正規表達式

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正規表達式是被用來匹配字串中字元組合的模式。在 JavaScript 中,正規表達式也是物件,這些模式在 RegExpexectest 方法中,以及 Stringmatchreplacesearchsplit 等方法中被運用。這一章節將解說 JavaScript 中的正規表達式。

創建正規表達式

您可透過下列兩種方法去創建一條正規表達式:

  • 使用 RegExp 常數,包含兩個 / 字元之間的模式如下:
            var re = /ab+c/;
        

    正規表達式常數在腳本載入時會被編譯,當正規表達式為定值時,使用此方法可獲得較佳效能。

  • 呼叫 RegExp 物件的建構子函數如下:
            var re = new RegExp("ab+c");
            

    使用建構子函數供即時編譯正則表達式,當模式會異動、事先未知匹配模式、或者您將從其他地方取得時,使用建構子函數將較為合適。

撰寫正規表達模式

正規表達模式由數個簡易字元組成,例如 /abc/,或是由簡易字元及特殊符號組合而成,例如 /ab*c//Chapter (\d+)\.\d*/ )。最後一個範例用到了括號,這在正規表達式中用作記憶組,使用括號的匹配將會被留到後面使用,在 Using Parenthesized Substring Matches 有更多解釋。

使用簡易模式

Simple patterns are constructed of characters for which you want to find a direct match. For example, the pattern /abc/ matches character combinations in strings only when exactly the characters 'abc' occur together and in that order. Such a match would succeed in the strings "Hi, do you know your abc's?" and "The latest airplane designs evolved from slabcraft." In both cases the match is with the substring 'abc'. There is no match in the string "Grab crab" because it does not contain the substring 'abc'.

使用特殊字元

When the search for a match requires something more than a direct match, such as finding one or more b's, or finding white space, the pattern includes special characters. For example, the pattern /ab*c/ matches any character combination in which a single 'a' is followed by zero or more 'b's (* means 0 or more occurrences of the preceding item) and then immediately followed by 'c'. In the string "cbbabbbbcdebc," the pattern matches the substring 'abbbbc'.

The following table provides a complete list and description of the special characters that can be used in regular expressions.

Table 4.1 Special characters in regular expressions.
字元 解說
\

Matches according to the following rules:

A backslash that precedes a non-special character indicates that the next character is special and is not to be interpreted literally. For example, a 'b' without a preceding '\' generally matches lowercase 'b's wherever they occur. But a '\b' by itself doesn't match any character; it forms the special word boundary character.

A backslash that precedes a special character indicates that the next character is not special and should be interpreted literally. For example, the pattern /a*/ relies on the special character '*' to match 0 or more a's. By contrast, the pattern /a\*/ removes the specialness of the '*' to enable matches with strings like 'a*'.

Do not forget to escape \ itself while using the RegExp("pattern") notation because \ is also an escape character in strings.

^

匹配輸入的開頭,如果 multiline flag 被設為 true,則會匹配換行字元後。

例如:/^A/ 不會匹配「an A」的 A,但會匹配「An E」中的 A。

^」出現在字元集模式的字首中有不同的意思,詳見補字元集

$

匹配輸入的結尾,如果 multiline flag 被設為 true,則會匹配換行字元。

例如:/t$/ 不會匹配「eater」中的 t,卻會匹配「eat」中的 t。

*

匹配前一字元 0 至多次。

例如:/bo*/ 匹配「A ghost booooed」中的 boooo、「A bird warbled」中的 b,但在「A goat grunted」中不會匹配任何字串。

+

匹配前一字元 1 至多次,等價於 {1,}

例如:/a+/ 匹配「candy」中的 a,以及所有「caaaaaaandy」中的 a。

?

匹配前一字元 0 至 1 次,等價於 {0,1}

例如:/e?le?/ 匹配「angel」中的 el、「angle」中的 le、以及「oslo」中的 l。

如果是使用在 *+?{} 等 quantifier 之後,將會使這些 quantifier non-greedy(也就是儘可能匹配最少的字元),與此相對的是 greedy(匹配儘可能多的字元)。例如:在「123abc」中應用 /\d+/ 可匹配「123」,但使用 /\d+?/ 在相同字串上只能匹配「1」。


Also used in lookahead assertions, as described in the x(?=y) and x(?!y) entries of this table.

