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lex(1)
NAME
lex - Generates programs for lexical tasks
SYNOPSIS
lex [-ct] [-n | -v] [file...]
[Tru64 UNIX] The following syntax applies when the CMD_ENV environment
variable is set to svr4:
lex [-crt] [-n | -v] [-V] [-Qy | -Qn] [file...]
STANDARDS
Interfaces documented on this reference page conform to industry standards
as follows:
lex: XPG4, XPG4-UNIX
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
OPTIONS
-c Writes C code to the file lex.yy.c. This is the default.
-n Suppresses the statistics summary. When you set your own table sizes
for the finite state machine, lex automatically produces this summary
if you do not select this flag.
-r [Tru64 UNIX] Writes RATFOR code to the file lex.yy.r. (There is no
RATFOR compiler for Tru64 UNIX.)
-t Writes to standard output instead of writing to a file.
-v Provides a summary of the generated finite state machine statistics.
-V [Tru64 UNIX] Outputs lex version number to standard error. Requires
the environment variable CMD_ENV to be set to svr4.
-Q[y|n]
[Tru64 UNIX] Determines whether the lex version number is written to
the output file. The -Qn option does not do so and is the default.
Requires the environment variable CMD_ENV to be set to svr4.
DESCRIPTION
The lex command uses the rules and actions contained in file to generate a
program, lex.yy.c, which can be compiled with the cc command. That program
can then receive input, break the input into the logical pieces defined by
the rules in file, and run program fragments contained in the actions in
file.
The generated program is a C Language function called yylex(). The lex
command stores yylex() in a file named lex.yy.c. You can use yylex() alone
to recognize simple, 1-word input, or you can use it with other C Language
programs to perform more difficult input analysis functions. For example,
you can use lex to generate a program that tokenizes an input stream before
sending it to a parser program generated by the yacc command.
The yylex() function analyzes the input stream using a program structure
called a finite state machine. This structure allows the program to exist
in only one state (or condition) at a time. A finite number of states are
allowed. The rules in file determine how the program moves from one state
to another in response to the input that the program receives.
The lex command reads its skeleton finite state machine from the file
/usr/ccs/lib/ncpform or /usr/ccs/lib/ncform. Use the environment variable
LEXER to specify another location for lex to read from.
If you do not specify a file, lex reads standard input. It treats multiple
files as a single file.
Input File Format
The input file can contain three sections: definitions, rules, and user
subroutines. Each section must be separated from the others by a line
containing only the delimiter, %%. The format is as follows:
definitions
%%
rules
%%
user_subroutines
The purpose and format of each of these sections are described under the
headings that follow.
Definitions Section
If you want to use variables in rules, you must define them in the
definitions section. The variables make up the left column, and their
definitions make up the right column. For example, to define D as a
numerical digit, enter:
D [0-9]
You can use a defined variable in the rules section by enclosing the
variable name in braces, {D}.
In the definitions section, you can set either of the following two
mutually exclusive declarations:
%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. Use of the %pointer definition selects the
/usr/ccs/lib/ncpform skeleton.
In the definitions section, you can also set table sizes for the resulting
finite state machine. The default sizes are large enough for small
programs. You may want to set larger sizes for more complex programs:
%p number
Number of positions is number (default 5000)
%n number
Number of states is number (default 2500)
%e number
Number of parse tree nodes is number (default 2000)
%a number
Number of transitions is number (default 5000)
%k number
Number of packed character classes is number (default 2000)
%o number
Number of output slots is number (default 5000)
If extended characters appear in regular expression strings, you may need
to reset the output array size with the %o parameter (possibly to array
sizes in the range 10,000 to 20,000). This reset reflects the much larger
number of extended characters relative to the number of ASCII characters.
Rules Section
The rules section is required, and it must be preceded by the %% delimiter,
even if you do not have a definitions section. The lex command does not
recognize rules without the delimiter.
In this section, the left column contains the pattern to be recognized in
an input file to yylex(). The right column contains the C program fragment
executed when that pattern is recognized.
Patterns can include extended characters with one exception: extended
characters may not appear in range specifications within character class
expressions surrounded by brackets.
