正则表达式-学习2 - 语法

2017年11月08日09:57:27再次重新系统的开始学习正则表达式,希望这次可以真正的学会!

语法学习

1. Character classes

Character classes match a character from a specific set. There are a number of predefined(预定的) character classes and you can also define your own sets.

用法 匹配
. 匹配任何字符,除了换行符(any character except newline)
\w\d\s 字符,数字,空白(word, digit, whitespace)
\W,\D,\S 非字符,非数字,非空白( not word, digit, whitespace)
[abc] any of a, b, or c 【中间没有任何字符分隔】
[^abc] not a,b or c (出了个a,b,c其余的)
[a-g] character between a & g
\b 匹配单词(word)边界位置,如空格,标点符号或字符串的开始/结尾。这匹配一个位置,而不是一个字符。(Matches a word boundary position such as whitespace, punctuation, or the start/end of the string. This matches a position, not a character.)
\B 匹配不是单词边界的任意位置,这匹配的是一个位置,不是一个字符。
\w 匹配任何单词字符(字母数字和下划线)。只匹配low-ascii字符(没有重音或非罗马字符)。相当于[A-Za-z0-9_](Matches any word character (alphanumeric & underscore). Only matches low-ascii characters (no accented or non-roman characters). Equivalent to [A-Za-z0-9_])
\W Matches any character that is not a word character (alphanumeric & underscore). Equivalent to [^A-Za-z0-9_]
\d Matches any digit character (0-9). Equivalent to [0-9].
\D Matches any character that is not a digit character (0-9). Equivalent to [^0-9].
\s Matches any whitespace character (spaces, tabs, line breaks(换行符)).
\S Matches any character that is not a whitespace character (spaces, tabs, line breaks).
character set(字符集)[aeiou] Match any character in the set.
negated set(非字符集)[^aeiou] Match any character that is not in the set.
range: [a-z] Matches a character having a character code between the two specified characters inclusive.(在两个指定的字符之间匹配具有字符代码的字符)

2. Anchors (锚)

Anchors are unique in that they match a position within a string, not a character.

是独一无二的,它们匹配的位置在一个字符串,而不是一个字符。

用法 匹配
begining : ^ Matches the beginning of the string, or the beginning of a line if the multiline flag (m) is enabled. This matches a position, not a character. e.g.: /^\w+/gmHe is a boy.
end: $ Matches the end of the string, or the end of a line if the multiline flag (m) is enabled. This matches a position, not a character. e.g.:/\w+$/g – I am a girl_
word boundary(边界)\b Matches a word boundary position such as whitespace(空白), punctuation(标点符号), or the start/end of the string. This matches a position, not a character. e.g.:/m\b/gm – me e**m** ;/\bm/gm – **m**e em ;
not word boundary \B Matches any position that is not a word boundary. This matches a position, not a character. e.g.:/\w\B/gm –I **a**m a **girl**_ **m**e **e**m

3. Escapaed characters(转义字符)

Some characters have special meaning in regular expressions and must be escaped(避免). All escaped characters begin with the \ character.

Within a character set, only , -, and ] need to be escaped.

用法 匹配
octal escape:\251 Octal(八进制) escaped character in the form \000. Value must be less than 255 (\377).(八进制需要转义的字符是从\000\377(十进制的255))e.g.: /\255/g – RegExr is ©2017.(其中©是被匹配的)
Hexadecimal(16进制) escape:\xA9 Hexadecimal escaped character in the form \xFF. e.g.: /\xA9/g – RegExr is ©2017.(其中©是被匹配的)
Unicode escape:\u00A9 Unicode escaped character in the form \uFFFF. e.g.: /\u00A9/g – RegExr is ©2017.(其中©是被匹配的)
control character escape Escaped control character in the form \cZ. This can range from \cA (NULL, char code 0) to \cZ (EM, char code 25). e.g.: \cI matches TAB (char code 9).
tab: \t Matches a TAB character (char code 9).
line feed : \n Matches a LINE FEED character (char code 10 - 换行键).
vertical tab: \v Matches a VERTICAL TAB character (char code 11 - 垂直制表符).
form feed:\f Matches a FORM FEED character (char code 12-换行键).
carriage return :\r Matches a CARRIAGE RETURN character (char code 13 - 回车键).
null : \0 Matches a NULL character (char code 0 - 空字符).
点.: \. Matches a “.” character (char code 46).
斜杠 \ : \\ Matches a “\” character (char code 92).
    \+
Matches a “+” character (char code 43).
    : \*
Matches a “*” character (char code 42).
?:\? Matches a “?” character (char code 63).
^: \^ Matches a “^” character (char code 94).

:$|Matchesa" ” character (char code 36).
[: \[ | Matches a “[” character (char code 91).
]: \] | Matches a “]” character (char code 93).
{:\{ | Matches a “{” character (char code 123).
}: \} | Matches a “}” character (char code 125).
(: \( | Matches a “(” character (char code 40).
): \) | Matches a “)” character (char code 41).
|: \| | Matches a “|” character (char code 124).
/: \/ | Matches a “\/” character (char code 47).

