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JavaTM 2 Platform Std. Ed. v1.4.2

java.util.regex
Class Pattern

java.lang.Object
  java.util.regex.Pattern

All Implemented Interfaces:: Serializable

public final class Pattern
extends Object
implements Serializable

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
`\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}]`
`\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]`

Classes for Unicode blocks and categories
`\p{InGreek}`	A character in the Greek block (simple block)
`\p{Lu}`	An uppercase letter (simple category)
`\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

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

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 (non-capturing)
`(?:`X`)`	X, as a non-capturing group
`(?idmsux-idmsux)`	Nothing, but turns match flags on - off
`(?idmsux-idmsux:`X`)`	X, as a non-capturing group with the given flags 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 by the Java Language Specification as either Unicode escapes or other character escapes. 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:

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

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.

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 pure, non-capturing groups that do not capture text and do not count towards the group total.

Unicode support

This class follows Unicode Technical Report #18: Unicode Regular Expression Guidelines, implementing its second level of support though with a slightly different concrete syntax.

Unicode escape sequences such as \u2014 in Java source code are processed as described in ?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.

Unicode blocks and categories 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. Blocks are specified with the prefix In, as in InMongolian. Categories may be specified with the optional prefix Is: Both \p{L} and \p{IsL} denote the category of Unicode letters. Blocks and categories can be used both inside and outside of a character class.

The supported blocks and categories are those of The Unicode Standard, Version 3.0. The block names are those defined in Chapter 14 and in the file Blocks-3.txt of the Unicode Character Database except that the spaces are removed; "Basic Latin", for example, becomes "BasicLatin". The category names are those defined in table 4-5 of the Standard (p. 88), both normative and informative.

Comparison to Perl 5

Perl constructs not supported by this class:

The conditional constructs (?{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:

Possessive quantifiers, which greedily match as much as they can and do not back off, even when doing so would allow the overall match to succeed.
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.
Perl is forgiving about malformed matching constructs, as in the expression *a, as well as dangling brackets, as in the expression abc], and treats them as literals. This class also accepts dangling brackets but is strict about dangling metacharacters like +, ? and *, and will throw a PatternSyntaxException if it encounters them.

For a more precise description of the behavior of regular expression constructs, please see Mastering Regular Expressions, 2nd Edition, Jeffrey E. F. Friedl, O'Reilly and Associates, 2002.

Since:: 1.4
See Also:: String.split(String, int), String.split(String), Serialized Form

Field Summary

static int CANON_EQ
          Enables canonical equivalence.

static int CASE_INSENSITIVE
          Enables case-insensitive matching.

static int COMMENTS
          Permits whitespace and comments in pattern.

static int DOTALL
          Enables dotall mode.

static int MULTILINE
          Enables multiline mode.

static int UNICODE_CASE
          Enables Unicode-aware case folding.

static int UNIX_LINES
          Enables Unix lines mode.

Method Summary

static Pattern compile(String regex)
          Compiles the given regular expression into a pattern.

static Pattern compile(String regex, int flags)
          Compiles the given regular expression into a pattern with the given flags.

int flags()
          Returns this pattern's match flags.

Matcher matcher(CharSequence input)
          Creates a matcher that will match the given input against this pattern.

static boolean matches(String regex, CharSequence input)
          Compiles the given regular expression and attempts to match the given input against it.

String pattern()
          Returns the regular expression from which this pattern was compiled.

String[] split(CharSequence input)
          Splits the given input sequence around matches of this pattern.

String[] split(CharSequence input, int limit)
          Splits the given input sequence around matches of this pattern.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

UNIX_LINES

public static final int UNIX_LINES

Enables Unix lines mode.

In this mode, only the '\n' line terminator is recognized in the behavior of ., ^, and $.

Unix lines mode can also be enabled via the embedded flag expression (?d).

See Also:: Constant Field Values

CASE_INSENSITIVE

public static final int CASE_INSENSITIVE

Enables case-insensitive matching.

By default, case-insensitive matching assumes that only characters in the US-ASCII charset are being matched. Unicode-aware case-insensitive matching can be enabled by specifying the UNICODE_CASE flag in conjunction with this flag.

Case-insensitive matching can also be enabled via the embedded flag expression (?i).

Specifying this flag may impose a slight performance penalty.

See Also:: Constant Field Values

COMMENTS

public static final int COMMENTS

Permits whitespace and comments in pattern.

In this mode, whitespace is ignored, and embedded comments starting with # are ignored until the end of a line.

Comments mode can also be enabled via the embedded flag expression (?x).

See Also:: Constant Field Values

MULTILINE

public static final int MULTILINE

Enables multiline mode.

In multiline mode the expressions ^ and $ match just after or just before, respectively, a line terminator or the end of the input sequence. By default these expressions only match at the beginning and the end of the entire input sequence.

Multiline mode can also be enabled via the embedded flag expression (?m).

See Also:: Constant Field Values

DOTALL

public static final int DOTALL

Enables dotall mode.