.

(小數點)匹配除了換行符號之外的單一字元。

例如:/.n/ 匹配「nay, an apple is on the tree」中的 an 和 on,但在「nay」中沒有匹配。

(x)

Matches 'x' and remembers the match, as the following example shows. The parentheses are called capturing parentheses.

The '(foo)' and '(bar)' in the pattern /(foo) (bar) \1 \2/ match and remember the first two words in the string "foo bar foo bar". The \1 and \2 in the pattern match the string's last two words.

(?:x) Matches 'x' but does not remember the match. The parentheses are called non-capturing parentheses, and let you define subexpressions for regular expression operators to work with. Consider the sample expression /(?:foo){1,2}/. Without the non-capturing parentheses, the {1,2} characters would apply only to the last 'o' in 'foo'. With the capturing parentheses, the {1,2} applies to the entire word 'foo'.
x(?=y)

Matches 'x' only if 'x' is followed by 'y'. This is called a lookahead.

For example, /Jack(?=Sprat)/ matches 'Jack' only if it is followed by 'Sprat'. /Jack(?=Sprat|Frost)/ matches 'Jack' only if it is followed by 'Sprat' or 'Frost'. However, neither 'Sprat' nor 'Frost' is part of the match results.

x(?!y)

Matches 'x' only if 'x' is not followed by 'y'. This is called a negated lookahead.

For example, /\d+(?!\.)/ matches a number only if it is not followed by a decimal point. The regular expression /\d+(?!\.)/.exec("3.141") matches '141' but not '3.141'.

x|y

符合「x」或「y」。

舉例來說, /green|red/  的話,會匹配 "green apple" 中的 "green" 以及 "red apple." 的 "red"

{n} Matches exactly n occurrences of the preceding character. N must be a positive integer.

For example, /a{2}/ doesn't match the 'a' in "candy," but it does match all of the a's in "caandy," and the first two a's in "caaandy."
{n,m}

Where n and m are positive integers. Matches at least n and at most m occurrences of the preceding character. When m is zero, it can be omitted.

For example, /a{1,3}/ matches nothing in "cndy", the 'a' in "candy," the first two a's in "caandy," and the first three a's in "caaaaaaandy" Notice that when matching "caaaaaaandy", the match is "aaa", even though the original string had more a's in it.

[xyz] 字元的集合。此格式會匹配中括號內所有字元, including escape sequences。特殊字元,例如點(.) 和米字號(*),在字元集合中不具特殊意義,所以不需轉換。若要設一個字元範圍的集合,可以使用橫線 "-" ,如下例所示:

[a-d] 等同於 [abcd]。會匹配 "brisket" 的 "b" 、"city" 的 'c' ……等。 而/[a-z.]+/ /[\w.]+/ 均可匹配字串 "test.i.ng" 。
[^xyz]

A negated or complemented character set. That is, it matches anything that is not enclosed in the brackets. You can specify a range of characters by using a hyphen. Everything that works in the normal character set also works here.

For example, [^abc] is the same as [^a-c]. They initially match 'r' in "brisket" and 'h' in "chop."

[\b] Matches a backspace (U+0008). (Not to be confused with \ b.)
\b

Matches a word boundary. A word boundary matches the position where a word character is not followed or preceeded by another word-character. Note that a matched word boundary is not included in the match. In other words, the length of a matched word boundary is zero. (Not to be confused with [\b].)

Examples:
/\bm/ matches the 'm' in "moon" ;
/oo\b/ does not match the 'oo' in "moon", because 'oo' is followed by 'n' which is a word character;
/oon\b/ matches the 'oon' in "moon", because 'oon' is the end of the string, thus not followed by a word character;
/\w\b\w/ will never match anything, because a word character can never be followed by both a non-word and a word character.

\B

Matches a non-word boundary. This matches a position where the previous and next character are of the same type: Either both must be words, or both must be non-words. The beginning and end of a string are considered non-words.

