Java 中的正则表达式

正则表达式（Regular Expression，简称regex或regexp）是一种用于匹配字符串的强大工具。 它通过使用特定的语法规则来描述字符串的模式，从而可以在文本中进行搜索、替换和验证等操作。

正则表达式语法结构

字符 (Characters)

字符集 (Character classes)

预定义字符集

POSIX 字符集 (仅限 US-ASCII)

java.lang.Character 字符集

简单 Java 字符类型

Unicode Scripts, blocks, categories, binary properties 字符集

Unicode 脚本、块、类别和二进制属性都是用 \p 和 \P 结构编写（和 Perl 中一样），可以在字符集内部和外部使用。

如果输入具有属性 prop，则 \p{prop} 匹配，\P{prop} 不匹配。

• 脚本（Scripts）

  可以用前缀 Is 来指定，例如 IsHiragana，或者使用关键字 script（或其缩写形式 sc）。例如：script=Hiragana 或 sc=Hiragana。

  有效的 Script 命名参考 UnicodeScript.forName.

• 块（Blocks）

  可以用前缀 In 来指定，例如 InMongolian，或者使用关键字 block（或其缩写形式 blk）。例如：block=Mongolian 或 blk=Mongolian。

  有效的 Block 命名参考 UnicodeBlock.forName.

• 类别（Categories）

  可以用前缀 Is 来指定。\p{L} 和 \p{IsL} 都表示 Unicode 字母。或者使用关键字 general_category（或其缩写形式 gc）。例如：general_category=Lu 或 gc=Lu。

• 二进制属性（Binary Properties）

  以 Is 为前缀指定，例如：IsAlphabetic。java.util.regex.Pattern 支持的二进制属性有：

  • Alphabetic
  • Ideographic
  • Letter
  • Lowercase
  • Uppercase
  • Titlecase
  • Punctuation
  • White_Space
  • Digit
  • Hex_Digit
  • Join_Control
  • Noncharacter_Code_Point
  • Assigned

匹配器

边界匹配器

换行匹配器

Unicode 扩展字形匹配器

字形是构成文字的最小单元，通常对应于一个可视字符，但它可能由多个 Unicode 字符组合而成（例如，表情符号、附加符号、复合字符等）。 Unicode 扩展字形匹配器的作用是识别并处理这些复杂的字符组合。

逻辑运算符

量词

贪婪量词

贪婪量词（Greedy Quantifiers）尽可能多地匹配字符，直到整个模式匹配成功。如果后面的模式不能匹配，它会回溯并增加匹配的字符数。

懒惰量词

懒惰量词（也称不情愿量词，Reluctant Quantifiers）尽量少地匹配字符，尽可能减少匹配的字符数，直到整个模式匹配成功。

语法：在贪婪量词后面接 ?。

占有量词

占有量词（Possessive Quantifiers）与贪婪量词类似，但不会回溯。匹配后无法撤销，如果无法匹配后续内容，则匹配失败。

语法：在贪婪量词后面接 +。

修饰符

各个修饰符简要说明：

示例，配置 Case insensitive 修饰符正则表达式写法：

• 启用修饰符 (?i)
• 禁用修饰符 (?-i)

分组与捕获

以 (? 开头的组要么是纯粹的非捕获组，不会捕获文本并且不计入组总数，要么是命名捕获组

分组组号

分组组号是通过左括号从左到右来编号的。例如，正则表达式 ((A)(B(C)))，有 4 个这样的组：

1. ((A)(B(C)))
2. (A)
3. (B(C))
4. (C)

0 组始终代表表达式捕获的整个字符串。

在匹配过程中，与该组匹配的输入序列的每个子序列都会被保存。捕获的子序列可以通过反向引用在表达式中使用，也可以在匹配操作完成后从匹配器中检索。

分组名

也可以为分组命名，即命名捕获组。后续也可以通过分组名进行反向引用，在匹配操作完成后也可以通过分组名从匹配器中获取。

使用命名捕获组后仍会为组进行编号。

组名由以下字符组成：（第一个字符必须是字母）

• 数字 0-9 (\u0030 - \u0039)
• 大写字母 A-Z (\u0041 - \u005a)
• 小写字母 a-z (\u0061 - \u007a)

