flexdoc.1

C++版 词法分析、语法分析器

.TH FLEX 1 "26 May 1990" "Version 2.3"
.SH NAME
flex - fast lexical analyzer generator
.SH SYNOPSIS
.B flex
.B [-bcdfinpstvFILT8 -C[efmF] -Sskeleton]
.I [filename ...]
.SH DESCRIPTION
.I flex
is a tool for generating
.I scanners:
programs which recognized lexical patterns in text.
.I flex
reads
the given input files, or its standard input if no file names are given,
for a description of a scanner to generate.  The description is in
the form of pairs
of regular expressions and C code, called
.I rules.  flex
generates as output a C source file,
.B lex.yy.c,
which defines a routine
.B yylex().
This file is compiled and linked with the
.B -lfl
library to produce an executable.  When the executable is run,
it analyzes its input for occurrences
of the regular expressions.  Whenever it finds one, it executes
the corresponding C code.
.SH SOME SIMPLE EXAMPLES
.LP
First some simple examples to get the flavor of how one uses
.I flex.
The following
.I flex
input specifies a scanner which whenever it encounters the string
"username" will replace it with the user's login name:
.nf

    %%
    username    printf( "%s", getlogin() );

.fi
By default, any text not matched by a
.I flex
scanner
is copied to the output, so the net effect of this scanner is
to copy its input file to its output with each occurrence
of "username" expanded.
In this input, there is just one rule.  "username" is the
.I pattern
and the "printf" is the
.I action.
The "%%" marks the beginning of the rules.
.LP
Here's another simple example:
.nf

        int num_lines = 0, num_chars = 0;

    %%
    \\n    ++num_lines; ++num_chars;
    .     ++num_chars;

    %%
    main()
        {
        yylex();
        printf( "# of lines = %d, # of chars = %d\\n",
                num_lines, num_chars );
        }

.fi
This scanner counts the number of characters and the number
of lines in its input (it produces no output other than the
final report on the counts).  The first line
declares two globals, "num_lines" and "num_chars", which are accessible
both inside
.B yylex()
and in the
.B main()
routine declared after the second "%%".  There are two rules, one
which matches a newline ("\\n") and increments both the line count and
the character count, and one which matches any character other than
a newline (indicated by the "." regular expression).
.LP
A somewhat more complicated example:
.nf

    /* scanner for a toy Pascal-like language */

    %{
    /* need this for the call to atof() below */
    #include <math.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 whitespace */

    .           printf( "Unrecognized character: %s\\n", yytext );

    %%

    main( argc, argv )
    int argc;
    char **argv;
        {
        ++argv, --argc;  /* skip over program name */
        if ( argc > 0 )
                yyin = fopen( argv[0], "r" );
        else
                yyin = stdin;
        
        yylex();
        }

.fi
This is the beginnings of a simple scanner for a language like
Pascal.  It identifies different types of
.I tokens
and reports on what it has seen.
.LP
The details of this example will be explained in the following
sections.
.SH FORMAT OF THE INPUT FILE
The
.I flex
input file consists of three sections, separated by a line with just
.B %%
in it:
.nf

    definitions
    %%
    rules
    %%
    user code

.fi
The
.I definitions
section contains declarations of simple
.I name
definitions to simplify the scanner specification, and declarations of
.I start conditions,
which are explained in a later section.
.LP
Name definitions have the form:
.nf

    name definition

.fi
The "name" is a word beginning with a letter or an underscore ('_')
followed by zero or more letters, digits, '_', or '-' (dash).
The definition is taken to begin at the first non-white-space character
following the name and continuing to the end of the line.
The definition can subsequently be referred to using "{name}", which
will expand to "(definition)".  For example,
.nf

    DIGIT    [0-9]
    ID       [a-z][a-z0-9]*

.fi
defines "DIGIT" to be a regular expression which matches a
single digit, and
"ID" to be a regular expression which matches a letter
followed by zero-or-more letters-or-digits.
A subsequent reference to
.nf

