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Java Regular Expressions - detailed manual

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
 \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}] or space
 \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 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



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 these expressions match just after or
just before, respectively, a line terminator or the end of the input sequence 
with the exception that the expression ^ never matches at the end of input, 
even if the last character is a newline. 

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. 

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