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,

<tr>
<td><tt class="literal">\u</tt></td>
<td>Force next character to uppercase ("titlecase" in Unicode).<a name="INDEX-572"></a></td>
</tr>

<tr>
<td><tt class="literal">\l</tt></td>
<td>Force next character to lowercase.<a name="INDEX-573"></a></td>
</tr>

<tr>
<td><tt class="literal">\U</tt></td>
<td>Force all following characters to uppercase.</td>
</tr>

<tr>
<td><tt class="literal">\L</tt></td>
<td>Force all following characters to lowercase.</td>
</tr>

<tr>
<td><tt class="literal">\Q</tt></td>
<td>Backslash all following nonalphanumeric characters.<a name="INDEX-574"></a></td>
</tr>

<tr>
<td><tt class="literal">\E</tt></td>
<td>End <tt class="literal">\U</tt>, <tt class="literal">\L</tt>, or <tt class="literal">\Q</tt>.<a name="INDEX-575"></a></td>
</tr>


</table>

<a name="INDEX-576"></a><a name="INDEX-577"></a>
<p>You may also embed newlines directly in your strings; that is, they
can begin and end on different lines.  This is often useful,
but it also means that if you forget a trailing quote, the error
will not be reported until Perl finds another line containing the
quote character, which may be much further on in the script.
Fortunately, this usually causes an immediate syntax error on the
same line, and Perl is then smart enough to warn you that you might
have a runaway string where it thought the string started.</p>

<p>
<a name="INDEX-578"></a><a name="INDEX-579"></a>
Besides the backslash escapes listed above, double-quoted strings are
subject to <em class="emphasis">variable interpolation</em> of scalar and list values.  This
means that you can insert the values of certain variables directly into
a string literal.  It's really just a handy form of string
concatenation.<a href="#FOOTNOTE-12">[12]</a> Variable
interpolation may be done for scalar variables, entire arrays (but not
hashes), single elements from an array or hash, or slices (multiple
subscripts) of an array or hash.  Nothing else interpolates.  In other
words, you may only interpolate expressions that begin with <tt class="literal">$</tt> or
<tt class="literal">@</tt>, because those are the two characters (along with backslash) that
the string parser looks for.  Inside strings, a literal <tt class="literal">@</tt> that is
not part of an array or slice identifier but is followed by an
alphanumeric character must be escaped with a backslash (<tt class="literal">\@</tt>), or
else a compilation error will result.  Although a complete hash
specified with a <tt class="literal">%</tt> may not be interpolated into the string, single
hash values or hash slices are okay, because they begin with <tt class="literal">$</tt> and
<tt class="literal">@</tt> respectively.
<a name="INDEX-580"></a><a name="INDEX-581"></a>
</p>
<blockquote class="footnote">

<a name="FOOTNOTE-12"></a>
<p>[12]With warnings enabled, Perl may report
undefined values interpolated into strings as using the concatenation
or join operations, even though you don't actually use those operators
there.  The compiler created them for you anyway.</p>

</blockquote>

<p>The following code segment prints out "<tt class="literal">The price is $100.</tt>":
<blockquote>
<pre class="programlisting">$Price = '$100';                    # not interpolated
print "The price is $Price.\n";     # interpolated</pre>
</blockquote>
<a name="INDEX-582"></a><a name="INDEX-583"></a><a name="INDEX-584"></a>
</p>

<p>As in some shells, you can put braces around the identifier to
distinguish it from following alphanumerics: <tt class="literal">"How ${verb}able!"</tt>.
An identifier within such braces is forced to be a string, as is
any single identifier within a hash subscript.  For

example:
<blockquote>
<pre class="programlisting">$days{'Feb'}</pre>
</blockquote>

can be written as:
<blockquote>
<pre class="programlisting">$days{Feb}</pre>
</blockquote>

and the quotes will be assumed.  Anything more complicated in the
subscript is interpreted as an expression, and then you'd have to
put in the quotes:
<blockquote>
<pre class="programlisting">$days{'February 29th'}   # Ok.
$days{"February 29th"}   # Also ok. "" doesn't have to interpolate.
$days{ February 29th }   # WRONG, produces parse error.</pre>
</blockquote>

In particular, you should always use quotes in slices such as:
<blockquote>
<pre class="programlisting">@days{'Jan','Feb'}       # Ok.
@days{"Jan","Feb"}       # Also ok.
@days{ Jan,  Feb }       # Kinda wrong (breaks under use strict)</pre>
</blockquote>
</p>

<p>
<a name="INDEX-585"></a>
Apart from the subscripts of interpolated array and hash variables,
there are no multiple levels of interpolation.  Contrary to the
expectations of shell programmers, backticks do not interpolate
within double quotes, nor do single quotes impede evaluation of
variables when used within double quotes.  Interpolation is extremely
powerful but strictly controlled in Perl.  It happens only inside
double quotes, and in certain other "double-quotish" operations
that we'll describe in the next section:
<blockquote>
<pre class="programlisting">print "\n";              # Ok, print a newline.
print  \n ;              # WRONG, no interpolative context.</pre>
</blockquote>
</p>

<a name="INDEX-586"></a>





<a name="ch02-sect-pick"></a>
<h3 class="sect2">2.6.3. Pick Your Own Quotes</h3>

<a name="INDEX-587"></a><a name="INDEX-588"></a><a name="INDEX-589"></a><a name="INDEX-590"></a>
<p>Although we usually think of quotes as literal values, in Perl they
function more like operators, providing various kinds of interpolating
and pattern-matching capabilities.  Perl provides the customary
quote characters for these behaviors, but also provides a more general way for
you to choose your quote character for any of them.  In
<a href="ch02_06.htm#perl2-ch-2-tab-5a">Table 2-3</a>, any nonalphanumeric, nonwhitespace
delimiter may be used in place of <tt class="literal">/</tt>.  (The newline
and space characters are no longer allowed as delimiters, although ancient
versions of Perl once allowed this.)</p>

