perlrequick.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.

.Sp
.Vb 1
\&    [0\-9a\-zA\-Z_]
.Ve
.IP "\(bu" 4
\&\eD is a negated \ed; it represents any character but a digit
.Sp
.Vb 1
\&    [^0\-9]
.Ve
.IP "\(bu" 4
\&\eS is a negated \es; it represents any non-whitespace character
.Sp
.Vb 1
\&    [^\es]
.Ve
.IP "\(bu" 4
\&\eW is a negated \ew; it represents any non-word character
.Sp
.Vb 1
\&    [^\ew]
.Ve
.IP "\(bu" 4
The period '.' matches any character but \*(L"\en\*(R"
.PP
The \f(CW\*(C`\ed\es\ew\eD\eS\eW\*(C'\fR abbreviations can be used both inside and outside
of character classes.  Here are some in use:
.PP
.Vb 7
\&    /\ed\ed:\ed\ed:\ed\ed/; # matches a hh:mm:ss time format
\&    /[\ed\es]/;         # matches any digit or whitespace character
\&    /\ew\eW\ew/;         # matches a word char, followed by a
\&                      # non\-word char, followed by a word char
\&    /..rt/;           # matches any two chars, followed by \*(Aqrt\*(Aq
\&    /end\e./;          # matches \*(Aqend.\*(Aq
\&    /end[.]/;         # same thing, matches \*(Aqend.\*(Aq
.Ve
.PP
The \fBword\ anchor\fR\  \f(CW\*(C`\eb\*(C'\fR matches a boundary between a word
character and a non-word character \f(CW\*(C`\ew\eW\*(C'\fR or \f(CW\*(C`\eW\ew\*(C'\fR:
.PP
.Vb 4
\&    $x = "Housecat catenates house and cat";
\&    $x =~ /\ebcat/;  # matches cat in \*(Aqcatenates\*(Aq
\&    $x =~ /cat\eb/;  # matches cat in \*(Aqhousecat\*(Aq
\&    $x =~ /\ebcat\eb/;  # matches \*(Aqcat\*(Aq at end of string
.Ve
.PP
In the last example, the end of the string is considered a word
boundary.
.Sh "Matching this or that"
.IX Subsection "Matching this or that"
We can match different character strings with the \fBalternation\fR
metacharacter \f(CW\*(Aq|\*(Aq\fR.  To match \f(CW\*(C`dog\*(C'\fR or \f(CW\*(C`cat\*(C'\fR, we form the regex
\&\f(CW\*(C`dog|cat\*(C'\fR.  As before, perl will try to match the regex at the
earliest possible point in the string.  At each character position,
perl will first try to match the first alternative, \f(CW\*(C`dog\*(C'\fR.  If
\&\f(CW\*(C`dog\*(C'\fR doesn't match, perl will then try the next alternative, \f(CW\*(C`cat\*(C'\fR.
If \f(CW\*(C`cat\*(C'\fR doesn't match either, then the match fails and perl moves to
the next position in the string.  Some examples:
.PP
.Vb 2
\&    "cats and dogs" =~ /cat|dog|bird/;  # matches "cat"
\&    "cats and dogs" =~ /dog|cat|bird/;  # matches "cat"
.Ve
.PP
Even though \f(CW\*(C`dog\*(C'\fR is the first alternative in the second regex,
\&\f(CW\*(C`cat\*(C'\fR is able to match earlier in the string.
.PP
.Vb 2
\&    "cats"          =~ /c|ca|cat|cats/; # matches "c"
\&    "cats"          =~ /cats|cat|ca|c/; # matches "cats"
.Ve
.PP
At a given character position, the first alternative that allows the
regex match to succeed will be the one that matches. Here, all the
alternatives match at the first string position, so the first matches.
.Sh "Grouping things and hierarchical matching"
.IX Subsection "Grouping things and hierarchical matching"
The \fBgrouping\fR metacharacters \f(CW\*(C`()\*(C'\fR allow a part of a regex to be
treated as a single unit.  Parts of a regex are grouped by enclosing
them in parentheses.  The regex \f(CW\*(C`house(cat|keeper)\*(C'\fR means match
\&\f(CW\*(C`house\*(C'\fR followed by either \f(CW\*(C`cat\*(C'\fR or \f(CW\*(C`keeper\*(C'\fR.  Some more examples
are
.PP
.Vb 2
\&    /(a|b)b/;    # matches \*(Aqab\*(Aq or \*(Aqbb\*(Aq
\&    /(^a|b)c/;   # matches \*(Aqac\*(Aq at start of string or \*(Aqbc\*(Aq anywhere
\&
\&    /house(cat|)/;  # matches either \*(Aqhousecat\*(Aq or \*(Aqhouse\*(Aq
\&    /house(cat(s|)|)/;  # matches either \*(Aqhousecats\*(Aq or \*(Aqhousecat\*(Aq or
\&                        # \*(Aqhouse\*(Aq.  Note groups can be nested.