    {DIGIT}+"."{DIGIT}*

.fi
is identical to
.nf

    ([0-9])+"."([0-9])*

.fi
and matches one-or-more digits followed by a '.' followed
by zero-or-more digits.
.LP
The
.I rules
section of the
.I flex
input contains a series of rules of the form:
.nf

    pattern   action

.fi
where the pattern must be unindented and the action must begin
on the same line.
.LP
See below for a further description of patterns and actions.
.LP
Finally, the user code section is simply copied to
.B lex.yy.c
verbatim.
It is used for companion routines which call or are called
by the scanner.  The presence of this section is optional;
if it is missing, the second
.B %%
in the input file may be skipped, too.
.LP
In the definitions and rules sections, any
.I indented
text or text enclosed in
.B %{
and
.B %}
is copied verbatim to the output (with the %{}'s removed).
The %{}'s must appear unindented on lines by themselves.
.LP
In the rules section,
any indented or %{} text appearing before the
first rule may be used to declare variables
which are local to the scanning routine and (after the declarations)
code which is to be executed whenever the scanning routine is entered.
Other indented or %{} text in the rule section is still copied to the output,
but its meaning is not well-defined and it may well cause compile-time
errors (this feature is present for
.I POSIX
compliance; see below for other such features).
.LP
In the definitions section, an unindented comment (i.e., a line
beginning with "/*") is also copied verbatim to the output up
to the next "*/".  Also, any line in the definitions section
beginning with '#' is ignored, though this style of comment is
deprecated and may go away in the future.
.SH PATTERNS
The patterns in the input are written using an extended set of regular
expressions.  These are:
.nf

    x          match the character 'x'
    .          any character except newline
    [xyz]      a "character class"; in this case, the pattern
                 matches either an 'x', a 'y', or a 'z'
    [abj-oZ]   a "character class" with a range in it; matches
                 an 'a', a 'b', any letter from 'j' through 'o',
                 or a 'Z'
    [^A-Z]     a "negated character class", i.e., any character
                 but those in the class.  In this case, any
                 character EXCEPT an uppercase letter.
    [^A-Z\\n]   any character EXCEPT an uppercase letter or
                 a newline
    r*         zero or more r's, where r is any regular expression
    r+         one or more r's
    r?         zero or one r's (that is, "an optional r")
    r{2,5}     anywhere from two to five r's
    r{2,}      two or more r's
    r{4}       exactly 4 r's
    {name}     the expansion of the "name" definition
               (see above)
    "[xyz]\\"foo"
               the literal string: [xyz]"foo
    \\X         if X is an 'a', 'b', 'f', 'n', 'r', 't', or 'v',
                 then the ANSI-C interpretation of \\x.
                 Otherwise, a literal 'X' (used to escape
                 operators such as '*')
    \\123       the character with octal value 123
    \\x2a       the character with hexadecimal value 2a
    (r)        match an r; parentheses are used to override
                 precedence (see below)


    rs         the regular expression r followed by the
                 regular expression s; called "concatenation"


    r|s        either an r or an s


    r/s        an r but only if it is followed by an s.  The
                 s is not part of the matched text.  This type
                 of pattern is called as "trailing context".
    ^r         an r, but only at the beginning of a line
    r$         an r, but only at the end of a line.  Equivalent
                 to "r/\\n".


    <s>r       an r, but only in start condition s (see
               below for discussion of start conditions)
    <s1,s2,s3>r
               same, but in any of start conditions s1,
               s2, or s3


    <<EOF>>    an end-of-file
    <s1,s2><<EOF>>
               an end-of-file when in start condition s1 or s2

.fi
The regular expressions listed above are grouped according to
precedence, from highest precedence at the top to lowest at the bottom.
Those grouped together have equal precedence.  For example,
.nf

    foo|bar*

.fi
is the same as
.nf

    (foo)|(ba(r*))

