perlfaq6.1

视频监控网络部分的协议ddns,的模块的实现代码,请大家大胆指正.

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.\" ========================================================================
.\"
.IX Title "PERLFAQ6 1"
.TH PERLFAQ6 1 "2007-12-18" "perl v5.10.0" "Perl Programmers Reference Guide"
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.SH "NAME"
perlfaq6 \- Regular Expressions ($Revision: 10126 $)
.SH "DESCRIPTION"
.IX Header "DESCRIPTION"
This section is surprisingly small because the rest of the \s-1FAQ\s0 is
littered with answers involving regular expressions.  For example,
decoding a \s-1URL\s0 and checking whether something is a number are handled
with regular expressions, but those answers are found elsewhere in
this document (in perlfaq9: \*(L"How do I decode or create those %\-encodings
on the web\*(R" and perlfaq4: \*(L"How do I determine whether a scalar is
a number/whole/integer/float\*(R", to be precise).
.Sh "How can I hope to use regular expressions without creating illegible and unmaintainable code?"
.IX Xref "regex, legibility regexp, legibility regular expression, legibility  x"
.IX Subsection "How can I hope to use regular expressions without creating illegible and unmaintainable code?"
Three techniques can make regular expressions maintainable and
understandable.
.IP "Comments Outside the Regex" 4
.IX Item "Comments Outside the Regex"
Describe what you're doing and how you're doing it, using normal Perl
comments.
.Sp
.Vb 3
\&        # turn the line into the first word, a colon, and the
\&        # number of characters on the rest of the line
\&        s/^(\ew+)(.*)/ lc($1) . ":" . length($2) /meg;
.Ve
.IP "Comments Inside the Regex" 4
.IX Item "Comments Inside the Regex"
The \f(CW\*(C`/x\*(C'\fR modifier causes whitespace to be ignored in a regex pattern
(except in a character class), and also allows you to use normal
comments there, too.  As you can imagine, whitespace and comments help
a lot.
.Sp
\&\f(CW\*(C`/x\*(C'\fR lets you turn this:
.Sp
.Vb 1
\&        s{<(?:[^>\*(Aq"]*|".*?"|\*(Aq.*?\*(Aq)+>}{}gs;
.Ve
.Sp
into this:
.Sp
.Vb 10
\&        s{ <                    # opening angle bracket
\&                (?:                 # Non\-backreffing grouping paren
\&                        [^>\*(Aq"] *        # 0 or more things that are neither > nor \*(Aq nor "
\&                                |           #    or else
\&                        ".*?"           # a section between double quotes (stingy match)
\&                                |           #    or else
\&                        \*(Aq.*?\*(Aq           # a section between single quotes (stingy match)
\&                ) +                 #   all occurring one or more times
\&                >                   # closing angle bracket
\&        }{}gsx;                 # replace with nothing, i.e. delete
.Ve
.Sp
It's still not quite so clear as prose, but it is very useful for
describing the meaning of each part of the pattern.
.IP "Different Delimiters" 4
.IX Item "Different Delimiters"
While we normally think of patterns as being delimited with \f(CW\*(C`/\*(C'\fR
characters, they can be delimited by almost any character.  perlre
describes this.  For example, the \f(CW\*(C`s///\*(C'\fR above uses braces as
delimiters.  Selecting another delimiter can avoid quoting the
delimiter within the pattern:
.Sp
.Vb 2
\&        s/\e/usr\e/local/\e/usr\e/share/g;  # bad delimiter choice
\&        s#/usr/local#/usr/share#g;              # better
.Ve
.Sh "I'm having trouble matching over more than one line.  What's wrong?"
.IX Xref "regex, multiline regexp, multiline regular expression, multiline"
.IX Subsection "I'm having trouble matching over more than one line.  What's wrong?"
Either you don't have more than one line in the string you're looking
at (probably), or else you aren't using the correct modifier(s) on
your pattern (possibly).