\&
\&    "20" =~ /(19|20|)\ed\ed/;  # matches the null alternative \*(Aq()\ed\ed\*(Aq,
\&                             # because \*(Aq20\ed\ed\*(Aq can\*(Aqt match
.Ve
.Sh "Extracting matches"
.IX Subsection "Extracting matches"
The grouping metacharacters \f(CW\*(C`()\*(C'\fR also allow the extraction of the
parts of a string that matched.  For each grouping, the part that
matched inside goes into the special variables \f(CW$1\fR, \f(CW$2\fR, etc.
They can be used just as ordinary variables:
.PP
.Vb 5
\&    # extract hours, minutes, seconds
\&    $time =~ /(\ed\ed):(\ed\ed):(\ed\ed)/;  # match hh:mm:ss format
\&    $hours = $1;
\&    $minutes = $2;
\&    $seconds = $3;
.Ve
.PP
In list context, a match \f(CW\*(C`/regex/\*(C'\fR with groupings will return the
list of matched values \f(CW\*(C`($1,$2,...)\*(C'\fR.  So we could rewrite it as
.PP
.Vb 1
\&    ($hours, $minutes, $second) = ($time =~ /(\ed\ed):(\ed\ed):(\ed\ed)/);
.Ve
.PP
If the groupings in a regex are nested, \f(CW$1\fR gets the group with the
leftmost opening parenthesis, \f(CW$2\fR the next opening parenthesis,
etc.  For example, here is a complex regex and the matching variables
indicated below it:
.PP
.Vb 2
\&    /(ab(cd|ef)((gi)|j))/;
\&     1  2      34
.Ve
.PP
Associated with the matching variables \f(CW$1\fR, \f(CW$2\fR, ... are
the \fBbackreferences\fR \f(CW\*(C`\e1\*(C'\fR, \f(CW\*(C`\e2\*(C'\fR, ...  Backreferences are
matching variables that can be used \fIinside\fR a regex:
.PP
.Vb 1
\&    /(\ew\ew\ew)\es\e1/; # find sequences like \*(Aqthe the\*(Aq in string
.Ve
.PP
\&\f(CW$1\fR, \f(CW$2\fR, ... should only be used outside of a regex, and \f(CW\*(C`\e1\*(C'\fR,
\&\f(CW\*(C`\e2\*(C'\fR, ... only inside a regex.
.Sh "Matching repetitions"
.IX Subsection "Matching repetitions"
The \fBquantifier\fR metacharacters \f(CW\*(C`?\*(C'\fR, \f(CW\*(C`*\*(C'\fR, \f(CW\*(C`+\*(C'\fR, and \f(CW\*(C`{}\*(C'\fR allow us
to determine the number of repeats of a portion of a regex we
consider to be a match.  Quantifiers are put immediately after the
character, character class, or grouping that we want to specify.  They
have the following meanings:
.IP "\(bu" 4
\&\f(CW\*(C`a?\*(C'\fR = match 'a' 1 or 0 times
.IP "\(bu" 4
\&\f(CW\*(C`a*\*(C'\fR = match 'a' 0 or more times, i.e., any number of times
.IP "\(bu" 4
\&\f(CW\*(C`a+\*(C'\fR = match 'a' 1 or more times, i.e., at least once
.IP "\(bu" 4
\&\f(CW\*(C`a{n,m}\*(C'\fR = match at least \f(CW\*(C`n\*(C'\fR times, but not more than \f(CW\*(C`m\*(C'\fR
times.