<a name="perl2-ch-2-tab-5a"></a>
<h4 class="objtitle">Table 2.3. Quote Constructs</h4>
<table border="1">

<tr>
<th>Customary</th>
<th>Generic</th>
<th>Meaning</th>
<th>Interpolates</th>
</tr>




<tr>
<td><tt class="literal">''</tt></td>
<td><tt class="literal">q//</tt></td>
<td>Literal string</td>
<td>No</td>
</tr>

<tr>
<td><tt class="literal">""</tt></td>
<td><tt class="literal">qq//</tt></td>
<td>Literal string</td>
<td>Yes</td>
</tr>

<tr>
<td><tt class="literal">``</tt></td>
<td><tt class="literal">qx//</tt></td>
<td>Command execution</td>
<td>Yes</td>
</tr>

<tr>
<td><tt class="literal">()</tt></td>
<td><tt class="literal">qw//</tt></td>
<td>Word list</td>
<td>No</td>
</tr>

<tr>
<td><tt class="literal">//</tt></td>
<td><tt class="literal">m//</tt></td>
<td>Pattern match</td>
<td>Yes</td>
</tr>

<tr>
<td><tt class="literal">s///</tt></td>
<td><tt class="literal">s///</tt></td>
<td>Pattern substitution</td>
<td>Yes</td>
</tr>

<tr>
<td><tt class="literal">y///</tt></td>
<td><tt class="literal">tr///</tt></td>
<td>Character translation</td>
<td>No</td>
</tr>

<tr>
<td><tt class="literal">""</tt></td>
<td><tt class="literal">qr//</tt></td>
<td>Regular expression</td>
<td>Yes</td>
</tr>


</table>

<p>
<a name="INDEX-591"></a>
Some of these are simply forms of "syntactic sugar" to let you avoid
putting too many backslashes into quoted strings, particularly into
pattern matches where your regular slashes and backslashes tend to
get all tangled.</p>

<p>
<a name="INDEX-592"></a>
If you choose single quotes for delimiters, no variable interpolation
is done even on those forms that ordinarily interpolate.  If the
opening delimiter is an opening parenthesis, bracket, brace, or
angle bracket, the closing delimiter will be the corresponding
closing character.  (Embedded occurrences of the delimiters must
match in pairs.)  Examples:
<blockquote>
<pre class="programlisting">$single = q!I said, "You said, 'She said it.'"!;

$double = qq(Can't we get some "good" $variable?);


$chunk_of_code = q {
    if ($condition) {
        print "Gotcha!";
    }
};</pre>
</blockquote>
<a name="INDEX-593"></a>
The last example demonstrates that you can use whitespace between the
quote specifier and its initial bracketing character.  For two-element
constructs like <tt class="literal">s///</tt> and <tt class="literal">tr///</tt>,
if the first pair of quotes is a bracketing pair, the second part gets
its own starting quote character.  In fact, the second pair needn't be
the same as the first pair.  So you can write things like
<tt class="literal">s&lt;foo&gt;(bar)</tt> or
<tt class="literal">tr(a-f)[A-F]</tt>.  Because whitespace is also allowed
between the two inner quote characters, you could even write that last
one as:
<blockquote>
<pre class="programlisting">tr (a-f)
   [A-F];</pre>
</blockquote>

Whitespace is not allowed, however, when <tt class="literal">#</tt> is being used as the
quoting character.  <tt class="literal">q#foo#</tt> is parsed as the string <tt class="literal">'foo'</tt>, while
<tt class="literal">q #foo#</tt> is parsed as the quote operator <tt class="literal">q</tt> followed by a comment.
Its delimiter will be taken from the next line.  Comments can also be
placed in the middle of two-element constructs, which allows you to
write:
<blockquote>
<pre class="programlisting">s {foo}   # Replace foo
  {bar};  #    with bar.

tr [a-f]  # Transliterate lowercase hex
   [A-F]; #            to uppercase hex</pre>
</blockquote>
<a name="INDEX-594"></a>
</p>






<h3 class="sect2">2.6.4. Or Leave the Quotes Out Entirely</h3>

<a name="INDEX-595"></a><a name="INDEX-596"></a><a name="INDEX-597"></a><a name="INDEX-598"></a>
<p>A name that has no other interpretation in the grammar will be
treated as if it were a quoted string.  These are known as
<em class="emphasis">barewords</em>.<a href="#FOOTNOTE-13">[13]</a> As with filehandles and labels, a
bareword that consists entirely of lowercase letters risks conflict
with future reserved words.  If you have warnings enabled, Perl will
warn you about barewords.  For example:
<blockquote>
<pre class="programlisting">@days = (Mon,Tue,Wed,Thu,Fri);
print STDOUT hello, ' ', world, "\n";</pre>
</blockquote>
<a name="INDEX-599"></a>
</p>
<blockquote class="footnote">

<a name="FOOTNOTE-13"></a>
<p>[13] Variable names, filehandles, labels, and the
like are not considered barewords because they have a meaning forced by
a preceding token or a following token (or both).  Predeclared names
such as subroutines aren't barewords either.  It's only a bareword when
the parser has no clue.</p>

</blockquote>

<p>sets the array <tt class="literal">@days</tt> to the short form of the weekdays