4. Groups & Lookaround(组和查看)

组允许您将一系列令牌组合在一起操作。捕获组可以通过反向引用来引用,并在结果中单独访问。
(Groups allow you to combine a sequence of tokens to operate on them together. Capture groups can be referenced by a backreference and accessed separately in the results.)

Lookaround让你匹配一个组,而不会在结果中包含它。
(Lookaround lets you match a group without including it in the result.)

用法 匹配
capturing group(捕获组): (ABC) 将多个标记组合在一起,并创建一个提取子字符串或使用反向引用的捕获组。(Groups multiple tokens together and creates a capture group for extracting a substring or using a backreference.) (备注1)
backreference(对捕获组的反向引用): \1,\3, \num Matches the results of a previous capture group. For example \1 matches the results of the first capture group & \3 matches the third. 对前面捕获分组的引用。 e.g. : var str = 'abccab abc aba cbc dbd;'结果:str.match(/(\w)b\1/g) -> (3) ["aba", "cbc", "dbd"](\num代表捕获到的第一个值)
non-capturing group:(?:ABC) Groups multiple tokens together without creating a capture group. (没有搞明白)
以下是断言
positive lookahead: (?=ABC) Matches a group after the main expression without including it in the result. 向指定xxx后边肯定会出现ABC,就用正先行断言,表达式:(?=ABC) e.g.: \d(?=px) – 1pt **2**px 3em **4**px
nagative lookahead: (?!ABC) Specifies a group that can not match after the main expression (if it matches, the result is discarded). 向指定xxx后边肯定不会出现ABC,就用正先行断言,表达式:(?!ABC) e.g.: \d(?=px) – **1**pt 2px **3**em 4px

备注1 - 捕获组:

捕获组就是把正则表达式中子表达式匹配的内容,保存到内存中以数字编号或显式命名的组里,方便后面引用。当然,这种引用既可以是在正则表达式内部,也可以是在正则表达式外部。

备注2 - 断言:
所谓断言,就是指明某个字符串前边或者后边,将会出现满足某种规律的字符串。

5. Quantifiers & Alternation

量词指示前面的标记必须匹配一定的次数。在默认情况下,量词是贪婪的,并且会匹配尽可能多的字符。

交替行为像一个布尔OR,匹配一个或另一个序列。(Alternation acts like a boolean OR, matching one sequence or another.)

用法 匹配
plus: + Matches 1 or more of the preceding token.
star: * Matches 0 or more of the preceding token.
quantifier: {1,3} Matches the specified quantity of the previous token. {1,3} will match 1 to 3. {3} will match exactly 3. {3,} will match 3 or more.
optional: ? Matches 0 or 1 of the preceding token(上述标记), effectively making it optional.(使其成为可选项) eg. : /colou?r/g – color colour
lazy: ? Makes the preceding(前) quantifier(量词) lazy, causing it to match as few characters as possible. By default, quantifiers are greedy(贪婪的), and will match as many characters as possible. eg. : b\w+? – b be **be**e **be**er **be**ers
alternation:| Acts like a boolean OR. Matches the expression before or after the |.It can operate within a group, or on a whole expression. The patterns will be tested in order.(它可以在一个小组内或者是整个表达内内运行,模式将按顺序进行测试) eg. :b(a|e|i)dbad bud bod bed bid

6. Substitution (代替)

These tokens are used in a substitution string to insert different parts of the match.(这些令牌用于替换字符串插入的不同部分匹配。)

用法 匹配
match:$& Inserts the matched text. eg. 题目:'意见-rightClick-modal'.replace(/([^-]*)-/,'===$&====') 结果:"===意见-====rightClick-modal"
capture group: $1 Inserts the results of the specified capture group. For example, $3 would insert the third capture group. 详细解释在下:1.2 捕获组编号规则
before match: $` Inserts the portion of the source string that precedes the match.(插入匹配之前的源字符串部分)
after match : $’ Inserts the portion of the source string that follows the match.
escaped: $ Inserts a dollar sign character ($). e.g.. > test = 'abcdefg'; > test.replace(/ab(\w+?)[\D\d]*e(\w+?).*/g, '$2$') < "f$"

7. Flags

用法 匹配
ignore case : i Makes the whole expression case-insensitive(不区分大小写). For example, /aBc/i would match AbC.
global search: g Retain the index of the last match, allowing subsequent(后续的) searches to start from the end of the previous match.
multiline: m When the multiline flag is enabled, beginning and end anchors (^ and )willmatchthestartandendofaline,insteadofthestartandendofthewholestring.((and ) 将匹配行的开始和结束,而不是整个字符串的开始和结束)
Unicode:u When the unicode flag is enabled, you can use extended(扩展) unicode escapes in the form \x{FFFFF}.It also makes other escapes stricter, causing unrecognized escapes (ex. \j) to throw an error.
sticky:y It also makes other escapes stricter, causing unrecognized escapes (ex. \j) to throw an error.(该表达式只能匹配lastIndex的位置,如果设置则忽略全局(g)标志。 由于RegExr中的每个搜索都是离散的,因此此标志对显示的结果没有进一步的影响。)