The columns are separated by a tab. For example, to search files for the
word LEAD and replace it with GOLD, perform the following steps:
1. Create a file called transmute.l containing the lines:
%%
(LEAD) printf("GOLD");
2. Then issue the following commands to the shell:
lex transmute.l
cc -o transmute lex.yy.c -ll
3. You can test the resulting program with the command:
transmute <transmute.l
This command echoes the contents of transmute.l, with the occurrences of
LEAD changed to GOLD.
Each pattern may have a corresponding action, that is, a fragment of C
source code to execute when the pattern is matched. Each statement must
end with a ; (semicolon). If you use more than one statement in an action,
you must enclose all of them in {} (braces). A second delimiter, %%, must
follow the rules section if you have a user subroutine section.
When yylex() matches a string in the input stream, it copies the matched
text to an external character array, yytext, before it executes any actions
in the rules section.
You can use the following operators to form patterns that you want to
match:
x, y
Matches the characters written.
[ ] Matches any one character in the enclosed range ([.-.]) or the enclosed
list ([...]). [abcx-z] matches a,b,c,x,y, or z.
" " Matches the enclosed character or string even if it is an operator.
"$" prevents lex from interpreting the $ character as an operator.
\ Acts the same as double quotes. \$ prevents lex from interpreting the
$ character as an operator.
* Matches zero or more occurrences of the single-character regular
expression immediately preceding it. x* matches zero or more repeated
literal characters x.
+ Matches one or more occurrences of the single-character regular
expression immediately preceding it.
? Matches either zero or one occurrence of the single-character regular
expression immediately preceding it.
^ Matches the character only at the beginning of a line. ^x matches an x
at the beginning of a line.
[^] Matches any character except for the characters following the ^.
[^xyz] matches any character but x, y, or z.
. Matches any character except the newline character.
$ Matches the end of a line.
| Matches either of two characters. x|y matches either x or y.
/ Matches one extended regular expression (ERE) only when followed by a
second ERE. It reads only the first token into yytext. Given the
regular expression a*b/cc and the input aaabcc, yytext would contain
the string aaab on this match.
( ) Matches the pattern in the ( ) (parentheses). This is used for
grouping. It reads the whole pattern into yytext. A group in
parentheses can be used in place of any single character in any other
pattern. (xyz123) matches the pattern xyz123 and reads the whole
string into yytext.
{} Matches the character as defined in the definitions section. If D is
defined as numeric digits, {D} matches all numeric digits.
{m,n}
Matches m-to-n occurrences of the specified character. x{2,4} matches
2, 3, or 4 occurrences of x.
If a line begins with only a space, lex copies it to the lex.yy.c output
file. If the line is in the definitions section of file, lex copies it to
the declarations section of lex.yy.c. If the line is in the rules section,
lex copies it to the program code section of lex.yy.c.
User Subroutines Section
The lex library has three subroutines defined as macros that you can use in
the rules.
input( )
Reads a character from yyin.
unput( )
Replaces a character after it is read.
output( )
Writes a character to yyout.
You can override these three macros by writing your own code for these
routines in the user subroutines section. But if you write your own
routines, you must undefine these macros in the definitions section as
follows:
%{
#undef input
#undef unput
#undef output
}%
When you are using lex as a simple transformer/recognizer for stdin to
stdout piping, you can avoid writing the framework by using libl.a (the lex
library). It has a main routine that calls yylex() for you.
External names generated by lex all begin with the prefix yy, as in yyin,
yyout, yylex, and yytext.
Putting Spaces in an Expression
Normally, spaces or tabs end a rule and, therefore, the expression that
defines a rule. However, you can enclose the spaces or tab characters in
"" (double quotes) to include them in the expression. Use quotes around all
spaces in expressions that are not already within sets of [ ] (brackets).
Other Special Characters
The lex program recognizes many of the normal C language special
characters. These character sequences are as follows:
Sequence Meaning
\n Newline
\t Tab
\b Backspace
\\ Backslash
\digits
The character whose encoding is represented
by the three-digit octal number
\xdigits
The character whose encoding is represented
by the hexadecimal integer
Do not use the actual newline character in an expression.
When using these special characters in an expression, you do not need to
enclose them in quotes. Every character, except these special characters
and the previously described operator symbols, is always a text character.
Matching Rules
When more than one expression can match the current input, lex chooses the
longest match first. Among rules that match the same number of characters,
the rule that occurs first is chosen. For example:
integer keyword action...;
[a-z]+ identifier action...;
If the preceding rules are given in that order and integers is the input
word, lex matches the input as an identifier because [a-z]+ matches eight
characters, while integer matches only seven. However, if the input is
integer, both rules match seven characters. The keyword rule is selected
because it occurs first. A shorter input, such as int, does not match the
expression rule integer and causes lex to select the rule identifier.
