Java regex one space
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 |
| \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 | The character with hexadecimal value 0xh. h ( Character.MIN_CODE_POINT 0xh. h |
| \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: 4 |
| \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 | A lower-case alphabetic character: [a-z] |
| \p | An upper-case alphabetic character: [A-Z] |
| \p | All ASCII: [\x00-\x7F] |
| \p | An alphabetic character: [\p\p] |
| \p | A decimal digit: 1 |
| \p | An alphanumeric character: [\p\p] |
| \p | Punctuation: One of !"#$%&'()*+,-./:;?@[\]^_`<|>~ |
| \p | A visible character: [\p\p] |
| \p | A printable character: [\p\x20] |
| \p | A space or a tab: [ \t] |
| \p | A control character: [\x00-\x1F\x7F] |
| \p | A hexadecimal digit: [0-9a-fA-F] |
| \p | A whitespace character: [ \t\n\x0B\f\r] |
| java.lang.Character classes (simple java character type) | |
| \p | Equivalent to java.lang.Character.isLowerCase() |
| \p | Equivalent to java.lang.Character.isUpperCase() |
| \p | Equivalent to java.lang.Character.isWhitespace() |
| \p | Equivalent to java.lang.Character.isMirrored() |
| Classes for Unicode scripts, blocks, categories and binary properties | |
| \p | A Latin script character (script) |
| \p | A character in the Greek block (block) |
| \p | An uppercase letter (category) |
| \p | An alphabetic character (binary property) |
| \p | A currency symbol |
| \P | Any character except one in the Greek block (negation) |
| [\p&&[^\p]] | 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 |
| Xn> | X, exactly n times |
| Xn,> | X, at least n times |
| Xn,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 |
| Xn>? | X, exactly n times |
| Xn,>? | X, at least n times |
| Xn,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 |
| Xn>+ | X, exactly n times |
| Xn,>+ | X, at least n times |
| Xn,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 |
| \kname> | 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) | |
| (?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 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
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:
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 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).
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:
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
- 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'),
Unicode support
- Alphabetic
- Ideographic
- Letter
- Lowercase
- Uppercase
- Titlecase
- Punctuation
- Control
- White_Space
- Digit
- Hex_Digit
- Join_Control
- Noncharacter_Code_Point
- Assigned
Comparison to Perl 5
- Predefined character classes (Unicode character) \X Match Unicode extended grapheme cluster
- The backreference constructs, \gn> for the n th capturing group and \gname> for named-capturing group.
- The named character construct, \Nname> 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:
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.