重点详解

1. 捕获组

1.1 what

捕获组就是把正则表达式中子表达式匹配的内容,保存到内存中以数字编号或显式命名的组里,方便后面引用。当然,这种引用既可以是在正则表达式内部,也可以是在正则表达式外部。

捕获组有两种形式,一种是普通捕获组,另一种是命名捕获组,通常所说的捕获组指的是普通捕获组。语法如下:

普通捕获组:(Expression)
命名捕获组:(?Expression)

普通捕获组在大多数支持正则表达式的语言或工具中都是支持的,而命名捕获组目前只有.NET、PHP、Python等部分语言支持,据说Java会在7.0中提供对这一特性的支持。上面给出的命名捕获组的语法是.NET中的语法,另外在.NET中使(?’name’Expression)与使用(?<name>Expression)等价的。在PHP和Python中命名捕获组语法为:(?P<name>Expression)

另外需要说明的一点是,除(Expression)(?<name>Expression)语法外,其它的(?...)语法都不是捕获组。

1.2 捕获组编号规则:$1\$2\$3...$n

编号规则指的是以数字为捕获组进行编号的规则,在普通捕获组命名捕获组单独出现的正则表达式中,编号规则比较清晰,在普通捕获组与命名捕获组混合出现的正则表达式中,捕获组的编号规则稍显复杂。

在展开讨论之前,需要说明的是,编号为0的捕获组,指的是正则表达式整体,这一规则在支持捕获组的语言中,基本上都是适用的。下面对其它编号规则逐一展开讨论。

通俗的解释:

1, 2…是表示的小括号里的内容 , 1, 2是第2个小括号里的
e.g.

var test = ‘abcde acerfade sdfjawide hello dfwaf’;
test.replace(/a(\w+?)[\D\d]he(\w+?)[\D\D]/g, ‘ 1 2’)
结果: “b-l”
第一个分组匹配出来的是“b”,第二个分组匹配出来的是”l”,所以将$1-$2的结果就是b-l
而[\D\d]是匹配任意字符

1.2.1普通捕获组

如果没有显式为捕获组命名,即没有使用命名捕获组,那么需要按数字顺序来访问所有捕获组。在只有普通捕获组的情况下,捕获组的编号是按照“(”出现的顺序,从左到右,从1开始进行编号的。

e.g.:正则表达式:(\d{4})-(\d{2}-(\d\d))
reg.png

上面的正则表达式可以用来匹配格式为yyyy-MM-dd的日期,为了在下表中得以区分,月和日分别采用了\d{2}和\d\d这两种写法。

用以上正则表达式匹配字符串:2008-12-31,匹配结果为:

编号 命名 捕获组 匹配内容
0 (\d{4})-(\d{2}-(\d\d)) 2008-12-31
1 (\d{4}) 2008
2 (\d{2}-(\d\d)) 12-31
3 (\d\d) 31

e.g.: 浏览器中执行的结果:

> var dateStr = '2008-12-31';

> datestr.match(/(\d{4})-(\d{2}-(\d\d))/g);
< ["2008-12-31"]

> datestr.match(/(\d{4})-(\d{2}-(\d\d))/)
< (4) ["2008-12-31", "2008", "12-31", "31", index: 0, input: "2008-12-31"]

为什么会有这两种不同的结果呢?

这是JavaScript中match()方法的特性。match方法的返回值存放匹配结果的数组。该数组的内容依赖于 regexp 是否具有全局标志 g。
* 如果 regexp 没有标志 g,那么 match() 方法就只能在 stringObject 中执行一次匹配。如果没有找到任何匹配的文本, match() 将返回 null。否则,它将返回一个数组,其中存放了与它找到的匹配文本有关的信息。该数组的第 0 个元素存放的是匹配文本,而其余的元素存放的是与正则表达式的子表达式匹配的文本。除了这些常规的数组元素之外,返回的数组还含有两个对象属性。index 属性声明的是匹配文本的起始字符在 stringObject 中的位置,input 属性声明的是对 stringObject 的引用

* 如果 regexp 具有标志 g,则 match() 方法将执行全局检索,找到 stringObject 中的所有匹配子字符串。若没有找到任何匹配的子串,则返回 null。如果找到了一个或多个匹配子串,则返回一个数组。不过全局匹配返回的数组的内容与前者大不相同,它的数组元素中存放的是 stringObject 中所有的匹配子串,而且也没有 index 属性或 input 属性。

注意:在全局检索模式下,match() 即不提供与子表达式匹配的文本的信息,也不声明每个匹配子串的位置。如果您需要这些全局检索的信息,可以使用 RegExp.exec()。

由于普通捕获组编号顺序从0开始,那么可以使用$1,$2,$..来进行表示。e.g:

> datestr.replace(/(\d{4})-(\d{2}-(\d\d))/, "$1$2号");
< "2008年12-31号"
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