.IP "\(bu" 4
\&\f(CW\*(C`a{n,}\*(C'\fR = match at least \f(CW\*(C`n\*(C'\fR or more times
.IP "\(bu" 4
\&\f(CW\*(C`a{n}\*(C'\fR = match exactly \f(CW\*(C`n\*(C'\fR times
.PP
Here are some examples:
.PP
.Vb 6
\&    /[a\-z]+\es+\ed*/;  # match a lowercase word, at least some space, and
\&                     # any number of digits
\&    /(\ew+)\es+\e1/;    # match doubled words of arbitrary length
\&    $year =~ /\ed{2,4}/;  # make sure year is at least 2 but not more
\&                         # than 4 digits
\&    $year =~ /\ed{4}|\ed{2}/;    # better match; throw out 3 digit dates
.Ve
.PP
These quantifiers will try to match as much of the string as possible,
while still allowing the regex to match.  So we have
.PP
.Vb 5
\&    $x = \*(Aqthe cat in the hat\*(Aq;
\&    $x =~ /^(.*)(at)(.*)$/; # matches,
\&                            # $1 = \*(Aqthe cat in the h\*(Aq
\&                            # $2 = \*(Aqat\*(Aq
\&                            # $3 = \*(Aq\*(Aq   (0 matches)
.Ve
.PP
The first quantifier \f(CW\*(C`.*\*(C'\fR grabs as much of the string as possible
while still having the regex match. The second quantifier \f(CW\*(C`.*\*(C'\fR has
no string left to it, so it matches 0 times.
.Sh "More matching"
.IX Subsection "More matching"
There are a few more things you might want to know about matching
operators.  In the code
.PP
.Vb 4
\&    $pattern = \*(AqSeuss\*(Aq;
\&    while (<>) {
\&        print if /$pattern/;
\&    }
.Ve
.PP
perl has to re-evaluate \f(CW$pattern\fR each time through the loop.  If
\&\f(CW$pattern\fR won't be changing, use the \f(CW\*(C`//o\*(C'\fR modifier, to only
perform variable substitutions once.  If you don't want any
substitutions at all, use the special delimiter \f(CW\*(C`m\*(Aq\*(Aq\*(C'\fR:
.PP
.Vb 3
\&    @pattern = (\*(AqSeuss\*(Aq);
\&    m/@pattern/; # matches \*(AqSeuss\*(Aq
\&    m\*(Aq@pattern\*(Aq; # matches the literal string \*(Aq@pattern\*(Aq
.Ve
.PP
The global modifier \f(CW\*(C`//g\*(C'\fR allows the matching operator to match
within a string as many times as possible.  In scalar context,
successive matches against a string will have \f(CW\*(C`//g\*(C'\fR jump from match
to match, keeping track of position in the string as it goes along.
You can get or set the position with the \f(CW\*(C`pos()\*(C'\fR function.
For example,
.PP
.Vb 4
\&    $x = "cat dog house"; # 3 words
\&    while ($x =~ /(\ew+)/g) {
\&        print "Word is $1, ends at position ", pos $x, "\en";
\&    }
.Ve
.PP
prints
.PP
.Vb 3
\&    Word is cat, ends at position 3
\&    Word is dog, ends at position 7
\&    Word is house, ends at position 13
.Ve
.PP
A failed match or changing the target string resets the position.  If
you don't want the position reset after failure to match, add the
\&\f(CW\*(C`//c\*(C'\fR, as in \f(CW\*(C`/regex/gc\*(C'\fR.
.PP
In list context, \f(CW\*(C`//g\*(C'\fR returns a list of matched groupings, or if
there are no groupings, a list of matches to the whole regex.  So
.PP
.Vb 4
\&    @words = ($x =~ /(\ew+)/g);  # matches,
\&                                # $word[0] = \*(Aqcat\*(Aq
\&                                # $word[1] = \*(Aqdog\*(Aq
\&                                # $word[2] = \*(Aqhouse\*(Aq
.Ve
.Sh "Search and replace"
.IX Subsection "Search and replace"
Search and replace is performed using \f(CW\*(C`s/regex/replacement/modifiers\*(C'\fR.
The \f(CW\*(C`replacement\*(C'\fR is a Perl double quoted string that replaces in the
string whatever is matched with the \f(CW\*(C`regex\*(C'\fR.  The operator \f(CW\*(C`=~\*(C'\fR is
also used here to associate a string with \f(CW\*(C`s///\*(C'\fR.  If matching
against \f(CW$_\fR, the \f(CW\*(C`$_\ =~\*(C'\fR\  can be dropped.  If there is a match,
\&\f(CW\*(C`s///\*(C'\fR returns the number of substitutions made, otherwise it returns
false.  Here are a few examples:
.PP
.Vb 5
\&    $x = "Time to feed the cat!";
\&    $x =~ s/cat/hacker/;   # $x contains "Time to feed the hacker!"