反向引用

捕获组与量词

当对捕获组使用量词时，捕获组会覆盖之前的捕获值，只保留最后一次迭代的捕获。如果二次捕获时失败则捕获组内是上一次捕获的结果（如果有）。

示例：

使用正则表达式 (a(b)?)+ 匹配字符串 "aba"。最终得到的捕获组为：

• 捕获组 1 "ab"
• 捕获组 2 "b"

TIP: 可以在重复量词外部使用捕获组来捕获所有的匹配。

非捕获组与独立非捕获组对比

非捕获组 (?:X) 与 独立非捕获组(?>X) 的区别：

零宽断言

转义

示例说明：

当需要匹配一个正则表达式元字符的字符串，如 a.b*c，如果不转义，就会遇到问题，因为正则表达式会试图将 . 和 * 解释为元字符。 这时可以通过如下方式进行匹配：

1. 使用转义字符 \

   regex

   1

   a\.b\*c

2. 使用 \Q 和 \E

   regex

   1

   \Qa.b*c\E

附录: 行终止符

原文参考：Line terminators

常用正则表达式

注意：正则表达式并未转义。比如 \d 在 Java 中应使用 \\d。

IPv4 地址

1

(?:(?:2(?:5[0-5]|[0-4]\d))|[0-1]?\d{1,2})(?:\.(?:(?:2(?:5[0-5]|[0-4]\d))|[0-1]?\d{1,2})){3}

中文

1

[\u4e00-\u9fa5]

email

1

[\w-\.]+@(?:[A-Za-z0-9-]+\.)+[a-z]+

附录: Java API

原文地址（Java 17）

A compiled representation of a regular expression.

A regular expression, specified as a string, must first be compiled into an instance of this class. The resulting pattern can then be used to create a Matcher object that can match arbitrary character sequences against the regular expression. All of the state involved in performing a match resides in the matcher, so many matchers can share the same pattern.

A typical invocation sequence is thus

1
2
3

Pattern p = Pattern.compile("a*b");
Matcher m = p.matcher("aaaaab");
boolean b = m.matches();

A matches method is defined by this class as a convenience for when a regular expression is used just once. This method compiles an expression and matches an input sequence against it in a single invocation. The statement

1

boolean b = Pattern.matches("a*b", "aaaaab");

is equivalent to the three statements above, though for repeated matches it is less efficient since it does not allow the compiled pattern to be reused.

Instances of this class are immutable and are safe for use by multiple concurrent threads. Instances of the Matcher class are not safe for such use.

Summary of regular-expression constructs

Characters

Character classes

Predefined character classes

POSIX character classes (US-ASCII only)

java.lang.Character classes

(simple java character type)

Classes for Unicode scripts, blocks, categories and binary properties

Boundary matchers

Linebreak matcher

Unicode Extended Grapheme matcher

Greedy quantifiers

Reluctant quantifiers

Possessive quantifiers

Logical operators

Back references

Quotation

Special constructs (named-capturing and non-capturing)

Backslashes, escapes, and quoting

The backslash character ('\') serves to introduce escaped constructs, as defined in the table above, as well as to quote characters that otherwise would be interpreted as unescaped constructs. Thus the expression \\ matches a single backslash and \{ matches a left brace.

It is an error to use a backslash prior to any alphabetic character that does not denote an escaped construct; these are reserved for future extensions to the regular-expression language. A backslash may be used prior to a non-alphabetic character regardless of whether that character is part of an unescaped construct.

