Pattern (Java Platform SE 7 ) (original) (raw)

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

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

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

*

Construct Matches
Characters
x The character x
\\ The backslash character
\0_n_ The character with octal value 0_n_ (0 <= n <= 7)
\0_nn_ The character with octal value 0_nn_ (0 <= n <= 7)
\0_mnn_ The character with octal value 0_mnn_ (0 <= m <= 3, 0 <= n <= 7)
\x_hh_ The character with hexadecimal value 0x_hh_
\u_hhhh_ The character with hexadecimal value 0x_hhhh_
\x_{h...h}_ The character with hexadecimal value 0x_h...h_ (

Character.MIN_CODE_POINT <= 0x_h...h_ <= Character.MAX_CODE_POINT)

\t

The tab character ('\u0009')

\n

The newline (line feed) character ('\u000A')

\r

The carriage-return character ('\u000D')

\f

The form-feed character ('\u000C')

\a

The alert (bell) character ('\u0007')

\e

The escape character ('\u001B')

\c_x_

The control character corresponding to x

Character classes

[abc]

a, b, or c (simple class)

[^abc]

Any character except a, b, or c (negation)

[a-zA-Z]

a through z or A through Z, inclusive (range)

[a-d[m-p]]

a through d, or m through p: [a-dm-p] (union)

[a-z&&[def]]

d, e, or f (intersection)

[a-z&&[^bc]]

a through z, except for b and c: [ad-z] (subtraction)

[a-z&&[^m-p]]

a through z, and not m through p: [a-lq-z](subtraction)

Predefined character classes

.

Any character (may or may not match line terminators)

\d

A digit: [0-9]

\D

A non-digit: [^0-9]

\s

A whitespace character: [ \t\n\x0B\f\r]

\S

A non-whitespace character: [^\s]

\w

A word character: [a-zA-Z_0-9]

\W

A non-word character: [^\w]

POSIX character classes (US-ASCII only)

\p{Lower}

A lower-case alphabetic character: [a-z]

\p{Upper}

An upper-case alphabetic character:[A-Z]

\p{ASCII}

All ASCII:[\x00-\x7F]

\p{Alpha}

An alphabetic character:[\p{Lower}\p{Upper}]

\p{Digit}

A decimal digit: [0-9]

\p{Alnum}

An alphanumeric character:[\p{Alpha}\p{Digit}]

\p{Punct}

Punctuation: One of !"#$%&'()*+,-./:;<=>?@[\]^_`{|}~

\p{Graph}

A visible character: [\p{Alnum}\p{Punct}]

\p{Print}

A printable character: [\p{Graph}\x20]

\p{Blank}

A space or a tab: [ \t]

\p{Cntrl}

A control character: [\x00-\x1F\x7F]

\p{XDigit}

A hexadecimal digit: [0-9a-fA-F]

\p{Space}

A whitespace character: [ \t\n\x0B\f\r]

java.lang.Character classes (simple java character type)

\p{javaLowerCase}

Equivalent to java.lang.Character.isLowerCase()

\p{javaUpperCase}

Equivalent to java.lang.Character.isUpperCase()

\p{javaWhitespace}

Equivalent to java.lang.Character.isWhitespace()

\p{javaMirrored}

Equivalent to java.lang.Character.isMirrored()

Classes for Unicode scripts, blocks, categories and binary properties

\p{IsLatin}

A Latin script character (script)

\p{InGreek}

A character in the Greek block (block)

\p{Lu}

An uppercase letter (category)

\p{IsAlphabetic}

An alphabetic character (binary property)

\p{Sc}

A currency symbol

\P{InGreek}

Any character except one in the Greek block (negation)

[\p{L}&&[^\p{Lu}]]

Any letter except an uppercase letter (subtraction)

Boundary matchers

^

The beginning of a line

$

The end of a line

\b

A word boundary

\B

A non-word boundary

\A

The beginning of the input

\G

The end of the previous match

\Z

The end of the input but for the finalterminator, if any

\z

The end of the input

Greedy quantifiers

X?

