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If the -t option is not specified:
When lex.yy.c is compiled and linked with the lex library (using the -l l operand with c89 or cc), the resulting program reads character input from the standard input and partitions it into strings that match the given expressions.
When an expression is matched, these actions will occur:
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:
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 definition area of the lex.yy.c file.
Any such input (beginning with a blank character or within %{ and %} delimiter lines) appearing at the beginning of the Rules section before any rules are specified will be written to lex.yy.c after the 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 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.
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:
The default type of yytext is char[]. If an application refers to yytext outside of the scanner source file (that is, via an extern), the application will include the appropriate %array or %pointer declaration in the scanner source file.
lex will accept declarations in the Definitions in lex section for setting certain internal table sizes. The declarations are shown in the following table.
+-----------------------------------------------------------+ |Declaration Description Default | +-----------------------------------------------------------+ | | | %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 10000 | | %o n 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 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.
When either the -e or -w option is used, the generated C program must be linked with the wide character library libw.a using the -lw linker flag.
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:
<state>r
Within an ERE, a backslash character (\\, \a, \b, \f, \n, \r, \t, \v) is considered to begin an escape sequence. In addition, the escape sequences in the following table will be recognized.
A literal newline character cannot occur within an ERE; the escape sequence \n can be used to represent a newline character. A newline character cannot be matched by a period operator.
The order of precedence given to extended regular expressions for lex is as shown in the following table, from high to low.
+---------------------------------------------------------+
| |
|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:
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 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 elsewhere 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.
The specification for an action, including C statements and special actions, can extend across several lines if enclosed in braces:
ERE <one or more blanks> { program statement
program statement }
The default action when a string in the input to a lex.yy.c program is not matched by any expression is to copy the string to the output. Because the default behavior of a program generated by lex is to read the input and copy it to the output, a minimal lex source program that has just %% generates a C program that simply copies the input to the output unchanged.
Four special actions are available:
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).
The following functions appear only in the lex library accessible through the -l l operand; they can therefore be redefined by a portable application:
The reason for breaking these functions into two lists is that only those functions in libl.a can be reliably redefined by a portable application.
Except for input, unput and main, all external and static names generated by lex begin with the prefix yy or YY.
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
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 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.)
%{
/* 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();
}
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Created by unroff & hp-tools. © by Hans-Peter Bischof. All Rights Reserved (1997).
Last modified 21/April/97