For example, /\B../ matches 'oo' in "noonday" (, and /y\B./ matches 'ye' in "possibly yesterday."

\cX

Where X is a character ranging from A to Z. Matches a control character in a string.

For example, /\cM/ matches control-M (U+000D) in a string.

\d

Matches a digit character. Equivalent to [0-9].

For example, /\d/ or /[0-9]/ matches '2' in "B2 is the suite number."

\D

Matches any non-digit character. Equivalent to [^0-9].

For example, /\D/ or /[^0-9]/ matches 'B' in "B2 is the suite number."

\f Matches a form feed (U+000C).
\n Matches a line feed (U+000A).
\r Matches a carriage return (U+000D).
\s

Matches a single white space character, including space, tab, form feed, line feed. Equivalent to [ \f\n\r\t\v​\u00a0\u1680​\u180e\u2000​\u2001\u2002​\u2003\u2004​\u2005\u2006​\u2007\u2008​\u2009\u200a​\u2028\u2029​​\u202f\u205f​\u3000].

For example, /\s\w*/ matches ' bar' in "foo bar."

\S

Matches a single character other than white space. Equivalent to [^ \f\n\r\t\v​\u00a0\u1680​\u180e\u2000​\u2001\u2002​\u2003\u2004​\u2005\u2006​\u2007\u2008​\u2009\u200a​\u2028\u2029​\u202f\u205f​\u3000].

For example, /\S\w*/ matches 'foo' in "foo bar."

\t Matches a tab (U+0009).
\v Matches a vertical tab (U+000B).
\w

Matches any alphanumeric character including the underscore. Equivalent to [A-Za-z0-9_].

For example, /\w/ matches 'a' in "apple," '5' in "$5.28," and '3' in "3D."

\W

Matches any non-word character. Equivalent to [^A-Za-z0-9_].

For example, /\W/ or /[^A-Za-z0-9_]/ matches '%' in "50%."

\n

Where n is a positive integer, a back reference to the last substring matching the n parenthetical in the regular expression (counting left parentheses).

For example, /apple(,)\sorange\1/ matches 'apple, orange,' in "apple, orange, cherry, peach."

\0 Matches a NULL (U+0000) character. Do not follow this with another digit, because \0<digits> is an octal escape sequence.
\xhh Matches the character with the code hh (two hexadecimal digits)
\uhhhh Matches the character with the code hhhh (four hexadecimal digits).

Escaping user input to be treated as a literal string within a regular expression can be accomplished by simple replacement:

function escapeRegExp(string){
return string.replace(/([.*+?^=!:${}()|\[\]\/\\])/g, "\\$1");
}

使用括號

Parentheses around any part of the regular expression pattern cause that part of the matched substring to be remembered. Once remembered, the substring can be recalled for other use, as described in Using Parenthesized Substring Matches.

For example, the pattern /Chapter (\d+)\.\d*/ illustrates additional escaped and special characters and indicates that part of the pattern should be remembered. It matches precisely the characters 'Chapter ' followed by one or more numeric characters (\d means any numeric character and + means 1 or more times), followed by a decimal point (which in itself is a special character; preceding the decimal point with \ means the pattern must look for the literal character '.'), followed by any numeric character 0 or more times (\d means numeric character, * means 0 or more times). In addition, parentheses are used to remember the first matched numeric characters.

This pattern is found in "Open Chapter 4.3, paragraph 6" and '4' is remembered. The pattern is not found in "Chapter 3 and 4", because that string does not have a period after the '3'.

To match a substring without causing the matched part to be remembered, within the parentheses preface the pattern with ?:. For example, (?:\d+) matches one or more numeric characters but does not remember the matched characters.

運用正規表達式

Regular expressions are used with the RegExp methods test and exec and with the String methods match, replace, search, and split. These methods are explained in detail in the JavaScript Reference.

