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 |
\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] |
\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} | block) |
\p{Lu} | 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 nth 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 |
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 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:
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 (e.g. $0, $1, $2, ... can be used to use a captured group as replacement).
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.
The replacement string may contain references to subsequences captured during the previous match: Each occurrence of $g will be replaced by the result of evaluating group(g). The first number after the $ is always treated as part of the group reference. Subsequent numbers are incorporated into g if they would form a legal group reference. Only the numerals '0' through '9' are considered as potential components of the group reference. If the second group matched the string "foo", for example, then passing the replacement string "$2bar" would cause "foobar" to be appended to the string buffer. A dollar sign ($) may be included as a literal in the replacement string by preceding it with a backslash (\$).
Capturing groups are indexed from left to right, starting at one. Group zero denotes the entire pattern.
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. 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 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 For a more precise description of the behavior of regular expression constructs,
please see "http://www.oreilly.com/catalog/regex2/" Mastering Regular Expressions,
2nd Edition, Jeffrey E. F. Friedl, O'Reilly and Associates, 2002. This text is a subset of the Java 1.4 Pattern JavaDocComparison to Perl 5
Matcher
class: Repeated invocations of the find
method will resume
where the last match left off, unless the matcher is reset. PatternSyntaxException
if it encounters them.