X, once or not at all

X*

X, zero or more times

X+

X, one or more times

X{n}

X, exactly n times

X{n,}

X, at least n times

X{n,m}

X, at least n but not more than m times

Reluctant quantifiers

X??

X, once or not at all

X*?

X, zero or more times

X+?

X, one or more times

X{n}?

X, exactly n times

X{n,}?

X, at least n times

X{n,m}?

X, at least n but not more than m times

Possessive quantifiers

X?+

X, once or not at all

X*+

X, zero or more times

X++

X, one or more times

X{n}+

X, exactly n times

X{n,}+

X, at least n times

X{n,m}+

X, at least n but not more than m times

Logical operators

XY

X followed by Y

X|Y

Either X or Y

(X)

X, as a capturing group

Back references

\n

Whatever the _n_th capturing group matched

\k<_name_>

Whatever thenamed-capturing group "name" matched

Quotation

\

Nothing, but quotes the following character

\Q

Nothing, but quotes all characters until \E

\E

Nothing, but ends quoting started by \Q

Special constructs (named-capturing and non-capturing)

(?<name>X)

X, as a named-capturing group

(?:X)

X, as a non-capturing group

(?idmsuxU-idmsuxU)

Nothing, but turns match flags i d m s u x U on - off

(?idmsux-idmsux:X)

X, as a non-capturing group with the given flags i d m s u x on - off

(?=X)

X, via zero-width positive lookahead

(?!X)

X, via zero-width negative lookahead

(?<=X)

X, via zero-width positive lookbehind

(?<!X)

X, via zero-width negative lookbehind

(?>X)

X, as an independent, non-capturing group

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 byThe 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:

1 Literal escape \x
2 Grouping [...]
3 Range a-z
4 Union [a-e][i-u]
5 Intersection [a-z&&[aeiou]]

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:

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.

A named-capturing group is still numbered as described inGroup 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 Expression, plus RL2.1 Canonical Equivalents.

Unicode escape sequences such as \u2014 in Java source code are processed as described in section 3.3 ofThe 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 in a regular-expression 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 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 inIsHiragana, 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 byUnicodeScript.forName.

Blocks are specified with the prefix In, as inInMongolian, 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 byUnicodeBlock.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 formgc) as in general_category=Lu or gc=Lu.

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

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

Predefined Character classes and POSIX character classes are in conformance with the recommendation of Annex C: Compatibility Properties of Unicode Regular Expression, when UNICODE_CHARACTER_CLASS flag is specified.

Classes Matches
\p{Lower} A lowercase character:\p{IsLowercase}
\p{Upper} An uppercase character:\p{IsUppercase}
\p{ASCII} All ASCII:[\x00-\x7F]
\p{Alpha} An alphabetic character:\p{IsAlphabetic}
\p{Digit} A decimal digit character:p{IsDigit}
\p{Alnum} An alphanumeric character:[\p{IsAlphabetic}\p{IsDigit}]
\p{Punct} A punctuation character:p{IsPunctuation}
\p{Graph} A visible character: [^\p{IsWhite_Space}\p{gc=Cc}\p{gc=Cs}\p{gc=Cn}]
\p{Print} A printable character: [\p{Graph}\p{Blank}&&[^\p{Cntrl}]]
\p{Blank} A space or a tab: [\p{IsWhite_Space}&&[^\p{gc=Zl}\p{gc=Zp}\x0a\x0b\x0c\x0d\x85]]
\p{Cntrl} A control character: \p{gc=Cc}
\p{XDigit} A hexadecimal digit: [\p{gc=Nd}\p{IsHex_Digit}]
\p{Space} A whitespace character:\p{IsWhite_Space}
\d A digit: \p{IsDigit}
\D A non-digit: [^\d]
\s A whitespace character: \p{IsWhite_Space}
\S A non-whitespace character: [^\s]
\w A word character: [\p{Alpha}\p{gc=Mn}\p{gc=Me}\p{gc=Mc}\p{Digit}\p{gc=Pc}]
\W A non-word character: [^\w]

Categories that behave like the java.lang.Character boolean is_methodname_ 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:

Constructs supported by this class but not by Perl:

Notable differences from Perl:

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