ch28_02.htm

编程珍珠,里面很多好用的代码,大家可以参考学习呵呵,

<pre class="programlisting">{
    local $/;
    $_ = &lt;FH&gt;;
}</pre>
</blockquote>

<p>Setting <tt class="literal">$/</tt> to a reference to either an integer, a scalar containing
an integer, or a scalar that's convertible to an integer will make
<tt class="literal">readline</tt> and <tt class="literal">&lt;FH&gt;</tt> operations read in fixed-length records
(with the maximum record size being the referenced integer) instead of
variable-length record terminated by a particular string.  So this:
<blockquote>
<pre class="programlisting">$/ = \32768; # or \"32768" or \$scalar_var_containing_32768
open(FILE, $myfile);
$record = &lt;FILE&gt;;</pre>
</blockquote>

will read a record of no more than 32,768 bytes from the
<tt class="literal">FILE</tt> handle.  If you're not reading from a
record-oriented file (or your operating system doesn't have
record-oriented files), then you'll likely get a full chunk of data
with every read.  If a record is larger than the record size you've
set, you'll get the record back in pieces.  Record mode mixes well
with line mode only on systems where standard I/O supplies a
<em class="emphasis">read</em>(3) function; VMS is a notable
exception.</p>

<p>Calling <tt class="literal">chomp</tt> when <tt class="literal">$/</tt> is set to
enable record mode--or when it is undefined--has no effect.  See also
the <span class="option">-0</span> (the digit) and the <span class="option">-l</span> (the
letter) command-line switches in <a href="ch19_01.htm">Chapter 19, "The Command-Line Interface"</a>.  (Mnemonic:
<tt class="literal">/</tt> is used to separate lines when quoting poetry.)</p>
</dd>


<dt>
<b><tt class="literal">@ISA</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4508"></a>
[PKG] This array contains names of other packages to look
through when a method call cannot be found in the current package.  That is,
it contains the base classes of the package.  The <tt class="literal">use base</tt>
pragma sets this implicitly.  It is not exempt from <tt class="literal">use strict</tt>.  See <a href="ch12_01.htm">Chapter 12, "Objects"</a>.</p>
</dd>


<dt>
<b><tt class="literal">@LAST_MATCH_END</tt></b>
</dt>
<dt>
<b><tt class="literal">@+</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4509"></a><a name="INDEX-4510"></a><a name="INDEX-4511"></a>
This array holds the offsets of the ends of the last successful
submatches in the currently active dynamic scope.
<tt class="literal">$+[0]</tt> is the offset of the end of the entire match.
This is the same value the <tt class="literal">pos</tt> function returns
when called on the variable that was matched against.  (When we say
"offset of the end", we really mean the offset to the first character
<em class="emphasis">following</em> the end of whatever matched, so that we
can subtract beginning offsets from end offsets and arrive at the
length.)  The <em class="emphasis">n</em>th element of this array
holds the offset of the <em class="emphasis">n</em>th submatch, so
<tt class="literal">$+[1]</tt> is the offset where <tt class="literal">$1</tt>
ends, <tt class="literal">$+[2]</tt> the offset where <tt class="literal">$2</tt>
ends, and so on.  You can use <tt class="literal">$#+</tt> to determine how
many subgroups were in the last successful match.  See also
<tt class="literal">@_</tt> (<tt class="literal">@LAST_MATCH_START</tt>).</p>

<p>After a successful match against some variable <tt class="literal">$var</tt>:</p>
</dd>
<ul>
<li><tt>$`</tt> is the same as <tt>substr($var, 0, $-[0])</tt>
<li><tt>$&amp;</tt> is the same as <tt>substr($var, $-[0], $+[0] - $-[0])</tt>
<li><tt>$'</tt> is the same as <tt>substr($var, $+[0])</tt>
<li><tt>$1</tt> is the same as <tt>substr($var, $-[1], $+[1] - $-[1])</tt>
<li><tt>$2</tt> is the same as <tt>substr($var, $-[2], $+[2] - $-[2])</tt>
<li><tt>$3</tt> is the same as <tt>substr($var, $-[3], $+[3] - $-[2])</tt>, and so on.
</ul>

