perlfaq4.html

《Perl 5教程》及《perl常问问题集》

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<H3><A NAME="How_do_I_expand_function_calls_i">How do I expand function calls in a string?

</A></H3>
This is documented in <A HREF="../../tppmsgs/msgs0.htm#59" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perlref.html">the perlref manpage</A>. In general, this is fraught with quoting and readability problems, but it
is possible. To interpolate a subroutine call (in a list context) into a
string:


<P>

<PRE>    print &quot;My sub returned @{[mysub(1,2,3)]} that time.\n&quot;;
</PRE>

<P>

If you prefer scalar context, similar chicanery is also useful for
arbitrary expressions:


<P>

<PRE>    print &quot;That yields ${\($n + 5)} widgets\n&quot;;
</PRE>

<P>

See also ``How can 
<FONT SIZE=-1>I</FONT> expand variables in text strings?'' in this section of the 
<FONT SIZE=-1>FAQ.</FONT>



<P>

<P>
<HR>
<H3><A NAME="How_do_I_find_matching_nesting_a">How do I find matching/nesting anything?

</A></H3>
This isn't something that can be tackled in one regular expression, no
matter how complicated. To find something between two single characters, a
pattern like <CODE>/x([^x]*)x/</CODE> will get the intervening bits in $1. For multiple ones, then something more
like <CODE>/alpha(.*?)omega/</CODE> would be needed. But none of these deals with nested patterns, nor can
they. For that you'll have to write a parser.


<P>

<P>
<HR>
<H3><A NAME="How_do_I_reverse_a_string_">How do I reverse a string?

</A></H3>
Use <CODE>reverse()</CODE> in a scalar context, as documented in
<A HREF="../../tppmsgs/msgs0.htm#68" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perlfunc.html#reverse">reverse</A>.


<P>

<PRE>    $reversed = reverse $string;
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_do_I_expand_tabs_in_a_string">How do I expand tabs in a string?

</A></H3>
You can do it the old-fashioned way:


<P>

<PRE>    1 while $string =~ s/\t+/' ' x (length($&amp;) * 8 - length($`) % 8)/e;
</PRE>

<P>

Or you can just use the Text::Tabs module (part of the standard perl
distribution).


<P>

<PRE>    use Text::Tabs;
    @expanded_lines = expand(@lines_with_tabs);
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_do_I_reformat_a_paragraph_">How do I reformat a paragraph?

</A></H3>
Use Text::Wrap (part of the standard perl distribution):


<P>

<PRE>    use Text::Wrap;
    print wrap(&quot;\t&quot;, '  ', @paragraphs);
</PRE>

<P>

The paragraphs you give to Text::Wrap may not contain embedded newlines.
Text::Wrap doesn't justify the lines (flush-right).


<P>

<P>
<HR>
<H3><A NAME="How_can_I_access_change_the_firs">How can I access/change the first N letters of a string?

</A></H3>
There are many ways. If you just want to grab a copy, use substr:


<P>

<PRE>    $first_byte = substr($a, 0, 1);
</PRE>

<P>

If you want to modify part of a string, the simplest way is often to use
<CODE>substr()</CODE> as an lvalue:


<P>

<PRE>    substr($a, 0, 3) = &quot;Tom&quot;;
</PRE>

<P>

Although those with a regexp kind of thought process will likely prefer


<P>

<PRE>    $a =~ s/^.../Tom/;
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_do_I_change_the_Nth_occurren">How do I change the Nth occurrence of something?

</A></H3>
You have to keep track. For example, let's say you want to change the fifth
occurrence of ``whoever'' or ``whomever'' into ``whosoever'' or
``whomsoever'', case insensitively.


<P>

<PRE>    $count = 0;
    s{((whom?)ever)}{
        ++$count == 5           # is it the 5th?
            ? &quot;${2}soever&quot;      # yes, swap
            : $1                # renege and leave it there
    }igex;
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_can_I_count_the_number_of_oc">How can I count the number of occurrences of a substring within a string?