Table 4.2 Methods that use regular expressions
Method Description
exec A RegExp method that executes a search for a match in a string. It returns an array of information.
test A RegExp method that tests for a match in a string. It returns true or false.
match A String method that executes a search for a match in a string. It returns an array of information or null on a mismatch.
search A String method that tests for a match in a string. It returns the index of the match, or -1 if the search fails.
replace A String method that executes a search for a match in a string, and replaces the matched substring with a replacement substring.
split A String method that uses a regular expression or a fixed string to break a string into an array of substrings.

When you want to know whether a pattern is found in a string, use the test or search method; for more information (but slower execution) use the exec or match methods. If you use exec or match and if the match succeeds, these methods return an array and update properties of the associated regular expression object and also of the predefined regular expression object, RegExp. If the match fails, the exec method returns null (which coerces to false).

In the following example, the script uses the exec method to find a match in a string.

var myRe = /d(b+)d/g;
var myArray = myRe.exec("cdbbdbsbz");

If you do not need to access the properties of the regular expression, an alternative way of creating myArray is with this script:

var myArray = /d(b+)d/g.exec("cdbbdbsbz");

If you want to construct the regular expression from a string, yet another alternative is this script:

var myRe = new RegExp("d(b+)d", "g");
var myArray = myRe.exec("cdbbdbsbz");

With these scripts, the match succeeds and returns the array and updates the properties shown in the following table.

Table 4.3 Results of regular expression execution.
物件 Property or index 說明 範例
myArray   The matched string and all remembered substrings. ["dbbd", "bb"]
index The 0-based index of the match in the input string. 1
input The original string. "cdbbdbsbz"
[0] The last matched characters. "dbbd"
myRe lastIndex The index at which to start the next match. (This property is set only if the regular expression uses the g option, described in Advanced Searching With Flags.) 5
source The text of the pattern. Updated at the time that the regular expression is created, not executed. "d(b+)d"

As shown in the second form of this example, you can use a regular expression created with an object initializer without assigning it to a variable. If you do, however, every occurrence is a new regular expression. For this reason, if you use this form without assigning it to a variable, you cannot subsequently access the properties of that regular expression. For example, assume you have this script:

var myRe = /d(b+)d/g;
var myArray = myRe.exec("cdbbdbsbz");
console.log("The value of lastIndex is " + myRe.lastIndex);

This script displays:

The value of lastIndex is 5

However, if you have this script:

var myArray = /d(b+)d/g.exec("cdbbdbsbz");
console.log("The value of lastIndex is " + /d(b+)d/g.lastIndex);

It displays:

The value of lastIndex is 0

The occurrences of /d(b+)d/g in the two statements are different regular expression objects and hence have different values for their lastIndex property. If you need to access the properties of a regular expression created with an object initializer, you should first assign it to a variable.

Using Parenthesized Substring Matches

Including parentheses in a regular expression pattern causes the corresponding submatch to be remembered. For example, /a(b)c/ matches the characters 'abc' and remembers 'b'. To recall these parenthesized substring matches, use the Array elements [1], ..., [n].

The number of possible parenthesized substrings is unlimited. The returned array holds all that were found. The following examples illustrate how to use parenthesized substring matches.

Example 1

The following script uses the replace() method to switch the words in the string. For the replacement text, the script uses the $1 and $2 in the replacement to denote the first and second parenthesized substring matches.

var re = /(\w+)\s(\w+)/;
var str = "John Smith";
var newstr = str.replace(re, "$2, $1");
console.log(newstr);

This prints "Smith, John".

Advanced Searching With Flags

Regular expressions have four optional flags that allow for global and case insensitive searching. These flags can be used separately or together in any order, and are included as part of the regular expression.

Table 4.4 Regular expression flags.
Flag Description
g Global search.
i Case-insensitive search.
m Multi-line search.
y Perform a "sticky" search that matches starting at the current position in the target string.

Firefox 3 note

Support for the y flag was added in Firefox 3. The y flag fails if the match doesn't succeed at the current position in the target string.

To include a flag with the regular expression, use this syntax:

var re = /pattern/flags;

or

var re = new RegExp("pattern", "flags");

Note that the flags are an integral part of a regular expression. They cannot be added or removed later.