<dt>
<b><tt class="literal">@LAST_MATCH_START</tt></b>
</dt>
<dt>
<b><tt class="literal">@-</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4512"></a><a name="INDEX-4513"></a><a name="INDEX-4514"></a>
[DYN,RO] This array holds the offsets of the beginnings of the last
successful submatches in the currently active dynamic scope.
<tt class="literal">$-[0]</tt> is the offset of the beginning of the entire
match.  The <em class="emphasis">n</em>th element of this array
holds the offset of the <em class="emphasis">n</em>th submatch, so
<tt class="literal">$-[1]</tt> is the offset where <tt class="literal">$1</tt>
begins, <tt class="literal">$-[2]</tt> the offset where
<tt class="literal">$2</tt> begins, and so on.  You can use
<tt class="literal">$#-</tt> to determine how many subgroups were in the
last successful match.  See also <tt class="literal">@+</tt>
(<tt class="literal">@LAST_MATCH_END</tt>).</p>
</dd>


<dt>
<b><tt class="literal">$LAST_PAREN_MATCH</tt></b>
</dt>
<dt>
<b><tt class="literal">$+</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4515"></a><a name="INDEX-4516"></a>
[DYN,RO] This returns the last parenthesized submatch from the last
successful pattern in the currently active dynamic scope.  This
is useful when you don't know (or care) which of a set of alternative
patterns matched.  (Mnemonic: be positive and forward looking.)
Example:
<blockquote>
<pre class="programlisting">$rev = $+   if /Version: (.*)|Revision: (.*)/;</pre>
</blockquote>
</p>
</dd>


<dt>
<b><tt class="literal">$LAST_REGEXP_CODE_RESULT</tt></b>
</dt>
<dt>
<b><tt class="literal">$^R</tt></b>
</dt>
<dd>
<p>[DYN] This contains the result of the last snippet of code executed
inside a successful pattern with the <tt class="literal">(?{&nbsp;</tt><em class="replaceable">CODE</em><tt class="literal">&nbsp;})</tt> construct.
<tt class="literal">$^R</tt> gives you a way to execute code and remember the result for use
later in the pattern, or even afterward.</p>

<p>As the Perl regular expression engine moves through the pattern, it
may encounter multiple <tt class="literal">(?{&nbsp;</tt><em class="replaceable">CODE</em><tt class="literal">&nbsp;})</tt> expressions.  As it does, it
remembers each value of <tt class="literal">$^R</tt> so that if it later has to backtrack
past an expression, it restores the previous value of <tt class="literal">$^R</tt>.  In
other words, <tt class="literal">$^R</tt> has a dynamic scope within the pattern, much like
<tt class="literal">$1</tt> and friends.</p>

<p>So <tt class="literal">$^R</tt> is not simply the result of the last
snippet of code executed inside a pattern.  It's the result of the
last snippet of code <em class="emphasis">leading to a successful match</em>.  A corollary is
that if the match was not successful, <tt class="literal">$^R</tt> will be restored to
whatever value it had before the match occurred.</p>

<p>If the <tt class="literal">(?{&nbsp;</tt><em class="replaceable">CODE</em><tt class="literal">&nbsp;})</tt> pattern is functioning directly
as the conditional of a <tt class="literal">(?(</tt><em class="replaceable">COND</em><tt class="literal">)</tt><em class="replaceable">IFTRUE</em><tt class="literal">|</tt><em class="replaceable">IFFALSE</em><tt class="literal">)</tt>
subpattern, <tt class="literal">$^R</tt> is not set.</p>
</dd>


<dt>
<b><tt class="literal">$LIST_SEPARATOR</tt></b>
</dt>
<dt>
<b><tt class="literal">$"</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4517"></a><a name="INDEX-4518"></a>
[ALL] When an array or slice is interpolated into a double-quoted
string (or the like), this variable specifies the string to put
between individual elements.  Default is a space.  (Mnemonic: obvious,
one hopes.)</p>
</dd>


