正则表达式
Lookaround
正则表达式中两个有用的结构: 向前查找 & 向后查找 (lookahead & lookbehind), 统称为 lookaround.
lookaround 去判断一个模式在字符串中是否存在, 不管匹配是否成功, 都不返回, 因此 lookaround 也称为断言(assertions).
lookaround 就像字符串的开始结束, 单词边界锚点(^ $ /b)一样, 是 "zero-width assertions".
Lookahead
Positive lookahead
my(?=here) 会匹配 myhere 成功，匹配其中的 my , (?=here) 用来判断 my 之后是否有 here ,有就匹配成功, 否则匹配失败; 匹配成功后 here 不返回;
my(?=here)$ 匹配 myhere 失败, 因为 (?=here) 只是判断 my 之后是否有 here, 他只是一种判定, 并不匹配, 因此 myehre 中 here
不能与 $ 匹配, 因此 my(?=here)$ 匹配 myhere 失败;
Negative lookahead
my(?!here) 若字符串中有 my, 且 my 后跟的不是 here, 则匹配成功
Lookbebind
Positive lookbehind
(?<=my)here 若字符串中有 here, 且之前是 my, 则匹配成功
Negative lookbind
(?<!my)here 若字符串中有 here, 且之前不是 my, 则匹配成功
JavaScript 中只有 lookahead, 没有 lookbehind
'myhere'.match(/my(?=here)/); // 匹配成功, 得到 my 'myhere'.match(/my(?=here)$/); // 匹配失败
PHP 中有 lookahead 和 lookbehind
$str = 'myhere'; $positive_lookahead = '/my(?=here)/'; $negative_lookahead = '/my(?!here)/'; $positive_lookbehind = '/(?<=my)here/'; $negative_lookbehind = '/(?<!my)here/'; preg_match( $positive_lookahead , $str, $matches1); // 匹配成功 preg_match( $negative_lookahead , $str, $matches2); // 匹配失败 preg_match( $positive_lookbehind, $str, $matches3); // 匹配成功 preg_match( $negative_lookbehind, $str, $matches4); // 匹配失败
Lookaround 和 non-capturing group 的区别:
1, lookaround 是判断, 不匹配字符(不消耗字符);他只是判断字符是否存在, lookaround 之后的正则会当 lookaround 不存在接着匹配(匹配成功的情况)
2, non-capturing group 匹配, 但是不编号
// 利用 lookaround 在字符串中去掉重复单词 var str= 'a aa a a-b a-a a-b '; var reg = /(?:^|/s)(/S+)(?=/s+(?:/S+/s+)*/1(?:/s|$))/g; console.log(str.replace(reg,"")); // 如果正则中的 lookahead 换成 non-capturing group 会达不到去重的效果, 因为 non-capturing group 会消耗字符。 // // 'a aa a a-b a-a a-b '.match(/(?:^|/s)(/S+)(?=/s+(?:/S+/s+)*/1(?:/s|$))/g); // ["a", " a-b"] // 'a aa a a-b a-a a-b '.match(/(?:^|/s)(/S+)(?=/s+(?:/S+/s+)*/1(?:/s|$))/g); // ["a aa a "] // // 正则来自(有改动) // http://www.cnblogs.com/rubylouvre/archive/2011/01/27/1946397.html
non-capturing group 与 capturing group 的异同:
1, 两者都是分组
2, non-capturing group 分组不编号
3, capturing group 分组编号
4, non-capturing group 用在需要对一些字符组合构成一个分组(一个整体), 但是不需要从匹配结果中单独获取这个分组(non-capturing group 不编号); non-capturing group 性能会比 capturing group 好
正则表达式的效率:
1, Eliminated unnecessary use of capturing group.
2, Eliminated unnecessary use of alternation.
3, Eliminated inefficient use of lazy quantifier.
4, Implemented "unrolling-the-loop" efficiency technique.
5, A successful match is significantly faster.
6, Added non-capture group and beginning of string anchor.
7, A successful non-match is significantly faster.
8, Added non-capture group and word boundary anchor.
9, Linefeed added to regex to allow table to fit on screen.
10, Negligible performance improvement.
11, Reduced code size.
12, Added "i" ignore-case modifier.
13, The old and new regexes can match different text.
14, Removed unnecessary backslash escapes.
15, Removed unnecessary callback function.
16, Removed unnecessary local variables.
17, Added non-capture group and exposed literal starting text.
18, Literal RegExp object should be cached.
参考:
Lookaround
正则表达式的效率