Backslashes within string literals in Java source code are interpreted as required by The Java Language Specification as either Unicode escapes (section 3.3) or other character escapes (section 3.10.6) It is therefore necessary to double backslashes in string literals that represent regular expressions to protect them from interpretation by the Java bytecode compiler. The string literal "\b", for example, matches a single backspace character when interpreted as a regular expression, while "\\b" matches a word boundary. The string literal "\(hello\)" is illegal and leads to a compile-time error; in order to match the string (hello) the string literal "\\(hello\\)" must be used.

Character Classes

Character classes may appear within other character classes, and may be composed by the union operator (implicit) and the intersection operator (&&). The union operator denotes a class that contains every character that is in at least one of its operand classes. The intersection operator denotes a class that contains every character that is in both of its operand classes.

The precedence of character-class operators is as follows, from highest to lowest:

Note that a different set of metacharacters are in effect inside a character class than outside a character class. For instance, the regular expression . loses its special meaning inside a character class, while the expression - becomes a range forming metacharacter.

Line terminators

A line terminator is a one- or two-character sequence that marks the end of a line of the input character sequence. The following are recognized as line terminators:

• A newline (line feed) character ('\n'),
• A carriage-return character followed immediately by a newline character ("\r\n"),
• A standalone carriage-return character ('\r'),
• A next-line character ('\u0085'),
• A line-separator character ('\u2028'), or
• A paragraph-separator character ('\u2029').

If UNIX_LINES mode is activated, then the only line terminators recognized are newline characters.

The regular expression . matches any character except a line terminator unless the DOTALL flag is specified.

By default, the regular expressions ^ and $ ignore line terminators and only match at the beginning and the end, respectively, of the entire input sequence. If MULTILINE mode is activated then ^ matches at the beginning of input and after any line terminator except at the end of input. When in MULTILINE mode $ matches just before a line terminator or the end of the input sequence.

Groups and capturing

Group number

Capturing groups are numbered by counting their opening parentheses from left to right. In the expression ((A)(B(C))), for example, there are four such groups:

1. ((A)(B(C)))
2. (A)
3. (B(C))
4. (C)

Group zero always stands for the entire expression.

Capturing groups are so named because, during a match, each subsequence of the input sequence that matches such a group is saved. The captured subsequence may be used later in the expression, via a back reference, and may also be retrieved from the matcher once the match operation is complete.

Group name

A capturing group can also be assigned a “name”, a named-capturing group, and then be back-referenced later by the “name”. Group names are composed of the following characters. The first character must be a letter.

• The uppercase letters 'A' through 'Z' ('\u0041' through '\u005a'),
• The lowercase letters 'a' through 'z' ('\u0061' through '\u007a'),
• The digits '0' through '9' ('\u0030' through '\u0039'),

A named-capturing group is still numbered as described in Group number.

The captured input associated with a group is always the subsequence that the group most recently matched. If a group is evaluated a second time because of quantification then its previously-captured value, if any, will be retained if the second evaluation fails. Matching the string "aba" against the expression (a(b)?)+, for example, leaves group two set to "b". All captured input is discarded at the beginning of each match.

Groups beginning with (? are either pure, non-capturing groups that do not capture text and do not count towards the group total, or named-capturing group.

Unicode support

This class is in conformance with Level 1 of Unicode Technical Standard #18: Unicode Regular Expressions, plus RL2.1 Canonical Equivalents and RL2.2 Extended Grapheme Clusters.

Unicode escape sequences such as \u2014 in Java source code are processed as described in section 3.3 of The Java Language Specification. Such escape sequences are also implemented directly by the regular-expression parser so that Unicode escapes can be used in expressions that are read from files or from the keyboard. Thus the strings "\u2014" and "\\u2014", while not equal, compile into the same pattern, which matches the character with hexadecimal value 0x2014.

A Unicode character can also be represented by using its Hex notation (hexadecimal code point value) directly as described in construct \x{...}, for example a supplementary character U+2011F can be specified as \x{2011F}, instead of two consecutive Unicode escape sequences of the surrogate pair \uD840``\uDD1F.