</A></H3>
There are a number of ways, with varying efficiency: If you want a count of a certain single character 
<FONT SIZE=-1>(X)</FONT> within a string, you can use the

<CODE>tr///</CODE> function like so:


<P>

<PRE>    $string = &quot;ThisXlineXhasXsomeXx'sXinXit&quot;:
    $count = ($string =~ tr/X//);
    print &quot;There are $count X charcters in the string&quot;;
</PRE>

<P>

This is fine if you are just looking for a single character. However, if
you are trying to count multiple character substrings within a larger
string, <CODE>tr///</CODE> won't work. What you can do is wrap a <CODE>while()</CODE> loop around a
global pattern match. For example, let's count negative integers:


<P>

<PRE>    $string = &quot;-9 55 48 -2 23 -76 4 14 -44&quot;;
    while ($string =~ /-\d+/g) { $count++ }
    print &quot;There are $count negative numbers in the string&quot;;
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_do_I_capitalize_all_the_word">How do I capitalize all the words on one line?

</A></H3>
To make the first letter of each word upper case:


<P>

<PRE>        $line =~ s/\b(\w)/\U$1/g;
</PRE>

<P>

This has the strange effect of turning ``<CODE>don't do it</CODE>'' into ``<CODE>Don'T
Do It</CODE>''. Sometimes you might want this, instead (Suggested by Brian Foy &lt;<A HREF="MAILTO:[e-mail]">[e-mail]</A>&gt;):


<P>

<PRE>    $string =~ s/ (
                 (^\w)    #at the beginning of the line
                   |      # or
                 (\s\w)   #preceded by whitespace
                   )
                /\U$1/xg;
    $string =~ /([\w']+)/\u\L$1/g;
</PRE>

<P>

To make the whole line upper case:


<P>

<PRE>        $line = uc($line);
</PRE>

<P>

To force each word to be lower case, with the first letter upper case:


<P>

<PRE>        $line =~ s/(\w+)/\u\L$1/g;
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_can_I_split_a_character_de">How can I split a [character] delimited string except when inside
[character]? (Comma-separated files)

</A></H3>
Take the example case of trying to split a string that is comma-separated
into its different fields. (We'll pretend you said comma-separated, not
comma-delimited, which is different and almost never what you mean.) You
can't use <CODE>split(/,/)</CODE> because you shouldn't split if the comma is inside quotes. For example,
take a data line like this:


<P>

<PRE>    SAR001,&quot;&quot;,&quot;Cimetrix, Inc&quot;,&quot;Bob Smith&quot;,&quot;CAM&quot;,N,8,1,0,7,&quot;Error, Core Dumped&quot;
</PRE>

<P>

Due to the restriction of the quotes, this is a fairly complex problem.
Thankfully, we have Jeffrey Friedl, author of a highly recommended book on
regular expressions, to handle these for us. He suggests (assuming your
string is contained in $text):


<P>

<PRE>     @new = ();
     push(@new, $+) while $text =~ m{
         &quot;([^\&quot;\\]*(?:\\.[^\&quot;\\]*)*)&quot;,?  # groups the phrase inside the quotes
       | ([^,]+),?
       | ,
     }gx;
     push(@new, undef) if substr($text,-1,1) eq ',';
</PRE>

<P>

If you want to represent quotation marks inside a quotation-mark-delimited
field, escape them with backslashes (eg,
C<``like \''this\``''). Unescaping them is a task addressed earlier in this
section.


<P>

Alternatively, the Text::ParseWords module (part of the standard perl
distribution) lets you say:


<P>

<PRE>    use Text::ParseWords;
    @new = quotewords(&quot;,&quot;, 0, $text);
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_do_I_strip_blank_space_from_">How do I strip blank space from the beginning/end of a string?

</A></H3>
The simplest approach, albeit not the fastest, is probably like this:


<P>

<PRE>    $string =~ s/^\s*(.*?)\s*$/$1/;
</PRE>

<P>

It would be faster to do this in two steps:


<P>

<PRE>    $string =~ s/^\s+//;
    $string =~ s/\s+$//;
</PRE>

<P>

Or more nicely written as:


<P>

<PRE>    for ($string) {
        s/^\s+//;
        s/\s+$//;
    }
</PRE>

<P>

<P>
<HR>
<H3><A NAME="How_do_I_extract_selected_column">How do I extract selected columns from a string?