In dotall mode, the expression . matches any character, including a line terminator. By default this expression does not match line terminators.

Dotall mode can also be enabled via the embedded flag expression (?s). (The s is a mnemonic for "single-line" mode, which is what this is called in Perl.)

See Also:: Constant Field Values

UNICODE_CASE

public static final int UNICODE_CASE

Enables Unicode-aware case folding.

When this flag is specified then case-insensitive matching, when enabled by the CASE_INSENSITIVE flag, is done in a manner consistent with the Unicode Standard. By default, case-insensitive matching assumes that only characters in the US-ASCII charset are being matched.

Unicode-aware case folding can also be enabled via the embedded flag expression (?u).

Specifying this flag may impose a performance penalty.

See Also:: Constant Field Values

CANON_EQ

public static final int CANON_EQ

Enables canonical equivalence.

When this flag is specified then two characters will be considered to match if, and only if, their full canonical decompositions match. The expression "a\u030A", for example, will match the string "?" when this flag is specified. By default, matching does not take canonical equivalence into account.

There is no embedded flag character for enabling canonical equivalence.

Specifying this flag may impose a performance penalty.

See Also:: Constant Field Values

Method Detail

compile

public static Pattern compile(String regex)

Compiles the given regular expression into a pattern.

Parameters:: regex - The expression to be compiled
Throws:: PatternSyntaxException - If the expression's syntax is invalid

compile

public static Pattern compile(String regex,
                              int flags)

Compiles the given regular expression into a pattern with the given flags.

Parameters:: regex - The expression to be compiled; flags - Match flags, a bit mask that may include CASE_INSENSITIVE, MULTILINE, DOTALL, UNICODE_CASE, and CANON_EQ
Throws:: IllegalArgumentException - If bit values other than those corresponding to the defined match flags are set in flags; PatternSyntaxException - If the expression's syntax is invalid

pattern

public String pattern()

Returns the regular expression from which this pattern was compiled.

Returns:: The source of this pattern

matcher

public Matcher matcher(CharSequence input)

Creates a matcher that will match the given input against this pattern.

Parameters:: input - The character sequence to be matched
Returns:: A new matcher for this pattern

flags

public int flags()

Returns this pattern's match flags.

Returns:: The match flags specified when this pattern was compiled

matches

public static boolean matches(String regex,
                              CharSequence input)

Compiles the given regular expression and attempts to match the given input against it.

An invocation of this convenience method of the form

 Pattern.matches(regex, input);

behaves in exactly the same way as the expression

 Pattern.compile(regex).matcher(input).matches()

If a pattern is to be used multiple times, compiling it once and reusing it will be more efficient than invoking this method each time.

Parameters:: regex - The expression to be compiled; input - The character sequence to be matched
Throws:: PatternSyntaxException - If the expression's syntax is invalid

split

public String[] split(CharSequence input,
                      int limit)

Splits the given input sequence around matches of this pattern.

The array returned by this method contains each substring of the input sequence that is terminated by another subsequence that matches this pattern or is terminated by the end of the input sequence. The substrings in the array are in the order in which they occur in the input. If this pattern does not match any subsequence of the input then the resulting array has just one element, namely the input sequence in string form.

The limit parameter controls the number of times the pattern is applied and therefore affects the length of the resulting array. If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n, and the array's last entry will contain all input beyond the last matched delimiter. If n is non-positive then the pattern will be applied as many times as possible and the array can have any length. If n is zero then the pattern will be applied as many times as possible, the array can have any length, and trailing empty strings will be discarded.

The input "boo:and:foo", for example, yields the following results with these parameters:

Regex

Limit

Result

: 2 { "boo", "and:foo" }

: 5 { "boo", "and", "foo" }

: -2 { "boo", "and", "foo" }

o 5 { "b", "", ":and:f", "", "" }

o -2 { "b", "", ":and:f", "", "" }

o 0 { "b", "", ":and:f" }

Parameters:: input - The character sequence to be split; limit - The result threshold, as described above
Returns:: The array of strings computed by splitting the input around matches of this pattern

split

public String[] split(CharSequence input)

Splits the given input sequence around matches of this pattern.

This method works as if by invoking the two-argument split method with the given input sequence and a limit argument of zero. Trailing empty strings are therefore not included in the resulting array.

The input "boo:and:foo", for example, yields the following results with these expressions:

Regex

Result

: { "boo", "and", "foo" }

o { "b", "", ":and:f" }

Parameters:: input - The character sequence to be split
Returns:: The array of strings computed by splitting the input around matches of this pattern

java.util.regex Class Pattern

Summary of regular-expression constructs

Backslashes, escapes, and quoting

Character Classes

Line terminators

Groups and capturing

Unicode support

Comparison to Perl 5

UNIX_LINES

CASE_INSENSITIVE

COMMENTS

MULTILINE

DOTALL

UNICODE_CASE

CANON_EQ

compile

compile

pattern

matcher

flags

matches

split

split

java.util.regex
Class Pattern