
and warning messages. Furthermore, there was a desire that lex be
allowed to output additional diagnostic messages. Leaving message for‐
mats unspecified avoids these formatting questions and problems with
internationalization.
Although the %x specifier for exclusive start conditions is not histor‐
ical practice, it is believed to be a minor change to historical imple‐
mentations and greatly enhances the usability of lex programs since it
permits an application to obtain the expected functionality with fewer
statements.
The %array and %pointer declarations were added as a compromise between
historical systems. The System V-based lex copies the matched text to a
yytext array. The flex program, supported in BSD and GNU systems, uses
a pointer. In the latter case, significant performance improvements are
available for some scanners. Most historical programs should require no
change in porting from one system to another because the string being
referenced is null-terminated in both cases. (The method used by flex
in its case is to null-terminate the token in place by remembering the
character that used to come right after the token and replacing it
before continuing on to the next scan.) Multi-file programs with exter‐
nal references to yytext outside the scanner source file should con‐
tinue to operate on their historical systems, but would require one of
the new declarations to be considered strictly portable.
The description of EREs avoids unnecessary duplication of ERE details
because their meanings within a lex ERE are the same as that for the
ERE in this volume of IEEE Std 1003.1-2001.
The reason for the undefined condition associated with text beginning
with a <blank> or within "%{" and "%}" delimiter lines appearing in the
Rules section is historical practice. Both the BSD and System V lex
copy the indented (or enclosed) input in the Rules section (except at
the beginning) to unreachable areas of the yylex() function (the code
is written directly after a break statement). In some cases, the System
V lex generates an error message or a syntax error, depending on the
form of indented input.
The intention in breaking the list of functions into those that may
appear in lex.yy.c versus those that only appear in libl.a is that only
those functions in libl.a can be reliably redefined by a conforming
application.
The descriptions of standard output and standard error are somewhat
complicated because historical lex implementations chose to issue diag‐
nostic messages to standard output (unless -t was given).
IEEE Std 1003.1-2001 allows this behavior, but leaves an opening for
the more expected behavior of using standard error for diagnostics.
Also, the System V behavior of writing the statistics when any table
sizes are given is allowed, while BSD-derived systems can avoid it. The
programmer can always precisely obtain the desired results by using
either the -t or -n options.
The OPERANDS section does not mention the use of - as a synonym for
standard input; not all historical implementations support such usage
for any of the file operands.
A description of the translation table was deleted from early proposals
because of its relatively low usage in historical applications.
The change to the definition of the input() function that allows
buffering of input presents the opportunity for major performance gains
in some applications.
The following examples clarify the differences between lex regular
expressions and regular expressions appearing elsewhere in this volume
of IEEE Std 1003.1-2001. 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 expres‐
sion "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 extends 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.
FUTURE DIRECTIONS
None.
SEE ALSO
c99, ed, yacc
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 LEX(1P)
