java.util.regex
Class Matcher

java.lang.Object
  |
  +--java.util.regex.Matcher

public final class Matcher

extends Object

An engine that performs match operations on a character sequence by interpreting a Pattern.

A matcher is created from a pattern by invoking the pattern's matcher method. Once created, a matcher can be used to perform three different kinds of match operations:

• The matches method attempts to match the entire input sequence against the pattern.

• The lookingAt method attempts to match the input sequence, starting at the beginning, against the pattern.

• The find method scans the input sequence looking for the next subsequence that matches the pattern.

Each of these methods returns a boolean indicating success or failure. More information about a successful match can be obtained by querying the state of the matcher.

This class also defines methods for replacing matched subsequences with new strings whose contents can, if desired, be computed from the match result. The appendReplacement and appendTail methods can be used in tandem in order to collect the result into an existing string buffer, or the more convenient replaceAll method can be used to create a string in which every matching subsequence in the input sequence is replaced.

The explicit state of a matcher includes the start and end indices of the most recent successful match. It also includes the start and end indices of the input subsequence captured by each capturing group in the pattern as well as a total count of such subsequences. As a convenience, methods are also provided for returning these captured subsequences in string form.

The explicit state of a matcher is initially undefined; attempting to query any part of it before a successful match will cause an IllegalStateException to be thrown. The explicit state of a matcher is recomputed by every match operation.

The implicit state of a matcher includes the input character sequence as well as the append position, which is initially zero and is updated by the appendReplacement method.

A matcher may be reset explicitly by invoking its reset() method or, if a new input sequence is desired, its reset(CharSequence) method. Resetting a matcher discards its explicit state information and sets the append position to zero.

Instances of this class are not safe for use by multiple concurrent threads.

Since:

1.4

Method Summary

Matcher	appendReplacement(StringBuffer sb, String replacement) Implements a non-terminal append-and-replace step.
StringBuffer	appendTail(StringBuffer sb) Implements a terminal append-and-replace step.
int	end() Returns the index of the last character matched, plus one.
int	end(int group) Returns the index of the last character, plus one, of the subsequence captured by the given group during the previous match operation.
boolean	find() Attempts to find the next subsequence of the input sequence that matches the pattern.
boolean	find(int start) Resets this matcher and then attempts to find the next subsequence of the input sequence that matches the pattern, starting at the specified index.
String	group() Returns the input subsequence matched by the previous match.
String	group(int group) Returns the input subsequence captured by the given group during the previous match operation.
int	groupCount() Returns the number of capturing groups in this matcher's pattern.
boolean	lookingAt() Attempts to match the input sequence, starting at the beginning, against the pattern.
boolean	matches() Attempts to match the entire input sequence against the pattern.
Pattern	pattern() Returns the pattern that is interpreted by this matcher.
String	replaceAll(String replacement) Replaces every subsequence of the input sequence that matches the pattern with the given replacement string.
String	replaceFirst(String replacement) Replaces the first subsequence of the input sequence that matches the pattern with the given replacement string.
Matcher	reset() Resets this matcher.
Matcher	reset(CharSequence input) Resets this matcher with a new input sequence.
int	start() Returns the start index of the previous match.
int	start(int group) Returns the start index of the subsequence captured by the given group during the previous match operation.

Methods inherited from class java.lang.Object

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

Method Detail

pattern

public Pattern pattern()

Returns the pattern that is interpreted by this matcher.

Returns:

The pattern for which this matcher was created

reset

public Matcher reset()

Resets this matcher.

Resetting a matcher discards all of its explicit state information and sets its append position to zero.

Returns:

This matcher

reset

public Matcher reset(CharSequence input)

Resets this matcher with a new input sequence.

Resetting a matcher discards all of its explicit state information and sets its append position to zero.

Parameters:

input - The new input character sequence

Returns:

This matcher

start

public int start()

Returns the start index of the previous match.

Returns:

The index of the first character matched

Throws:

IllegalStateException - If no match has yet been attempted, or if the previous match operation failed

start

public int start(int group)

Returns the start index of the subsequence captured by the given group during the previous match operation.

Capturing groups are indexed from left to right, starting at one. Group zero denotes the entire pattern, so the expression m.start(0) is equivalent to m.start().

Parameters:

group - The index of a capturing group in this matcher's pattern

Returns:

The index of the first character captured by the group, or -1 if the match was successful but the group itself did not match anything

Throws:

IllegalStateException - If no match has yet been attempted, or if the previous match operation failed

IndexOutOfBoundsException - If there is no capturing group in the pattern with the given index

end

public int end()

Returns the index of the last character matched, plus one.

Returns:

The index of the last character matched, plus one

Throws:

IllegalStateException - If no match has yet been attempted, or if the previous match operation failed

end

public int end(int group)

Returns the index of the last character, plus one, of the subsequence captured by the given group during the previous match operation.

Capturing groups are indexed from left to right, starting at one. Group zero denotes the entire pattern, so the expression m.end(0) is equivalent to m.end().