.PP
There are many ways to get multiline data into a string.  If you want
it to happen automatically while reading input, you'll want to set $/
(probably to '' for paragraphs or \f(CW\*(C`undef\*(C'\fR for the whole file) to
allow you to read more than one line at a time.
.PP
Read perlre to help you decide which of \f(CW\*(C`/s\*(C'\fR and \f(CW\*(C`/m\*(C'\fR (or both)
you might want to use: \f(CW\*(C`/s\*(C'\fR allows dot to include newline, and \f(CW\*(C`/m\*(C'\fR
allows caret and dollar to match next to a newline, not just at the
end of the string.  You do need to make sure that you've actually
got a multiline string in there.
.PP
For example, this program detects duplicate words, even when they span
line breaks (but not paragraph ones).  For this example, we don't need
\&\f(CW\*(C`/s\*(C'\fR because we aren't using dot in a regular expression that we want
to cross line boundaries.  Neither do we need \f(CW\*(C`/m\*(C'\fR because we aren't
wanting caret or dollar to match at any point inside the record next
to newlines.  But it's imperative that $/ be set to something other
than the default, or else we won't actually ever have a multiline
record read in.
.PP
.Vb 6
\&        $/ = \*(Aq\*(Aq;                # read in more whole paragraph, not just one line
\&        while ( <> ) {
\&                while ( /\eb([\ew\*(Aq\-]+)(\es+\e1)+\eb/gi ) {   # word starts alpha
\&                        print "Duplicate $1 at paragraph $.\en";
\&                }
\&        }
.Ve
.PP
Here's code that finds sentences that begin with \*(L"From \*(R" (which would
be mangled by many mailers):
.PP
.Vb 6
\&        $/ = \*(Aq\*(Aq;                # read in more whole paragraph, not just one line
\&        while ( <> ) {
\&                while ( /^From /gm ) { # /m makes ^ match next to \en
\&                print "leading from in paragraph $.\en";
\&                }
\&        }
.Ve
.PP
Here's code that finds everything between \s-1START\s0 and \s-1END\s0 in a paragraph:
.PP
.Vb 6
\&        undef $/;               # read in whole file, not just one line or paragraph
\&        while ( <> ) {
\&                while ( /START(.*?)END/sgm ) { # /s makes . cross line boundaries
\&                    print "$1\en";
\&                }
\&        }
.Ve
.Sh "How can I pull out lines between two patterns that are themselves on different lines?"
.IX Xref ".."
.IX Subsection "How can I pull out lines between two patterns that are themselves on different lines?"
You can use Perl's somewhat exotic \f(CW\*(C`..\*(C'\fR operator (documented in
perlop):
.PP
.Vb 1
\&        perl \-ne \*(Aqprint if /START/ .. /END/\*(Aq file1 file2 ...
.Ve
.PP
If you wanted text and not lines, you would use
.PP
.Vb 1
\&        perl \-0777 \-ne \*(Aqprint "$1\en" while /START(.*?)END/gs\*(Aq file1 file2 ...
.Ve
.PP
But if you want nested occurrences of \f(CW\*(C`START\*(C'\fR through \f(CW\*(C`END\*(C'\fR, you'll
run up against the problem described in the question in this section
on matching balanced text.
.PP
Here's another example of using \f(CW\*(C`..\*(C'\fR:
.PP
.Vb 7
\&        while (<>) {
\&                $in_header =   1  .. /^$/;
\&                $in_body   = /^$/ .. eof;
\&        # now choose between them
\&        } continue {
\&                $. = 0 if eof;  # fix $.
\&        }
.Ve
.Sh "I put a regular expression into $/ but it didn't work. What's wrong?"
.IX Xref "$ , regexes in $INPUT_RECORD_SEPARATOR, regexes in $RS, regexes in"
.IX Subsection "I put a regular expression into $/ but it didn't work. What's wrong?"
$/ has to be a string.  You can use these examples if you really need to 
do this.
.PP
If you have File::Stream, this is easy.
.PP
.Vb 1
\&        use File::Stream;