ch02_06.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

and prints "<tt class="literal">hello world</tt>" followed by a newline on <tt class="literal">STDOUT</tt>.  If you leave the
filehandle out, Perl tries to interpret <tt class="literal">hello</tt> as a filehandle,
resulting in a syntax error.  Because this is so error-prone, some
people may wish to avoid barewords entirely.  The quoting operators
listed earlier provide many convenient forms, including the <tt class="literal">qw//</tt> "quote
words" construct which nicely quotes a list of space-separated words:
<blockquote>
<pre class="programlisting">@days = qw(Mon Tue Wed Thu Fri);
print STDOUT "hello world\n";</pre>
</blockquote>

You can go as far as to outlaw barewords entirely.  If you say:
<blockquote>
<pre class="programlisting">use strict 'subs';</pre>
</blockquote>

then any bareword will produce a compile-time error.  The restriction
lasts through the end of the enclosing scope.  An inner scope may
countermand this by saying:
<blockquote>
<pre class="programlisting">no strict 'subs';</pre>
</blockquote>

Note that the bare identifiers in constructs like:
<blockquote>
<pre class="programlisting">"${verb}able"
$days{Feb}</pre>
</blockquote>

are not considered barewords since they're allowed by explicit rule
rather than by having "no other interpretation in the grammar".
<a name="INDEX-600"></a><a name="INDEX-601"></a>
</p>

<p>
<a name="INDEX-602"></a><a name="INDEX-603"></a>
An unquoted name with a trailing double colon, such as
<tt class="literal">main::</tt> or <tt class="literal">Dog::</tt>, is always
treated as the package name.  Perl turns the would-be bareword
<tt class="literal">Camel::</tt> into the string "<tt class="literal">Camel</tt>"
at compile time, so this usage is not subject to rebuke by
<tt class="literal">use strict</tt>.</p>






<h3 class="sect2">2.6.5. Interpolating Array Values</h3>

<a name="INDEX-604"></a><a name="INDEX-605"></a><a name="INDEX-606"></a><a name="INDEX-607"></a><a name="INDEX-608"></a>
<p>Array variables are interpolated into double-quoted strings by joining
all elements of the array with the separator specified in the
<tt class="literal">$"</tt> variable<a href="#FOOTNOTE-14">[14]</a> (which contains a space by
default).  The following are equivalent:
<blockquote>
<pre class="programlisting">$temp = join( $", @ARGV );
print $temp;

print "@ARGV";</pre>
</blockquote>
<a name="INDEX-609"></a><a name="INDEX-610"></a><a name="INDEX-611"></a>
Within search patterns, which also undergo double-quotish interpolation,
there is an unfortunate ambiguity:  is <tt class="literal">/$foo[bar]/</tt> to be interpreted
as <tt class="literal">/${foo}[bar]/</tt> (where <tt class="literal">[bar]</tt> is a character class for the
regular expression) or as <tt class="literal">/${foo[bar]}/</tt> (where <tt class="literal">[bar]</tt> is the
subscript to array <tt class="literal">@foo</tt>)?  If <tt class="literal">@foo</tt> doesn't otherwise exist,
it's obviously a character class.  If <tt class="literal">@foo</tt> exists, Perl takes a
good guess about <tt class="literal">[bar]</tt>, and is almost always right.<a href="#FOOTNOTE-15">[15]</a>  If it does guess wrong,
or if you're just plain paranoid, you can force the correct
interpretation with braces as shown earlier.  Even if you're merely
prudent, it's probably not a bad idea.</p>
<blockquote class="footnote">

<a name="FOOTNOTE-14"></a>
<p>[14]<tt class="literal">$LIST_SEPARATOR</tt> if you use the English module
bundled with Perl.</p>

</blockquote>
<blockquote class="footnote">

<a name="FOOTNOTE-15"></a>
<p>[15]The guesser is too boring to describe in full, but basically takes
a weighted average of all the things that look like character classes
(<tt class="literal">a-z</tt>, <tt class="literal">\w</tt>, initial <tt class="literal">^</tt>) versus things that look like expressions
(variables or reserved words).</p>

