regular expression

 

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

 

1 search patterns告诉我们怎么去match

1）.*^${n,m}()[]

2）/w/W/s/S/d/D

 

2 replacement patterns告诉我们将匹配的内容做如何处理

 

 

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

 

 

 

1.3.2.1 Search patterns

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

Character	Pattern
.	Match any single character except newline. Can match newline in awk.
*	Match any number (or none) of the single character that immediately precedes it. The preceding character can also be a regular expression. For example, since . (dot) means any character, .* means "match any number of any character."
^	Match the following regular expression at the beginning of the line or string.
$	Match the preceding regular expression at the end of the line or string.
/	Turn off the special meaning of the following character.
[ ]	Match any one of the enclosed characters. A hyphen (-) indicates a range of consecutive characters. A circumflex (^) as the first character in the brackets reverses the sense: it matches any one character not in the list. A hyphen or close bracket (]) as the first character is treated as a member of the list. All other metacharacters are treated as members of the list (i.e., literally).
{n,m}	Match a range of occurrences of the single character that immediately precedes it. The preceding character can also be a metacharacter. {n} matches exactly n occurrences; {n,} matches at least n occurrences; and {n,m} matches any number of occurrences between n and m. n and m must be between 0 and 255, inclusive.
/{n,m/}	Just like {n,m}, but with backslashes in front of the braces.
/( /)	Save the pattern enclosed between /( and /) into a special holding space. Up to nine patterns can be saved on a single line. The text matched by the subpatterns can be "replayed" in substitutions by the escape sequences /1 to /9.
/n	Replay the nth sub-pattern enclosed in /( and /) into the pattern at this point. n is a number from 1 to 9, with 1 starting on the left. 在perl regularEX中在()中的内容可以是一个group，也可以将其中匹配内容保存在一个变量中，变量名可以在serach pattern中使用，也可以在replacement pattern中使用。
/< />	Match characters at beginning (/<) or end (/>) of a word.
+	Match one or more instances of preceding regular expression.
?	Match zero or one instances of preceding regular expression.
|	Match the regular expression specified before or after.
( )	Apply a match to the enclosed group of regular expressions.

/w	Word character
/W	Non-word character
/d	Digit character
/D	Non-digit character
/s	Whitespace character
/S	Non-whitespace character

 

 

 

1.3.2.2 Replacement patterns

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

Character	Pattern
/	Turn off the special meaning of the following character.
/n	Restore the text matched by the nth pattern previously saved by /( and /). n is a number from 1 to 9, with 1 starting on the left.
&	Reuse the text matched by the search pattern as part of the replacement pattern.
~	Reuse the previous replacement pattern in the current replacement pattern. Must be the only character in the replacement pattern (ex and vi).
%	Reuse the previous replacement pattern in the current replacement pattern. Must be the only character in the replacement pattern (ed).
/u	Convert first character of replacement pattern to uppercase.
/U	Convert entire replacement pattern to uppercase.
/l	Convert first character of replacement pattern to lowercase.
/L	Convert entire replacement pattern to lowercase.
/E	Turn off previous /U or /L.
/e	Turn off previous /u or /l.

 

 

 

 

1.3.2 Regular Expression Operators Perl provides the built-in regular expression operators qr//, m//, and s///, as well as the split function. Each operator accepts a regular expression pattern string that is run through string and variable interpolation and then compiled. Regular expressions are often delimited with the forward slash, but you can pick any non-alphanumeric, non-whitespace character. Here are some examples: qr#...# m!...! m{...} s|...|...| s[...][...] s<...>/.../ A match delimited by slashes (/.../) doesn't require a leading m: /.../ #same as m/.../ Using the single quote as a delimiter suppresses interpolation of variables and the constructs /N{name}, /u, /l, /U, /L, /Q, /E. Normally these are interpolated before being passed to the regular expression engine. qr// (Quote Regex)

---

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// (Matching)

---

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/// (Substitution)

---

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

---

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.

 

 

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;

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". 

/([a-zA-Z])/1/注意这里的/1，他代表了前面()中匹配的内容