regular expression

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今天学习了perl的正则表达式，它重要吗？已经很久没有用过了，但是需要的时候就急了，因为每次用过就仍了，没有好好的理解过。
regular expression' world is full of interesting. believe that and study it in the core.

regular expression帮助我们match、replace文本。

我将regular expression 分为三部分，search patterns，replacement patterns，opreator

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1 search patterns告诉我们怎么去match

1）.*^${n,m}()[]

2）/w/W/s/S/d/D

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2 replacement patterns告诉我们将匹配的内容做如何处理

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3 opreator其实与regular expression无关，他与perl相关。

m// (Matching)

s/// (Substitution)

split

另外perl常用的是基于命令行格式，如perl -ne ‘s///ig’ file.txt

n代表在while（<>）循环

e代表命令行格式

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1.3.2.1 Search patterns

The characters in the following table have special meaning only in search patterns:

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1.3.2.2 Replacement patterns

The characters in the following table have special meaning only in replacement patterns:

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qr#...#       m!...!        m{...}
s|...|...|    s[...][...]   s<...>/.../

/.../      #same as m/.../

qr/PATTERN/ismxo

Quote and compile PATTERN as a regular expression. The returned value may be used in a later pattern match or substitution. This saves time if the regular expression is going to be repeatedly interpolated. The match modes (or lack of),/ismxo,are locked in.

m/PATTERN/imsxocg

Match PATTERN against input string. In list context,returns a list of substrings matched by capturing parentheses,or else (1) for a successful match or ( ) for a failed match. In scalar context,returns 1 for success or "" for failure. /imsxo are optional mode modifiers. /cg are optional match modifiers. /g in scalar context causes the match to start from the end of the previous match. In list context,a /g match returns all matches or all captured substrings from all matches. A failed /g match will reset the match start to the beginning of the string unless the match is in combined /cg mode.

s/PATTERN/REPLACEMENT/egimosx

Match PATTERN in the input string and replace the match text with REPLACEMENT,returning the number of successes. /imosx are optional mode modifiers. /g substitutes all occurrences of PATTERN. Each /e causes an evaluation of REPLACEMENT as Perl code.

split /PATTERN/,EXPR,LIMIT
split /PATTERN/,EXPR
split /PATTERN/
split

Return a list of substrings surrounding matches of PATTERN in EXPR. If LIMIT,the list contains substrings surrounding the first LIMIT matches. The pattern argument is a match operator,so use m if you want alternate delimiters (e.g.,split m{PATTERN}). The match permits the same modifiers as m{}. Table 1-8 lists the after-match variables.

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1.3.4 Examples

Example 1-1. Simple match

# Match Spider-Man,Spiderman,SPIDER-MAN,etc.
my $dailybugle = "Spider-Man Menaces City!";
if ($dailybugle =~ m/spider[- ]?man/i) { do_something(  ); }

Example 1-2. Match,capture group,and qr

# Match dates formatted like MM/DD/YYYY,MM-DD-YY,...
my $date  = "12/30/1969";
my $regex = qr!(/d/d)[-/](/d/d)[-/](/d/d(?:/d/d)?)!;
if ($date =~ m/$regex/) {
  print "Day=  ",$1,"Month=",$2,"Year= ",$3;
}

Example 1-3. Simple substitution

# Convert <br> to <br /> for XHTML compliance
my $text = "Hello World! <br>";
$text =~ s#<br>#<br />#ig;

Example 1-4. Harder substitution

# urlify - turn URL's into HTML links
$text = "Check the website,http://www.oreilly.com/catalog/repr.";
$text =~ 
    s{
      /b                         # start at word boundary
      (                          # capture to $1
       (https?|telnet|gopher|file|wais|ftp) : 
                                 # resource and colon
       [/w/#~:.?+=&%@!/-] +?     # one or more valid
                                 # characters 
                                 # but take as little as
                                 # possible
      )
      (?=                        # lookahead   
        [.:?/-] *                #  for possible punctuation
        (?: [^/w/#~:.?+=&%@!/-]  #  invalid character
          | $ )                  #  or end of string
      )
     }{<a href="$1">$1</a>}igox;

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Any word (a word is defined as a sequence of alphanumerics - no whitespace) that contains a

 double letter,for example "book" has a double "o" and "feed" has a double "e". 

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??/([a-zA-Z])/1/注意这里的/1，他代表了前面()中匹配的内容