</blockquote>






<h3 class="sect2">2.6.6. "Here" Documents</h3>

<a name="INDEX-612"></a><a name="INDEX-613"></a><a name="INDEX-614"></a><a name="INDEX-615"></a><a name="INDEX-616"></a><a name="INDEX-617"></a><a name="INDEX-618"></a>
<p>A line-oriented form of quoting is based on the Unix shell's
<em class="emphasis">here-document</em> syntax.  It's line-oriented in the
sense that the delimiters are lines rather than characters.  The
starting delimiter is the current line, and the terminating delimiter
is a line consisting of the string you specify.  Following a
<tt class="literal">&lt;&lt;</tt>, you specify the string to terminate the
quoted material, and all lines following the current line down to but
not including the terminating line are part of the string.  The
terminating string may be either an identifier (a word) or some quoted
text.  If quoted, the type of quote determines the treatment of the
text, just as it does in regular quoting.  An unquoted identifier
works as though it were in double quotes. A backslashed identifier
works as though it were in single quotes (for compatibility with shell
syntax).  There must be no space between the
<tt class="literal">&lt;&lt;</tt> and an unquoted identifier, although
whitespace is permitted if you specify a quoted string instead of the
bare identifier.  (If you insert a space, it will be treated as a null
identifier, which is valid but deprecated, and matches the first blank
line--see the first <tt class="literal">Hurrah!</tt> example below.)  The
terminating string must appear by itself, unquoted and with no extra
whitespace on either side, on the terminating line.
<blockquote>
<pre class="programlisting">print &lt;&lt;EOF;    # same as earlier example
The price is $Price.
EOF

print &lt;&lt;"EOF";  # same as above, with explicit quotes
The price is $Price.
EOF

print &lt;&lt;'EOF';    # single-quoted quote
All things (e.g. a camel's journey through
A needle's eye) are possible, it's true.
But picture how the camel feels, squeezed out
In one long bloody thread, from tail to snout.
                                -- C.S. Lewis
EOF

print &lt;&lt; x 10;    # print next line 10 times
The camels are coming!  Hurrah!  Hurrah!

print &lt;&lt;"" x 10;  # the preferred way to write that
The camels are coming!  Hurrah!  Hurrah!

print &lt;&lt;`EOC`;    # execute commands
echo hi there
echo lo there
EOC

print &lt;&lt;"dromedary", &lt;&lt;"camelid";   # you can stack them
I said bactrian.
dromedary
She said llama.
camelid

funkshun(&lt;&lt;"THIS", 23, &lt;&lt;'THAT');   # doesn't matter if they're in parens
Here's a line
or two.
THIS
And here's another.
THAT</pre>
</blockquote>

Just don't forget that you have to put a semicolon on the end to finish
the statement, because Perl doesn't know you're not going to try to do this:
<blockquote>
<pre class="programlisting">print &lt;&lt;'odd'
2345
odd
    + 10000;   # prints 12345</pre>
</blockquote>
<a name="INDEX-619"></a><a name="INDEX-620"></a>
If you want your here docs to be indented with the rest of the code,
you'll need to remove leading whitespace from each line manually:
<blockquote>
<pre class="programlisting">($quote = &lt;&lt;'QUOTE') =~ s/^\s+//gm;
    The Road goes ever on and on,
    down from the door where it began.
QUOTE</pre>
</blockquote>

You could even populate an array with the lines of a here document as
follows:
<blockquote>
<pre class="programlisting">@sauces = &lt;&lt;End_Lines =~ m/(\S.*\S)/g;
   normal tomato
   spicy tomato
   green chile
   pesto
   white wine
End_Lines</pre>
</blockquote>
</p>






<h3 class="sect2">2.6.7. V-String Literals</h3>

<p>
<a name="INDEX-621"></a><a name="INDEX-622"></a><a name="INDEX-623"></a><a name="INDEX-624"></a>
A literal that begins with a <tt class="literal">v</tt> and is followed by one or more
dot-separated integers is treated as a string literal composed of
characters with the specified ordinal values:
<blockquote>
<pre class="programlisting">$crlf = v13.10;           # ASCII carriage return, line feed</pre>
</blockquote>

