Pattern (Java Platform SE 8 )

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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.

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.

Construct
Matches

 
Characters
x
The character x
\\
The backslash character
\0n
The character with octal value 0n
(0 <= n <= 7)
\0nn
The character with octal value 0nn
(0 <= n <= 7)
\0mnn
The character with octal value 0mnn
(0 <= m <= 3,
0 <= n <= 7)
\xhh
The character with hexadecimal value 0xhh
\uhhhh
The character with hexadecimal value 0xhhhh
\x{h…h}
The character with hexadecimal value 0xh…h
(Character.MIN_CODE_POINT
 <= 0xh…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')
\cx
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]
\h
A horizontal whitespace character:
[ \t\xA0\u1680\u180e\u2000-\u200a\u202f\u205f\u3000]
\H
A non-horizontal whitespace character: [^\h]
\s
A whitespace character: [ \t\n\x0B\f\r]
\S
A non-whitespace character: [^\s]
\v
A vertical whitespace character: [\n\x0B\f\r\x85\u2028\u2029]
\V
A non-vertical whitespace character: [^\v]
\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 final
terminator, if any
\z
The end of the input
 
Linebreak matcher
\R
Any Unicode linebreak sequence, is equivalent to
\u000D\u000A|[\u000A\u000B\u000C\u000D\u0085\u2028\u2029]

 
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 nth
capturing group matched
\k<name>
Whatever the
named-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

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 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.

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.

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.

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 Expression, plus RL2.1
Canonical Equivalents.

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 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 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 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}\p{IsJoin_Control}]
\W
A non-word character: [^\w]

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 javamethodname.

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:

  • Predefined character classes (Unicode character)

    \X    Match Unicode

    extended grapheme cluster

  • The backreference constructs, \g{n} for
    the nthcapturing group and
    \g{name} for
    named-capturing group.

  • The named character construct, \N{name}
    for a Unicode character by its name.

  • 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, 3nd Edition, Jeffrey E. F. Friedl,
O’Reilly and Associates, 2006.

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