.fi
since the '*' operator has higher precedence than concatenation,
and concatenation higher than alternation ('|').  This pattern
therefore matches
.I either
the string "foo"
.I or
the string "ba" followed by zero-or-more r's.
To match "foo" or zero-or-more "bar"'s, use:
.nf

    foo|(bar)*

.fi
and to match zero-or-more "foo"'s-or-"bar"'s:
.nf

    (foo|bar)*

.fi
.LP
Some notes on patterns:
.IP -
A negated character class such as the example "[^A-Z]"
above
.I will match a newline
unless "\\n" (or an equivalent escape sequence) is one of the
characters explicitly present in the negated character class
(e.g., "[^A-Z\\n]").  This is unlike how many other regular
expression tools treat negated character classes, but unfortunately
the inconsistency is historically entrenched.
Matching newlines means that a pattern like [^"]* can match an entire
input (overflowing the scanner's input buffer) unless there's another
quote in the input.
.IP -
A rule can have at most one instance of trailing context (the '/' operator
or the '$' operator).  The start condition, '^', and "<<EOF>>" patterns
can only occur at the beginning of a pattern, and, as well as with '/' and '$',
cannot be grouped inside parentheses.  A '^' which does not occur at
the beginning of a rule or a '$' which does not occur at the end of
a rule loses its special properties and is treated as a normal character.
.IP
The following are illegal:
.nf

    foo/bar$
    <sc1>foo<sc2>bar

.fi
Note that the first of these, can be written "foo/bar\\n".
.IP
The following will result in '$' or '^' being treated as a normal character:
.nf

    foo|(bar$)
    foo|^bar

.fi
If what's wanted is a "foo" or a bar-followed-by-a-newline, the following
could be used (the special '|' action is explained below):
.nf

    foo      |
    bar$     /* action goes here */

.fi
A similar trick will work for matching a foo or a
bar-at-the-beginning-of-a-line.
.SH HOW THE INPUT IS MATCHED
When the generated scanner is run, it analyzes its input looking
for strings which match any of its patterns.  If it finds more than
one match, it takes the one matching the most text (for trailing
context rules, this includes the length of the trailing part, even
though it will then be returned to the input).  If it finds two
or more matches of the same length, the
rule listed first in the
.I flex
input file is chosen.
.LP
Once the match is determined, the text corresponding to the match
(called the
.I token)
is made available in the global character pointer
.B yytext,
and its length in the global integer
.B yyleng.
The
.I action
corresponding to the matched pattern is then executed (a more
detailed description of actions follows), and then the remaining
input is scanned for another match.
.LP
If no match is found, then the
.I default rule
is executed: the next character in the input is considered matched and
copied to the standard output.  Thus, the simplest legal
.I flex
input is:
.nf

    %%

.fi
which generates a scanner that simply copies its input (one character
at a time) to its output.
.SH ACTIONS
Each pattern in a rule has a corresponding action, which can be any
arbitrary C statement.  The pattern ends at the first non-escaped
whitespace character; the remainder of the line is its action.  If the
action is empty, then when the pattern is matched the input token
is simply discarded.  For example, here is the specification for a program
which deletes all occurrences of "zap me" from its input:
.nf

    %%
    "zap me"

.fi
(It will copy all other characters in the input to the output since
they will be matched by the default rule.)
.LP
Here is a program which compresses multiple blanks and tabs down to
a single blank, and throws away whitespace found at the end of a line:
.nf

    %%
    [ \\t]+        putchar( ' ' );
    [ \\t]+$       /* ignore this token */

.fi
.LP
If the action contains a '{', then the action spans till the balancing '}'
is found, and the action may cross multiple lines.
.I flex 
knows about C strings and comments and won't be fooled by braces found
within them, but also allows actions to begin with
.B %{
and will consider the action to be all the text up to the next
.B %}
(regardless of ordinary braces inside the action).
.LP
An action consisting solely of a vertical bar ('|') means "same as
the action for the next rule."  See below for an illustration.