Package java.util.regex

Class Pattern

java.lang.Object
java.util.regex.Pattern
All Implemented Interfaces:
Serializable
public final class Pattern
extends Object
implements Serializable
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 (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:

  • 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 accepted as input to Character.UnicodeBlock.forName(java.lang.String), prefixed by In. For example \p{InHebrew} for all characters in the Hebrew block.
  • Character method names. These are all non-deprecated methods from 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(int) 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, Serialized Form

  • Field Summary

    Fields
    Modifier and Type Field 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.

  • Method Summary

    Modifier and Type Method 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.

    Methods inherited from class java.lang.Object

    clone, equals, getClass, hashCode, notify, notifyAll, wait, wait, wait

  • Field Details

    • UNIX_LINES

      public static final int UNIX_LINES
      This constant specifies that a pattern matches Unix line endings ('\n') only against the '.', '^', and '$' meta characters. Corresponds to (?d).
      See Also:
      Constant Field Values

    • CASE_INSENSITIVE

      public static final int CASE_INSENSITIVE
      This constant specifies that a Pattern is matched case-insensitively. That is, the patterns "a+" and "A+" would both match the string "aAaAaA". See UNICODE_CASE. Corresponds to (?i).
      See Also:
      Constant Field Values

    • COMMENTS

      public static final int COMMENTS
      This constant specifies that a Pattern may contain whitespace or comments. Otherwise comments and whitespace are taken as literal characters. Corresponds to (?x).
      See Also:
      Constant Field Values

    • MULTILINE

      public static final int MULTILINE
      This constant specifies that the meta characters '^' and '$' match only the beginning and end of an input line, respectively. Normally, they match the beginning and the end of the complete input. Corresponds to (?m).
      See Also:
      Constant Field Values

    • LITERAL

      public static final int LITERAL
      This constant specifies that the whole Pattern is to be taken literally, that is, all meta characters lose their meanings.
      See Also:
      Constant Field Values

    • DOTALL

      public static final int DOTALL
      This constant specifies that the '.' meta character matches arbitrary characters, including line endings, which is normally not the case. Corresponds to (?s).
      See Also:
      Constant Field Values

    • UNICODE_CASE

      public static final int UNICODE_CASE
      This constant specifies that a 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).
      See Also:
      Constant Field Values

    • CANON_EQ

      public static final 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. It is (currently) not supported in Android.
      See Also:
      Constant Field Values

  • Method Details

    • matcher

      public Matcher matcher​(CharSequence input)
      Returns a 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.

    • split

      public String[] split​(CharSequence input, int limit)
      Splits the given 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.

      Parameters:
      limit - Determines the maximum number of entries in the resulting array, and the treatment of trailing empty strings.
      • For n > 0, the resulting array contains at most n entries. If this is fewer than the number of matches, the final entry will contain all remaining input.
      • For n < 0, the length of the resulting array is exactly the number of occurrences of the Pattern plus one for the text after the final separator. All entries are included.
      • For n == 0, the result is as for n < 0, except trailing empty strings will not be returned. (Note that the case where the input is itself an empty string is special, as described above, and the limit parameter does not apply there.)

    • split

      public String[] split​(CharSequence input)
      Equivalent to split(input, 0).

    • pattern

      public String pattern()
      Returns the regular expression supplied to compile.

    • toString

      public String toString()
      Description copied from class: Object
      Returns a string containing a concise, human-readable description of this object. Subclasses are encouraged to override this method and provide an implementation that takes into account the object's type and data. The default implementation is equivalent to the following expression: 
         getClass().getName() + '@' + Integer.toHexString(hashCode())

      See Writing a useful toString method if you intend implementing your own toString method.

      Overrides:
      toString in class Object
      Returns:
      a printable representation of this object.

    • flags

      public int flags()
      Returns the flags supplied to compile.

    • compile

      public static Pattern compile​(String regularExpression, int flags) throws PatternSyntaxException
      Returns a compiled form of the given regularExpression, as modified by the given flags. See the flags overview for more on flags.
      Throws:
      PatternSyntaxException - if the regular expression is syntactically incorrect.
      See Also:
      CANON_EQ, CASE_INSENSITIVE, COMMENTS, DOTALL, LITERAL, MULTILINE, UNICODE_CASE, UNIX_LINES

    • compile

      public static Pattern compile​(String pattern)
      Equivalent to Pattern.compile(pattern, 0).

    • matches

      public static boolean matches​(String regularExpression, CharSequence input)
      Tests whether the given 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.
      See Also:
      compile(java.lang.String, int), Matcher.matches()

    • quote

      public static String quote​(String string)
      Quotes the given 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.

    • finalize

      protected void finalize() throws Throwable
      Description copied from class: Object
      Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

      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.

      Overrides:
      finalize in class Object
      Throws:
      Throwable