【待翻译】java.util.regex.Pattern

java.util.regex.Pattern

Patterns are compiled regular expressions. In many cases, convenience methods such as String.matches, String.replaceAll and String.split will be preferable, but if you need to do a lot of work with the same regular expression, it may be more efficient to compile it once and reuse it. The Pattern class and its companion, Matcher, also offer more functionality than the small amount exposed by String.

// String convenience methods:
boolean sawFailures = s.matches("Failures: \\d+");
String farewell = s.replaceAll("Hello, (\\S+)", "Goodbye, $1");
String[] fields = s.split(":");

// Direct use of Pattern:
Pattern p = Pattern.compile("Hello, (\\S+)");
Matcher m = p.matcher(inputString);
while (m.find()) { // Find each match in turn; String can't do this.
String name = m.group(1); // Access a submatch group; String can't do this.
}

Regular expression syntax
Java supports a subset of Perl 5 regular expression syntax. An important gotcha is that Java has no regular expression literals, and uses plain old string literals instead. This means that you need an extra level of escaping. For example, the regular expression \s+ has to be represented as the string "\\s+".

Escape sequences
\ Quote the following metacharacter (so \. matches a literal .).
\Q Quote all following metacharacters until \E.
\E Stop quoting metacharacters (started by \Q).
\\ A literal backslash.
\uhhhh The Unicode character U+hhhh (in hex).
\xhh The Unicode character U+00hh (in hex).
\cx The ASCII control character ^x (so \cH would be ^H, U+0008).
\a The ASCII bell character (U+0007).
\e The ASCII ESC character (U+001b).
\f The ASCII form feed character (U+000c).
\n The ASCII newline character (U+000a).
\r The ASCII carriage return character (U+000d).
\t The ASCII tab character (U+0009).

Character classes
It's possible to construct arbitrary character classes using set operations: [abc] Any one of a, b, or c. (Enumeration.)
[a-c] Any one of a, b, or c. (Range.)
[^abc] Any character except a, b, or c. (Negation.)
[[a-f][0-9]] Any character in either range. (Union.)
[[a-z]&&[jkl]] Any character in both ranges. (Intersection.)

Most of the time, the built-in character classes are more useful: \d Any digit character.
\D Any non-digit character.
\s Any whitespace character.
\S Any non-whitespace character.
\w Any word character.
\W Any non-word character.
\p{NAME} Any character in the class with the given NAME.
\P{NAME} Any character not in the named class.

There are a variety of named classes:

Unicode category names, prefixed by Is. For example \p{IsLu}} for all uppercase letters.
POSIX class names. These are 'Alnum', 'Alpha', 'ASCII', 'Blank', 'Cntrl', 'Digit', 'Graph', 'Lower', 'Print', 'Punct', 'Upper', 'XDigit'.
Unicode block names, as used by java.lang.Character.UnicodeBlock.forName prefixed by In. For example \p{InHebrew}} for all characters in the Hebrew block.
Character method names. These are all non-deprecated methods from java.lang.Character whose name starts with is, but with the is replaced by java. For example, \p{javaLowerCase}}.
Quantifiers
Quantifiers match some number of instances of the preceding regular expression. * Zero or more.
? Zero or one.
+ One or more.
{n} Exactly n.
{n,} At least n.
{n,m} At least n but not more than m.

Quantifiers are "greedy" by default, meaning that they will match the longest possible input sequence. There are also non-greedy quantifiers that match the shortest possible input sequence. They're same as the greedy ones but with a trailing ?: *? Zero or more (non-greedy).
?? Zero or one (non-greedy).
+? One or more (non-greedy).
{n}? Exactly n (non-greedy).
{n,}? At least n (non-greedy).
{n,m}? At least n but not more than m (non-greedy).

Quantifiers allow backtracking by default. There are also possessive quantifiers to prevent backtracking. They're same as the greedy ones but with a trailing +: *+ Zero or more (possessive).
?+ Zero or one (possessive).
++ One or more (possessive).
{n}+ Exactly n (possessive).
{n,}+ At least n (possessive).
{n,m}+ At least n but not more than m (possessive).

Zero-width assertions
^ At beginning of line.
$ At end of line.
\A At beginning of input.
\b At word boundary.
\B At non-word boundary.
\G At end of previous match.
\z At end of input.
\Z At end of input, or before newline at end.

Look-around assertions
Look-around assertions assert that the subpattern does (positive) or doesn't (negative) match after (look-ahead) or before (look-behind) the current position, without including the matched text in the containing match. The maximum length of possible matches for look-behind patterns must not be unbounded.

(?=a) Zero-width positive look-ahead.
(?!a) Zero-width negative look-ahead.
(?<=a) Zero-width positive look-behind.
(?<!a) Zero-width negative look-behind.

Groups
(a) A capturing group.
(?:a) A non-capturing group.
(?>a) An independent non-capturing group. (The first match of the subgroup is the only match tried.)
\n The text already matched by capturing group n.

See Matcher.group for details of how capturing groups are numbered and accessed.

Operators
ab Expression a followed by expression b.
a|b Either expression a or expression b.

Flags
(?dimsux-dimsux:a) Evaluates the expression a with the given flags enabled/disabled.
(?dimsux-dimsux) Evaluates the rest of the pattern with the given flags enabled/disabled.

The flags are: i CASE_INSENSITIVE case insensitive matching
d UNIX_LINES only accept '\n' as a line terminator
m MULTILINE allow ^ and $ to match beginning/end of any line
s DOTALL allow . to match '\n' ("s" for "single line")
u UNICODE_CASE enable Unicode case folding
x COMMENTS allow whitespace and comments

Either set of flags may be empty. For example, (?i-m) would turn on case-insensitivity and turn off multiline mode, (?i) would just turn on case-insensitivity, and (?-m) would just turn off multiline mode.

Note that on Android, UNICODE_CASE is always on: case-insensitive matching will always be Unicode-aware.

There are two other flags not settable via this mechanism: CANON_EQ and LITERAL. Attempts to use CANON_EQ on Android will throw an exception.

Implementation notes
The regular expression implementation used in Android is provided by ICU. The notation for the regular expressions is mostly a superset of those used in other Java language implementations. This means that existing applications will normally work as expected, but in rare cases Android may accept a regular expression that is not accepted by other implementations.

In some cases, Android will recognize that a regular expression is a simple special case that can be handled more efficiently. This is true of both the convenience methods in String and the methods in Pattern.

See Also:
Matcher