Perl : 正则注记

学习perl总结

• (?=<RE>), look ahead匹配但不吃掉
• (?!<RE>),下一组文本不match时,才匹配
• (?<=<RE>), lood behind, also (?<!<RE>)

$a = " cat housecat catch crazycats "; 

$a =~ s/ (?<= \s ) cat (?= \s ) / CAT /g ; # 不变 

$a =~ s/ (?

• (?xims-xims: <RE>)

/ Answer: ((?i) yes ) / ; # 'Answer: yes', '…YES', '…Yes', etc

/ Answer: ((? -i ) yes ) /i ; # 'answer: yes' only, not "…YES"

• (?:<RE>),使用(…)但不计入反向引用中
• (?#)comment, replaced by m/…/x now.
• ?RE? Match only once between reset call.

关于Perl里面正则规范
其中介绍下面这些的部分不错：
(?#text)
(?:pattern)
(?imsx:pattern)
(?=pattern)
(?!pattern)
(?<=pattern)
(? (?{code})
(?>pattern)
(!<=pattern)
(!=pattern)
(?(condition)yes-pattern|no-pattern)
(?(condition)yes-pattern)
(?imsx)
(?-imsx)
与换行有关(\n)的修饰符
/s 使得.可以匹配任何字符，包括换行符。例如：/Barney.*Fred/s
/m 将字符串视作多行，允许根据字符串内部的换行符进行匹配。

正则式的描述符介绍：The Effective Perler-Know the difference between regex and match operator flags