\&    $y = "\*(Aqquoted words\*(Aq";
\&    $y =~ s/^\*(Aq(.*)\*(Aq$/$1/;  # strip single quotes,
\&                           # $y contains "quoted words"
.Ve
.PP
With the \f(CW\*(C`s///\*(C'\fR operator, the matched variables \f(CW$1\fR, \f(CW$2\fR, etc.
are immediately available for use in the replacement expression. With
the global modifier, \f(CW\*(C`s///g\*(C'\fR will search and replace all occurrences
of the regex in the string:
.PP
.Vb 4
\&    $x = "I batted 4 for 4";
\&    $x =~ s/4/four/;   # $x contains "I batted four for 4"
\&    $x = "I batted 4 for 4";
\&    $x =~ s/4/four/g;  # $x contains "I batted four for four"
.Ve
.PP
The evaluation modifier \f(CW\*(C`s///e\*(C'\fR wraps an \f(CW\*(C`eval{...}\*(C'\fR around the
replacement string and the evaluated result is substituted for the
matched substring.  Some examples:
.PP
.Vb 3
\&    # reverse all the words in a string
\&    $x = "the cat in the hat";
\&    $x =~ s/(\ew+)/reverse $1/ge;   # $x contains "eht tac ni eht tah"
\&
\&    # convert percentage to decimal
\&    $x = "A 39% hit rate";
\&    $x =~ s!(\ed+)%!$1/100!e;       # $x contains "A 0.39 hit rate"
.Ve
.PP
The last example shows that \f(CW\*(C`s///\*(C'\fR can use other delimiters, such as
\&\f(CW\*(C`s!!!\*(C'\fR and \f(CW\*(C`s{}{}\*(C'\fR, and even \f(CW\*(C`s{}//\*(C'\fR.  If single quotes are used
\&\f(CW\*(C`s\*(Aq\*(Aq\*(Aq\*(C'\fR, then the regex and replacement are treated as single quoted
strings.
.Sh "The split operator"
.IX Subsection "The split operator"
\&\f(CW\*(C`split /regex/, string\*(C'\fR splits \f(CW\*(C`string\*(C'\fR into a list of substrings
and returns that list.  The regex determines the character sequence
that \f(CW\*(C`string\*(C'\fR is split with respect to.  For example, to split a
string into words, use
.PP
.Vb 4
\&    $x = "Calvin and Hobbes";
\&    @word = split /\es+/, $x;  # $word[0] = \*(AqCalvin\*(Aq
\&                              # $word[1] = \*(Aqand\*(Aq
\&                              # $word[2] = \*(AqHobbes\*(Aq
.Ve
.PP
To extract a comma-delimited list of numbers, use
.PP
.Vb 4
\&    $x = "1.618,2.718,   3.142";
\&    @const = split /,\es*/, $x;  # $const[0] = \*(Aq1.618\*(Aq
\&                                # $const[1] = \*(Aq2.718\*(Aq
\&                                # $const[2] = \*(Aq3.142\*(Aq
.Ve
.PP
If the empty regex \f(CW\*(C`//\*(C'\fR is used, the string is split into individual
characters.  If the regex has groupings, then the list produced contains
the matched substrings from the groupings as well:
.PP
.Vb 6
\&    $x = "/usr/bin";
\&    @parts = split m!(/)!, $x;  # $parts[0] = \*(Aq\*(Aq
\&                                # $parts[1] = \*(Aq/\*(Aq
\&                                # $parts[2] = \*(Aqusr\*(Aq
\&                                # $parts[3] = \*(Aq/\*(Aq
\&                                # $parts[4] = \*(Aqbin\*(Aq
.Ve
.PP
Since the first character of \f(CW$x\fR matched the regex, \f(CW\*(C`split\*(C'\fR prepended
an empty initial element to the list.
.SH "BUGS"
.IX Header "BUGS"
None.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
This is just a quick start guide.  For a more in-depth tutorial on
regexes, see perlretut and for the reference page, see perlre.
.SH "AUTHOR AND COPYRIGHT"
.IX Header "AUTHOR AND COPYRIGHT"
Copyright (c) 2000 Mark Kvale
All rights reserved.
.PP
This document may be distributed under the same terms as Perl itself.
.Sh "Acknowledgments"
.IX Subsection "Acknowledgments"
The author would like to thank Mark-Jason Dominus, Tom Christiansen,
Ilya Zakharevich, Brad Hughes, and Mike Giroux for all their helpful
comments.