</A></H3>
Use <CODE>substr()</CODE> or <CODE>unpack(),</CODE> both documented in <A HREF="../../tppmsgs/msgs0.htm#68" tppabs="http://www.perl.org/CPAN/doc/manual/html/pod/perlfunc.html">the perlfunc manpage</A>.


<P>

<P>
<HR>
<H3><A NAME="How_do_I_find_the_soundex_value_">How do I find the soundex value of a string?

</A></H3>
Use the standard Text::Soundex module distributed with perl.


<P>

<P>
<HR>
<H3><A NAME="How_can_I_expand_variables_in_te">How can I expand variables in text strings?

</A></H3>
Let's assume that you have a string like:


<P>

<PRE>    $text = 'this has a $foo in it and a $bar';
    $text =~ s/\$(\w+)/${$1}/g;
</PRE>

<P>

Before version 5 of perl, this had to be done with a double-eval
substitution:


<P>

<PRE>    $text =~ s/(\$\w+)/$1/eeg;
</PRE>

<P>

Which is bizarre enough that you'll probably actually need an 
<FONT SIZE=-1>EEG</FONT> afterwards. :-)


<P>

See also ``How do 
<FONT SIZE=-1>I</FONT> expand function calls in a string?'' in this section of the 
<FONT SIZE=-1>FAQ.</FONT>



<P>

<P>
<HR>
<H3><A NAME="What_s_wrong_with_always_quoting">What's wrong with always quoting "$vars"?

</A></H3>
The problem is that those double-quotes force stringification, coercing
numbers and references into strings, even when you don't want them to be.


<P>

If you get used to writing odd things like these:


<P>

<PRE>    print &quot;$var&quot;;       # BAD
    $new = &quot;$old&quot;;      # BAD
    somefunc(&quot;$var&quot;);   # BAD
</PRE>

<P>

You'll be in trouble. Those should (in 99.8% of the cases) be the simpler
and more direct:


<P>

<PRE>    print $var;
    $new = $old;
    somefunc($var);
</PRE>

<P>

Otherwise, besides slowing you down, you're going to break code when the
thing in the scalar is actually neither a string nor a number, but a
reference:


<P>

<PRE>    func(\@array);
    sub func {
        my $aref = shift;
        my $oref = &quot;$aref&quot;;  # WRONG
    }
</PRE>

<P>

You can also get into subtle problems on those few operations in Perl that
actually do care about the difference between a string and a number, such
as the magical <CODE>++</CODE> autoincrement operator or the <CODE>syscall()</CODE> function.


<P>

<P>
<HR>
<H3><A NAME="Why_don_t_my_HERE_documents_wo">Why don't my <<HERE documents work?

</A></H3>
Check for these three things:


<P>

<OL>
<LI><STRONG><A NAME="item_There_must_be_no_space_after_the"> There must be no space after the << part.

</A></STRONG>
<LI><STRONG><A NAME="item_There_probably_should_be_a_sem"> There (probably) should be a semicolon at the end.

</A></STRONG>
<LI><STRONG><A NAME="item_You_can_t_easily_have_any_spac"> You can't (easily) have any space in front of the tag.

</A></STRONG>
</OL>
<P>
<HR>
<H2><A NAME="Data_Arrays">Data: Arrays

</A></H2>
<P>
<HR>
<H3><A NAME="What_is_the_difference_between_">What is the difference between $array[1] and @array[1]?

</A></H3>
The former is a scalar value, the latter an array slice, which makes it a
list with one (scalar) value. You should use $ when you want a scalar value
(most of the time) and @ when you want a list with one scalar value in it
(very, very rarely; nearly never, in fact).


<P>

Sometimes it doesn't make a difference, but sometimes it does. For example,
compare:


<P>

<PRE>    $good[0] = `some program that outputs several lines`;
</PRE>

<P>

with


<P>

<PRE>    @bad[0]  = `same program that outputs several lines`;
</PRE>

<P>

The <STRONG>-w</STRONG> flag will warn you about these matters.


<P>

<P>
<HR>
<H3><A NAME="How_can_I_extract_just_the_uniqu">How can I extract just the unique elements of an array?

</A></H3>
There are several possible ways, depending on whether the array is ordered
and whether you wish to preserve the ordering.