Regular Expressions Regular Expression通常是用來尋找特定的字串樣式(pattern)，也就是所謂格式辨認(pattern-matching)的功能。它的運算子是『=~』和『!~』，可以把它念做match和not match。 Syntax: $string =~ /regular expression/expression modifier 例：$sentence =~ /Hello/
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(a) Modifiers：修飾選項可有可無，它是用來對整個敘述作修正的。| g | Match globally, i.e. find all occurrences. |
| i | Makes the search case-insensitive. |
| m | If the string has new-line characters embedded within it, the metacharacters ^ and $ will not work correctly. This modifier tells Perl to treat this line as a multiple line. |
| o | Only compile pattern once. |
| s | The character . matches any character except a new line. This modifier treats this line as a single line, which allows . to match a new-line character. |
| x | Allows white space in the expression. |
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(b) Metacharacter：下面這些字元都具有特殊意義，可以讓你建立更複雜的搜尋樣式(searching pattern)。| \ | Tells Perl to accept the following characters as a regular character; this removes special meanings from any metacharacter. |
| ^ | Matches the beginning of the string, unless /m is used. |
| . | Matches any character except a new line character, unless /s is used. |
| $ | Matches the end of the string, unless /m is used. |
| | | Expresses alternation. This means the expressions will search for multiple patterns in the same string. |
| ( ) | Groups expressions to assist in alternation and back referencing. |
| [ ] | Looks for a set of characters. |
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(c) Pattern Quantifier：用來表示字元的數量關係。| * | Matchs 0 or more times. |
| + | Matchs 1 or more times. |
| ? | Matchs 0 or 1 times. |
| {n} | Matches exactly n times. |
| {n,} | Matches at least n times. |
| {n,m} | Matches at least n times but no more than m times. |
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(d) Character Patterns：下列的sequence用來match一些特定格式的字元：| \r | Carriage return(CR), ASCII 13(十進位) |
| \n | New line, UNIX中代表ASCII 10(十進位), DOS(Windows)系統中則是ASCII 13 + ASCII 10(十進位). |
| \t | Tab, ASCII 9(十進位) |
| \w | Matches an alphanumericcharacter. Alphanumeric also includes _. 即 [A-Za-z0-9_]. |
| \W | Matches a nonalphanumeric character. 即 [^A-Za-z0-9_]. |
| \s | Matches a white space character. This includes space, tab, FormFeed and CR/LF. 即 [\ \t\f\r\n]. |
| \S | Matches a non-whote space character. 即 [^\ \t\f\r\n]. |
| \d | Matches a digit. 即 [0-9]. |
| \D | Matches a nondigit character. 即 [^0-9]. |
| \b | Matches a word boundary. |
| \B | Matches a nonword boundary. |
| \033 | octal char |
| \x1B | hex char |
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(e) Examples：Regular Expression這個東東非常強大、非常重要，但是對初學者來說簡直是個惡夢，記得我當初剛接觸時也是霧煞煞的，就算現在的我也不敢說全懂了:p 但你若瞭解了它的基本技巧後，包你愛不釋手，每每為它強大的功能讚歎。上面那些表格相信你也是有看沒有懂，這種東西要借由範例入門比較快，下面我列出一些基本範例，希望能幫助你瞭解它的基本技巧。
