This is a summary of the documentattion of the Java Pattern class.
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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{hhhh} | The character with hexadecimal value 0xh..h (between 0x0000 and 0x10FFFF) |
\N{name} | The character with Unicode character name 'name' (e.g. \N{GREATER-THAN SIGN}) |
\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}] |
\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 scripts, blocks, categories and binary properties | |
\p{IsLatin} | A Latin script character (script) |
\p{InGreek} | A character in the Greek block (simple block) |
\p{Lu} | An uppercase letter (simple 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 |
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:
Precedence | Name | Example |
---|---|---|
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.
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.
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.
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.
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
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. 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]]
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, 3rd Edition, Jeffrey E. F. Friedl, O'Reilly and Associates, 2006.