Matching a String with Wildcard Characters
Because lex chooses the longest match first, do not use rules containing
expressions like .* (for example: '.*').
The preceding rule might seem like a good way to recognize a string in
single quotes. However, the lexical analyzer reads far ahead, looking for
a distant single quote to complete the long match. If a lexical analyzer
with such a rule gets the following input, it matches the whole string:
'first' quoted string here, 'second' here
To find the smaller strings, first and second, use the following rule:
'[^'\n]*'
This rule stops after matching 'first'.
Errors of this type are not far-reaching because the . (dot) operator does
not match a newline character. Therefore, expressions like .* stop on the
current line. Do not try to defeat this with expressions like [.\n] +. The
lexical analyzer tries to read the entire input file, and an internal
buffer overflow occurs.
Finding Strings within Strings
The lex program partitions the input stream and does not search for all
possible matches of each expression. Each character is accounted for once
and only once. For example, to count occurrences of both she and he in an
input text, try the following rules:
she s++;
he h++;
\n |
. ;
The last two rules ignore everything besides he and she. However, because
she includes he, lex does not recognize the instances of he that are
included in she.
To override this choice, use the REJECT action. This directive tells lex
to go to the next rule. The lex command then adjusts the position of the
input pointer to where it was before the first rule was executed, and
executes the second choice rule. For example, to count the included
instances of he, use the following rules:
she {s++; REJECT;}
he {h++; REJECT;}
\n |
. ;
After counting the occurrences of she, lex rejects the input stream and
then counts the occurrences of he. In this case, you can omit the REJECT
action on he because she includes he but not vice versa. In other cases, it
may be difficult to determine which input characters are in both classes.
In general, REJECT is useful whenever the purpose of lex is not to
partition the input stream but to detect all examples of some items in the
input, and the instances of these items may overlap or include each other.
NOTES
Because lex uses fixed names for intermediate and output files, you can
have only one lex-generated program in a given directory. If the -t option
is not specified, informational, error, and warning messages are written to
stdout. If the -t option is specified, informational, error, and warning
messages are written to stderr.
[Tru64 UNIX] The yytext array has a default dimension of 200, controlled
by the constant YYLMAX. If the programmer needs to allow a larger array,
the YYLMAX constant may be redefined as follows from within the lex command
file:
{
#undef YYLMAX
#define YYLMAX 8192
}
Two other arrays use YYLMAX, yysubf, and yylstate.
The lex program can be compiled as a C program with -std0, -std, or -std1
mode. It can also be compiled as a C++ program. If YY_NOPROTO is defined on
the compilation command line, function prototypes are not generated.
EXAMPLES
1. The following command draws lex instructions from the file lexcommands
and places the output in lex.yy.c:
lex lexcommands
2. The file lexcommands contains an example of a lex program that would
be put into a lex command file. The following program converts
uppercase to lowercase, removes spaces at the end of a line, and
replaces multiple spaces with single spaces:
%%
[A-Z] putchar(tolower(yytext[0]));
[ ]+$ ;
[ ]+ putchar(' ');
ENVIRONMENT VARIABLES
The following environment variables affect the behavior of lex():
LANG
Provides a default value for the locale category variables that are not
set or null.
LC_ALL
If set, overrides the values of all other locale variables.
LC_COLLATE
Determines the order in which output is sorted for the -x option.
LC_CTYPE
Determines the locale for the interpretation of byte sequences as
characters (single-byte or multi-byte) in input parameters and files.
LC_MESSAGES
Determines the locale used to affect the format and contents of
diagnostic messages displayed by the command.
NLSPATH
Determines the location of message catalogs for the processing of
LC_MESSAGES.
FILES
/usr/ccs/lib/libl.a
Run-time library.
/usr/ccs/lib/ncform
Default C language skeleton finite state machine for lex.
/usr/ccs/lib/ncpform
Default C language skeleton finite state machine for lex, implemented
with the pointer definition of yytext.
/usr/ccs/lib/nrform
Default RATFOR language skeleton finite state machine for lex.
SEE ALSO
Commands: yacc(1)
Standards: standards(5)
Programming Support Tools
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