public final class Pattern extends Object implements Serializable
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. }
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+"
.
\ | 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). |
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 (see note below). |
\D | Any non-digit character (see note below). |
\s | Any whitespace character (see note below). |
\S | Any non-whitespace character (see note below). |
\w | Any word character (see note below). |
\W | Any non-word character (see note below). |
\p{NAME} | Any character in the class with the given NAME. |
\P{NAME} | Any character not in the named class. |
Note that these built-in classes don't just cover the traditional ASCII range. For example,
\w
is equivalent to the character class [\p{Ll}\p{Lu}\p{Lt}\p{Lo}\p{Nd}]
.
For more details see Unicode TR-18,
and bear in mind that the set of characters in each class can vary between Unicode releases.
If you actually want to match only ASCII characters, specify the explicit characters you want;
if you mean 0-9 use [0-9]
rather than \d
, which would also include
Gurmukhi digits and so forth.
There are also a variety of named classes:
Is
. For example \p{IsLu}
for all uppercase letters.
Character.UnicodeBlock.forName(java.lang.String)
prefixed
by In
. For example \p{InHebrew}
for all characters in the Hebrew block.
Character
whose name starts with is
, but with the is
replaced by java
.
For example, \p{javaLowerCase}
.
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). |
^ | 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 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. |
(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(int)
for details of how capturing groups are numbered and accessed.
ab | Expression a followed by expression b. |
a|b | Either expression a or expression b. |
(?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.
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
.
Matcher
,
Serialized FormModifier and Type | Field and Description |
---|---|
static int |
CANON_EQ
This constant specifies that a character in a
Pattern and a
character in the input string only match if they are canonically
equivalent. |
static int |
CASE_INSENSITIVE
This constant specifies that a
Pattern is matched
case-insensitively. |
static int |
COMMENTS
This constant specifies that a
Pattern may contain whitespace or
comments. |
static int |
DOTALL
This constant specifies that the '.' meta character matches arbitrary
characters, including line endings, which is normally not the case.
|
static int |
LITERAL
This constant specifies that the whole
Pattern is to be taken
literally, that is, all meta characters lose their meanings. |
static int |
MULTILINE
This constant specifies that the meta characters '^' and '$' match only
the beginning and end of an input line, respectively.
|
static int |
UNICODE_CASE
This constant specifies that a
Pattern that uses case-insensitive matching
will use Unicode case folding. |
static int |
UNIX_LINES
This constant specifies that a pattern matches Unix line endings ('\n')
only against the '.', '^', and '$' meta characters.
|
Modifier and Type | Method and Description |
---|---|
static Pattern |
compile(String pattern)
Equivalent to
Pattern.compile(pattern, 0) . |
static Pattern |
compile(String regularExpression,
int flags)
Returns a compiled form of the given
regularExpression , as modified by the
given flags . |
protected void |
finalize()
Invoked when the garbage collector has detected that this instance is no longer reachable.
|
int |
flags()
Returns the flags supplied to
compile . |
Matcher |
matcher(CharSequence input)
Returns a
Matcher for this pattern applied to the given input . |
static boolean |
matches(String regularExpression,
CharSequence input)
Tests whether the given
regularExpression matches the given input . |
String |
pattern()
Returns the regular expression supplied to
compile . |
static String |
quote(String string)
Quotes the given
string using "\Q" and "\E", so that all
meta-characters lose their special meaning. |
String[] |
split(CharSequence input)
Equivalent to
split(input, 0) . |
String[] |
split(CharSequence input,
int limit)
Splits the given
input at occurrences of this pattern. |
String |
toString()
Returns a string containing a concise, human-readable description of this
object.
|
public static final int UNIX_LINES
(?d)
.public static final int CASE_INSENSITIVE
Pattern
is matched
case-insensitively. That is, the patterns "a+" and "A+" would both match
the string "aAaAaA". See UNICODE_CASE
. Corresponds to (?i)
.public static final int COMMENTS
Pattern
may contain whitespace or
comments. Otherwise comments and whitespace are taken as literal
characters. Corresponds to (?x)
.public static final int MULTILINE
(?m)
.public static final int LITERAL
Pattern
is to be taken
literally, that is, all meta characters lose their meanings.public static final int DOTALL
(?s)
.public static final int UNICODE_CASE
Pattern
that uses case-insensitive matching
will use Unicode case folding. On Android, UNICODE_CASE
is always on:
case-insensitive matching will always be Unicode-aware. If your code is intended to
be portable and uses case-insensitive matching on non-ASCII characters, you should
use this flag. Corresponds to (?u)
.public static final int CANON_EQ
Pattern
and a
character in the input string only match if they are canonically
equivalent. It is (currently) not supported in Android.public Matcher matcher(CharSequence input)
Matcher
for this pattern applied to the given input
.
The Matcher
can be used to match the Pattern
against the
whole input, find occurrences of the Pattern
in the input, or
replace parts of the input.public String[] split(CharSequence input, int limit)
input
at occurrences of this pattern.
If this pattern does not occur in the input, the result is an
array containing the input (converted from a CharSequence
to
a String
).
Otherwise, the limit
parameter controls the contents of the
returned array as described below.
limit
- Determines the maximum number of entries in the resulting
array, and the treatment of trailing empty strings.
Pattern
plus one for the text after the final separator.
All entries are included.
public String[] split(CharSequence input)
split(input, 0)
.public String pattern()
compile
.public String toString()
Object
getClass().getName() + '@' + Integer.toHexString(hashCode())
See Writing a useful
toString
method
if you intend implementing your own toString
method.
public int flags()
compile
.public static Pattern compile(String regularExpression, int flags) throws PatternSyntaxException
regularExpression
, as modified by the
given flags
. See the flags overview for more on flags.PatternSyntaxException
- if the regular expression is syntactically incorrect.CANON_EQ
,
CASE_INSENSITIVE
,
COMMENTS
,
DOTALL
,
LITERAL
,
MULTILINE
,
UNICODE_CASE
,
UNIX_LINES
public static boolean matches(String regularExpression, CharSequence input)
regularExpression
matches the given input
.
Equivalent to Pattern.compile(regularExpression).matcher(input).matches()
.
If the same regular expression is to be used for multiple operations, it may be more
efficient to reuse a compiled Pattern
.public static String quote(String string)
string
using "\Q" and "\E", so that all
meta-characters lose their special meaning. This method correctly
escapes embedded instances of "\Q" or "\E". If the entire result
is to be passed verbatim to compile(java.lang.String, int)
, it's usually clearer
to use the LITERAL
flag instead.protected void finalize() throws Throwable
Object
Note that objects that override finalize
are significantly more expensive than
objects that don't. Finalizers may be run a long time after the object is no longer
reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup.
Note also that finalizers are run on a single VM-wide finalizer thread,
so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary
for a class that has a native peer and needs to call a native method to destroy that peer.
Even then, it's better to provide an explicit close
method (and implement
Closeable
), and insist that callers manually dispose of instances. This
works well for something like files, but less well for something like a BigInteger
where typical calling code would have to deal with lots of temporaries. Unfortunately,
code that creates lots of temporaries is the worst kind of code from the point of view of
the single finalizer thread.
If you must use finalizers, consider at least providing your own
ReferenceQueue
and having your own thread process that queue.
Unlike constructors, finalizers are not automatically chained. You are responsible for
calling super.finalize()
yourself.
Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.