<dt>
<b><tt class="literal">$^M</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4519"></a><a name="INDEX-4520"></a><a name="INDEX-4521"></a><a name="INDEX-4522"></a><a name="INDEX-4523"></a><a name="INDEX-4524"></a>
[ALL] By default, running out of memory is not trappable.  However, if your
<em class="emphasis">perl</em> was compiled to take advantage of <tt class="literal">$^M</tt>, you may use it as an
emergency memory pool.  If your Perl is compiled with
<tt class="userinput"><b>-DPERL_EMERGENCY_SBRK</b></tt> and uses Perl's <tt class="literal">malloc</tt>, then:
<blockquote>
<pre class="programlisting">$^M = 'a' x (1 &lt;&lt; 16);</pre>
</blockquote>

would allocate a 64K buffer for emergency use.  See the <em class="emphasis">INSTALL</em> file
in the Perl source distribution directory for information on how to
enable this option.  As a disincentive to casual use of this advanced
feature, there is no <tt class="literal">use English</tt> long name for this variable (and
we won't tell you what the mnemonic is).</p>
</dd>


<dt>
<b><tt class="literal">$MATCH</tt></b>
</dt>
<dt>
<b><tt class="literal">$&amp;</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4525"></a><a name="INDEX-4526"></a>
[DYN,RO] The string matched by the last successful pattern match in
the currently active dynamic scope.  (Mnemonic: like
<tt class="literal">&amp;</tt> in some editors.)</p>
</dd>


<dt>
<b><tt class="literal">$OLD_PERL_VERSION</tt></b>
</dt>
<dt>
<b><tt class="literal">$]</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4527"></a><a name="INDEX-4528"></a><a name="INDEX-4529"></a>
[ALL] Returns the version + patchlevel/1000.  It can be used to
determine at the beginning of a script whether the Perl interpreter
executing the script is in the right range of versions.  (Mnemonic: is
this version of Perl in the right bracket?) Example:
<blockquote>
<pre class="programlisting">warn "No checksumming!\n" if $] &lt; 3.019;
die "Must have prototyping available\n" if $] &lt; 5.003;</pre>
</blockquote>

See also the documentation of <tt class="literal">use</tt><em class="replaceable">VERSION</em> and <tt class="literal">require</tt><em class="replaceable">VERSION</em> for a convenient way to fail if the
Perl interpreter is too old.  See <tt class="literal">$^V</tt> for a more
flexible UTF-8 representation of the Perl version.</p>
</dd>


<dt>
<b><tt class="literal">$OSNAME</tt></b>
</dt>
<dt>
<b><tt class="literal">$^O</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4530"></a><a name="INDEX-4531"></a><a name="INDEX-4532"></a><a name="INDEX-4533"></a>
[ALL] This variable contains the name of the platform (usually the operating
system) the current <em class="emphasis">perl</em> binary was compiled for.  It's a cheap
alternative to pulling it out of the <tt class="literal">Config</tt> module.</p>
</dd>


<dt>
<b><tt class="literal">$OS_ERROR</tt></b>
</dt>
<dt>
<b><tt class="literal">$ERRNO</tt></b>
</dt>
<dt>
<b><tt class="literal">$!</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4534"></a><a name="INDEX-4535"></a><a name="INDEX-4536"></a><a name="INDEX-4537"></a>
[ALL] If used in a numeric context, yields the current value of the last
syscall error, with all the usual caveats.  (This means that you
shouldn't depend on the value of <tt class="literal">$!</tt> to be anything in particular
unless you've gotten a specific error return indicating a system
error.)  If used in a string context, <tt class="literal">$!</tt> yields the corresponding system
error string.  You can assign an error number to <tt class="literal">$!</tt> if, for instance, you want
<tt class="literal">$!</tt> to return the string for that particular error, or you want to set
the exit value for <tt class="literal">die</tt>.  See also the <tt class="literal">Errno</tt> module in <a href="ch32_01.htm">Chapter 32, "Standard Modules"</a>.  (Mnemonic: what just went bang?)</p>
</dd>