These are called <em class="emphasis">v-strings</em>, short for "vector strings" or "version
strings" or anything else you can think of that starts with "v" and
deals with lists of integers.  They provide an alternate and more
legible way to construct strings when you want to specify the numeric
values of each character.  Thus, <tt class="literal">v1.20.300.4000</tt> is a more winsome way to
produce the same string value as any of:
<blockquote>
<pre class="programlisting">"\x{1}\x{14}\x{12c}\x{fa0}"
pack("U*", 1, 20, 300, 4000)
chr(1) . chr(20) . chr(300) . chr(4000)</pre>
</blockquote>

If such a literal has two or more dots (three or more integers), the
leading <tt class="literal">v</tt> may be omitted.
<blockquote>
<pre class="programlisting">print v9786;              # prints UTF-8 encoded SMILEY, "\x{263a}"
print v102.111.111;       # prints "foo"
print 102.111.111;        # same thing

use 5.6.0;                # require a particular Perl version (or later)

$ipaddr = 204.148.40.9;   # the IPv4 address of oreilly.com</pre>
</blockquote>

V-strings are useful for representing IP address and version numbers.
In particular, since characters can have an ordinal value larger than
255 these days, v-strings provide a way to represent version numbers of
any size that can be correctly compared with a simple string
comparison.</p>

<p>Version numbers and IP addresses stored in v-strings are not human
readable, since the individual integers are stored as arbitrary
characters.  To produce something legible, use the
<tt class="literal">v</tt> flag in a <tt class="literal">printf</tt> mask, like
<tt class="literal">"%vd"</tt>, as described under
<tt class="literal">sprintf</tt> in <a href="ch29_01.htm">Chapter 29, "Functions"</a>.
For more on Unicode strings, see <a href="ch15_01.htm">Chapter 15, "Unicode"</a>
and the <tt class="literal">use bytes</tt> pragma in
<a href="ch31_01.htm">Chapter 31, "Pragmatic Modules"</a>; for comparing version
strings using string comparison operators, see <tt class="literal">$^V</tt> in 
<a href="ch28_01.htm">Chapter 28, "Special Names"</a>; and for representing IPv4
addresses, see <tt class="literal">gethostbyaddr</tt> in
<a href="ch29_01.htm">Chapter 29, "Functions"</a>.</p>






<h3 class="sect2">2.6.8. Other Literal Tokens</h3>

<a name="INDEX-625"></a><a name="INDEX-626"></a><a name="INDEX-627"></a><a name="INDEX-628"></a><a name="INDEX-629"></a><a name="INDEX-630"></a><a name="INDEX-631"></a><a name="INDEX-632"></a><a name="INDEX-633"></a><a name="INDEX-634"></a>
<p>You should consider any identifier that both begins and ends with a
double underscore to be reserved for special syntactic use by Perl.
Two such special literals are <tt class="literal">__LINE__</tt> and <tt class="literal">__FILE__</tt>, which
represent the current line number and filename at that point in your
program.  They may only be used as separate tokens; they will not be
interpolated into strings.  Likewise, <tt class="literal">__PACKAGE__</tt> is the name of
the package the current code is being compiled into.  If there is no
current package (due to an empty <tt class="literal">package;</tt> directive),
<tt class="literal">__PACKAGE__</tt> is the undefined value.  The token <tt class="literal">__END__</tt> (or
alternatively, a Control-D or Control-Z character) may be used to
indicate the logical end of the script before the real end-of-file.
Any following text is ignored, but may be read via the <tt class="literal">DATA</tt>
filehandle.
<a name="INDEX-635"></a><a name="INDEX-636"></a>
</p>

<p>
<a name="INDEX-637"></a> The <tt class="literal">__DATA__</tt> token
functions similarly to the <tt class="literal">__END__</tt> token, but opens
the <tt class="literal">DATA</tt> filehandle within the current package's
namespace, so that files you <tt class="literal">require</tt> can each have
their own <tt class="literal">DATA</tt> filehandles open simultaneously.
For more information, see <tt class="literal">DATA</tt> in
<a href="ch28_01.htm">Chapter 28, "Special Names"</a>.</p>

<a name="INDEX-638"></a>



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