For example, re = /\w+\s/g creates a regular expression that looks for one or more characters followed by a space, and it looks for this combination throughout the string.

var re = /\w+\s/g;
var str = "fee fi fo fum";
var myArray = str.match(re);
console.log(myArray);

This displays ["fee ", "fi ", "fo "]. In this example, you could replace the line:

var re = /\w+\s/g;

with:

var re = new RegExp("\\w+\\s", "g");

and get the same result.

The m flag is used to specify that a multiline input string should be treated as multiple lines. If the m flag is used, ^ and $ match at the start or end of any line within the input string instead of the start or end of the entire string.

例子

The following examples show some uses of regular expressions.

Changing the Order in an Input String

The following example illustrates the formation of regular expressions and the use of string.split() and string.replace(). It cleans a roughly formatted input string containing names (first name first) separated by blanks, tabs and exactly one semicolon. Finally, it reverses the name order (last name first) and sorts the list.

// The name string contains multiple spaces and tabs,
// and may have multiple spaces between first and last names.
var names = "Harry Trump ;Fred Barney; Helen Rigby ; Bill Abel ; Chris Hand ";

var output = ["---------- Original String\n", names + "\n"];

// Prepare two regular expression patterns and array storage.
// Split the string into array elements.

// pattern: possible white space then semicolon then possible white space
var pattern = /\s*;\s*/;

// Break the string into pieces separated by the pattern above and
// store the pieces in an array called nameList
var nameList = names.split(pattern);

// new pattern: one or more characters then spaces then characters.
// Use parentheses to "memorize" portions of the pattern.
// The memorized portions are referred to later.
pattern = /(\w+)\s+(\w+)/;

// New array for holding names being processed.
var bySurnameList = [];

// Display the name array and populate the new array
// with comma-separated names, last first.
//
// The replace method removes anything matching the pattern
// and replaces it with the memorized string—second memorized portion
// followed by comma space followed by first memorized portion.
//
// The variables $1 and $2 refer to the portions
// memorized while matching the pattern.

output.push("---------- After Split by Regular Expression");

var i, len;
for (i = 0, len = nameList.length; i < len; i++){
output.push(nameList[i]);
bySurnameList[i] = nameList[i].replace(pattern, "$2, $1");
}

// Display the new array.
output.push("---------- Names Reversed");
for (i = 0, len = bySurnameList.length; i < len; i++){
output.push(bySurnameList[i]);
}

// Sort by last name, then display the sorted array.
bySurnameList.sort();
output.push("---------- Sorted");
for (i = 0, len = bySurnameList.length; i < len; i++){
output.push(bySurnameList[i]);
}

output.push("---------- End");

console.log(output.join("\n"));

使用特殊字元驗證輸入

In the following example, the user is expected to enter a phone number. When the user presses the "Check" button, the script checks the validity of the number. If the number is valid (matches the character sequence specified by the regular expression), the script shows a message thanking the user and confirming the number. If the number is invalid, the script informs the user that the phone number is not valid.

The regular expression looks for zero or one open parenthesis \(?, followed by three digits \d{3}, followed by zero or one close parenthesis \)?, followed by one dash, forward slash, or decimal point and when found, remember the character ([-\/\.]), followed by three digits \d{3}, followed by the remembered match of a dash, forward slash, or decimal point \1, followed by four digits \d{4}.

The Change event activated when the user presses Enter sets the value of RegExp.input.

<!DOCTYPE html>
<html>  
<head>  
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">  
<meta http-equiv="Content-Script-Type" content="text/javascript">  
<script type="text/javascript">  
var re = /\(?\d{3}\)?([-\/\.])\d{3}\1\d{4}/;  
function testInfo(phoneInput){  
var OK = re.exec(phoneInput.value);  
if (!OK)  
window.alert(RegExp.input + " isn't a phone number with area code!");  
else
window.alert("Thanks, your phone number is " + OK[0]);  
}  
</script>  
</head>  
<body>  
<p>Enter your phone number (with area code) and then click "Check".
<br>The expected format is like ###-###-####.</p>
<form action="#">  
<input id="phone"><button onclick="testInfo(document.getElementById('phone'));">Check</button>
</form>  
</body>  
</html>

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