<dt>
<b><tt class="literal">%OS_ERROR</tt></b>
</dt>
<dt>
<b><tt class="literal">%ERRNO</tt></b>
</dt>
<dt>
<b><tt class="literal">%!</tt></b>
</dt>
<dd>
<p>[ALL] This hash is defined only if you've loaded the standard
<tt class="literal">Errno</tt> module described in <a href="ch32_01.htm">Chapter 32, "Standard Modules"</a>.  Once you've done this, you
can subscript into <tt class="literal">%!</tt> using a particular error
string, and its value is true only if that's the current error.  For
example, <tt class="literal">$!{ENOENT}</tt> is true only if the C
<tt class="literal">errno</tt> variable is currently set to the C
<tt class="literal">#define</tt> value, <tt class="literal">ENOENT</tt>.  This is
convenient for accessing vendor-specific symbols.</p>
</dd>


<dt>
<b><tt class="literal">autoflush</tt> <em class="replaceable">HANDLE EXPR</em></b>
</dt>
<dt>
<b><tt class="literal">$OUTPUT_AUTOFLUSH</tt></b>
</dt>
<dt>
<b><tt class="literal">$AUTOFLUSH</tt></b>
</dt>
<dt>
<b><tt class="literal">$|</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4538"></a><a name="INDEX-4539"></a><a name="INDEX-4540"></a><a name="INDEX-4541"></a><a name="INDEX-4542"></a>
[FHA] If set to true, forces a buffer flush after every <tt class="literal">print</tt>,
<tt class="literal">printf</tt>, and <tt class="literal">write</tt> on the currently selected output handle.
(We call this <em class="emphasis">command buffering</em>.  Contrary to popular belief, setting
this variable does not turn off buffering.)  The default is false, which on
many systems means that <tt class="literal">STDOUT</tt> will be line buffered if
output is to the terminal, and block buffered otherwise, even on pipes
and sockets.  Setting this variable is useful when you are
outputting to a pipe, such as when you are running a Perl script under
<em class="emphasis">rsh</em>(1) and want to see the output as it's happening.  If you have
pending, unflushed data in the currently selected filehandle's output
buffer when this variable is set to true, that buffer will be immediately
flushed as a side-effect of assignment.   See the one-argument form of
<tt class="literal">select</tt> for examples of controlling buffering on filehandles other
than <tt class="literal">STDOUT</tt>.  (Mnemonic: when you want your pipes to be piping hot.)</p>

<p>This variable has no effect on input buffering; for that, see <tt class="literal">getc</tt>
in <a href="ch29_01.htm">Chapter 29, "Functions"</a> or the example in the <tt class="literal">POSIX</tt> module
in <a href="ch32_01.htm">Chapter 32, "Standard Modules"</a>.</p>
</dd>


<dt>
<b><tt class="literal">$OUTPUT_FIELD_SEPARATOR</tt></b>
</dt>
<dt>
<b><tt class="literal">$OFS</tt></b>
</dt>
<dt>
<b><tt class="literal">$,</tt></b>
</dt>
<dd>
<p>
<a name="INDEX-4543"></a><a name="INDEX-4544"></a><a name="INDEX-4545"></a><a name="INDEX-4546"></a><a name="INDEX-4547"></a><a name="INDEX-4548"></a><a name="INDEX-4549"></a>
[ALL] The output field separator (terminator, actually) for <tt class="literal">print</tt>.  Ordinarily, <tt class="literal">print</tt>
simply prints out the list elements you specify without anything
between them.  Set this variable as you would set <em class="emphasis">awk</em>'s
<tt class="literal">OFS</tt> variable to specify what is printed between fields.  (Mnemonic:
what is printed when there is a "<tt class="literal">,</tt>" in your <tt class="literal">print</tt> statement.)</p>
</dd>


<dt>
<b><tt class="literal">$OUTPUT_RECORD_SEPARATOR</tt></b>
</dt>