Unicode character names are supported by the named character construct \N{…}, for example, \N{WHITE SMILING FACE} specifies character \u263A. The character names supported by this class are the valid Unicode character names matched by Character.codePointOf(name).

Unicode extended grapheme clusters are supported by the grapheme cluster matcher \X and the corresponding boundary matcher \b{g}.

Unicode scripts, blocks, categories and binary properties are written with the \p and \P constructs as in Perl. \p{prop} matches if the input has the property prop, while \P{prop} does not match if the input has that property.

Scripts, blocks, categories and binary properties can be used both inside and outside of a character class.

Scripts are specified either with the prefix Is, as in IsHiragana, or by using the script keyword (or its short form sc) as in script=Hiragana or sc=Hiragana.

The script names supported by Pattern are the valid script names accepted and defined by UnicodeScript.forName.

Blocks are specified with the prefix In, as in InMongolian, or by using the keyword block (or its short form blk) as in block=Mongolian or blk=Mongolian.

The block names supported by Pattern are the valid block names accepted and defined by UnicodeBlock.forName.

Categories may be specified with the optional prefix Is: Both \p{L} and \p{IsL} denote the category of Unicode letters. Same as scripts and blocks, categories can also be specified by using the keyword general_category (or its short form gc) as in general_category=Lu or gc=Lu.

The supported categories are those of The Unicode Standard in the version specified by the Character class. The category names are those defined in the Standard, both normative and informative.

Binary properties are specified with the prefix Is, as in IsAlphabetic. The supported binary properties by Pattern are

• Alphabetic
• Ideographic
• Letter
• Lowercase
• Uppercase
• Titlecase
• Punctuation
• Control
• White_Space
• Digit
• Hex_Digit
• Join_Control
• Noncharacter_Code_Point
• Assigned

The following Predefined Character classes and POSIX character classes are in conformance with the recommendation of Annex C: Compatibility Properties of Unicode Technical Standard #18: Unicode Regular Expressions, when UNICODE_CHARACTER_CLASS flag is specified.

Categories that behave like the java.lang.Character boolean ismethodname methods (except for the deprecated ones) are available through the same \p{prop} syntax where the specified property has the name java*methodname*.

Comparison to Perl 5

The Pattern engine performs traditional NFA-based matching with ordered alternation as occurs in Perl 5.

Perl constructs not supported by this class:

• The backreference constructs, \g{n} for the nthcapturing group and \g{name} for named-capturing group.
• The conditional constructs (?(condition)X) and (?(condition)X|Y),
• The embedded code constructs (?{code}) and (??{code}),
• The embedded comment syntax (?#comment), and
• The preprocessing operations \l \u, \L, and \U.

Constructs supported by this class but not by Perl:

• Character-class union and intersection as described above.

Notable differences from Perl:

• In Perl, \1 through \9 are always interpreted as back references; a backslash-escaped number greater than 9 is treated as a back reference if at least that many subexpressions exist, otherwise it is interpreted, if possible, as an octal escape. In this class octal escapes must always begin with a zero. In this class, \1 through \9 are always interpreted as back references, and a larger number is accepted as a back reference if at least that many subexpressions exist at that point in the regular expression, otherwise the parser will drop digits until the number is smaller or equal to the existing number of groups or it is one digit.
• Perl uses the g flag to request a match that resumes where the last match left off. This functionality is provided implicitly by the Matcher class: Repeated invocations of the find method will resume where the last match left off, unless the matcher is reset.
• In Perl, embedded flags at the top level of an expression affect the whole expression. In this class, embedded flags always take effect at the point at which they appear, whether they are at the top level or within a group; in the latter case, flags are restored at the end of the group just as in Perl.

For a more precise description of the behavior of regular expression constructs, please see Mastering Regular Expressions, 3rd Edition, Jeffrey E. F. Friedl, O’Reilly and Associates, 2006.