[size=+1]/abc/ 找到含有abc的字串
[size=+1]/^abc/ 找到開頭是abc的字串
[size=+1]/abc$/ 找到結尾是abc的字串
[size=+1]/a|b/ 找到有a或b的字串，也可以用來找整個字(word)
[size=+1]/ab{2,4}c/ 找到a後面跟著2-4個b，再跟著c的字串，若只有/ab{2,}c/則會找二個以上的b
[size=+1]/ab*c/ 找到a後面跟著0個或多個b，再跟著c的字串，如同/ab{0,}c/[size=+1]
/ab+c/ 找到a後面跟著一個以上的b，再跟著c的字串，如同/ab{1,}c/[size=+1]
/a.c/ .可以代表任何字元，除了new line字元(\n)外。[size=+1]
/[abc]/ 找到含有這三個字元中任何一個的字串[size=+1]
/\d/ 找到含有數字的字串，如同/[0-9]/[size=+1]
/\w/ 找到含有字母的字串，如同/[a-zA-Z0-9_]/[size=+1]
/\s/ 找到含有white space的字串，如同/[ \t\r\n\f]/[size=+1]
/[^abc]/ 找到沒有abc任一字元的字串[size=+1]
/\*/ 找到含有字元*的字串，在反斜線"\"後面的字元Perl會把它當作普通字元看待。若你不確定這個符號是否為特殊字元，乾脆全加上\以策安全。[size=+1]
/abc/i 忽略abc的大小寫[size=+1]
/(\d+)\.(\d+)\.(\d+)\.(\d+)/ 找到類似IP的字串，並將IP的四個數字分別存在$1,$2,$3,$4四個特殊變數中，以便在其後加以利用。例：
[size=+1]if ($x =~ /(\d+\.\d+)\.\d+\.\d+/) {
print "海洋大學" if ($1 eq "140.121");
}
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m//gimosx m命令可以讓你自訂pattern的分隔符號，而gimosx則是它的修飾選項，請參看(a)Modifiers。例如： [size=+1]
$url="my.machine.tw:8080/cgi-bin/test.pl";
($host, $port, $file)=($url=~m|http://([^/:]+):{0,1}(\d*)(\S*)$|); 這個Regular Expression相當複雜，主要目的是分析指定的URL，然後取得host名稱、port號碼及對應的檔案。我一項項慢慢解釋： [size=+1]$url=~m||
m後面跟著的就是分隔符號，| |裡面的就是pattern。
[size=+1]([^/:]+)
match一個字串，裡面沒有/和:字元。找到的字串存在$1中。
[size=+1]:{0,1}(\d*)
match 0或1個:，後面跟著一串數字或nothing。找到的字串存在$2中，若找不到，$2就是空的。
[size=+1](\S*)$
match一串非空白字元，並以找到的字串為結尾。找到的字串存在$3中。
[size=+1]()=()
($host, $port, $file)=($1, $2, $3)
即$host="my.machine.tw"
$port=8080
$file="/cgi-bin/test.pl"
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s/PATTERN/REPLACEMENT/egimox 沒錯，這就是取代的命令。它會尋找符合PATTERN的字串，並取代成REPLACEMENT字串。它的修飾選項多了e選項，其他的和上面都一樣，我將它列表如下：
| e | Evaluate the right side as an expression. |
| g | Replace globally, i.e. all occurrences. |
| i | Do case-insensitive pattern matching. |
| m | Treat string as multiple lines. |
| o | Only compile pattern once. |
| s | Treat string as single line. |
| x | Use extended regular expressions. |
例：
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$x =~ s/\s+//g
把所有的white space全部去除掉 [size=+1]
$x =~ s/([^ ]*):*([^ ]*)/$2
1/
把用":"分開的兩個欄位互相對調 [size=+1]
$path =~ s|/usr/bin|/usr/local/bin|
它也可以讓你自訂分隔符號哦
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tr/SEARCHLIST/REPLACEMENTLIST/cds 這是也是取代的命令，和上一個不同的是SEARCHLIST和REPLACEMENTLIST只能是普通字串，而不是Regular Expression，所以速度比較快。它的修飾選項也比較少：
| c | Complement the SEARCHLIST. |
| d | Delete found but unreplaced characters. |
| s | Squash duplicate replaced characters. |
例：
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$x =~ tr/this/that/
把"this"替換成"that" [size=+1]
$x =~ tr/a-z/A-Z/
把小寫字母全部替換成大寫字母 [size=+1]
$count = $x =~ tr/*/*/
計算$x中有幾個"*"