Parameters:

group - The index of a capturing group in this matcher's pattern

Returns:

The index of the last character captured by the group, plus one, or -1 if the match was successful but the group itself did not match anything

Throws:

IllegalStateException - If no match has yet been attempted, or if the previous match operation failed

IndexOutOfBoundsException - If there is no capturing group in the pattern with the given index

group

public String group()

Returns the input subsequence matched by the previous match.

For a matcher m with input sequence s, the expressions m.group() and s.substring(m.start(), m.end()) are equivalent.

Note that some patterns, for example a*, match the empty string. This method will return the empty string when the pattern successfully matches the empty string in the input.

Returns:

The (possibly empty) subsequence matched by the previous match, in string form

Throws:

IllegalStateException - If no match has yet been attempted, or if the previous match operation failed

group

public String group(int group)

Returns the input subsequence captured by the given group during the previous match operation.

For a matcher m, input sequence s, and group index g, the expressions m.group(g) and s.substring(m.start(g), m.end(g)) are equivalent.

Capturing groups are indexed from left to right, starting at one. Group zero denotes the entire pattern, so the expression m.group(0) is equivalent to m.group().

If the match was successful but the group specified failed to match any part of the input sequence, then null is returned. Note that some groups, for example (a*), match the empty string. This method will return the empty string when such a group successfully matches the emtpy string in the input.

Parameters:

group - The index of a capturing group in this matcher's pattern

Returns:

The (possibly empty) subsequence captured by the group during the previous match, or null if the group failed to match part of the input

Throws:

IllegalStateException - If no match has yet been attempted, or if the previous match operation failed

IndexOutOfBoundsException - If there is no capturing group in the pattern with the given index

groupCount

public int groupCount()

Returns the number of capturing groups in this matcher's pattern.

Any non-negative integer smaller than the value returned by this method is guaranteed to be a valid group index for this matcher.

Returns:

The number of capturing groups in this matcher's pattern

matches

public boolean matches()

Attempts to match the entire input sequence against the pattern.

If the match succeeds then more information can be obtained via the start, end, and group methods.

Returns:

true if, and only if, the entire input sequence matches this matcher's pattern

find

public boolean find()

Attempts to find the next subsequence of the input sequence that matches the pattern.

This method starts at the beginning of the input sequence or, if a previous invocation of the method was successful and the matcher has not since been reset, at the first character not matched by the previous match.

If the match succeeds then more information can be obtained via the start, end, and group methods.

Returns:

true if, and only if, a subsequence of the input sequence matches this matcher's pattern

find

public boolean find(int start)

Resets this matcher and then attempts to find the next subsequence of the input sequence that matches the pattern, starting at the specified index.

If the match succeeds then more information can be obtained via the start, end, and group methods, and subsequent invocations of the find() method will start at the first character not matched by this match.

Returns:

true if, and only if, a subsequence of the input sequence starting at the given index matches this matcher's pattern

Throws:

IndexOutOfBoundsException - If start is less than zero or if start is greater than the length of the input sequence.

lookingAt

public boolean lookingAt()

Attempts to match the input sequence, starting at the beginning, against the pattern.

Like the matches method, this method always starts at the beginning of the input sequence; unlike that method, it does not require that the entire input sequence be matched.

If the match succeeds then more information can be obtained via the start, end, and group methods.

Returns:

true if, and only if, a prefix of the input sequence matches this matcher's pattern

appendReplacement

public Matcher appendReplacement(StringBuffer sb,
                                 String replacement)

Implements a non-terminal append-and-replace step.

This method performs the following actions:

1. It reads characters from the input sequence, starting at the append position, and appends them to the given string buffer. It stops after reading the last character preceding the previous match, that is, the character at index start() - 1.

2. It appends the given replacement string to the string buffer.

3. It sets the append position of this matcher to the index of the last character matched, plus one, that is, to end().

The replacement string may contain references to subsequences captured during the previous match: Each occurrence of $g will be replaced by the result of evaluating group(g). If the second group matched the string "foo", for example, then passing the replacement string "$2bar" would cause "foobar" to be appended to the string buffer. A dollar sign ($) may be included as a literal in the replacement string by preceding it with a backslash (\$).

This method is intended to be used in a loop together with the appendTail and find methods. The following code, for example, writes one dog two dogs in the yard to the standard-output stream:

 Pattern p = Pattern.compile("cat");
 Matcher m = p.matcher("one cat two cats in the yard");
 StringBuffer sb = new StringBuffer();
 boolean result = m.find();
 while (m.appendReplacement(sb, "dog")) {
     result = m.find();
 }
 m.appendTail(sb);
 System.out.println(sb.toString());

Parameters:

sb - The target string buffer

replacement - The replacement string

Returns:

This matcher

Throws:

IllegalStateException - If no match has yet been attempted, or if the previous match operation failed

IndexOutOfBoundsException - If the replacement string refers to a capturing group that does not exist in the pattern

appendTail

public StringBuffer appendTail(StringBuffer sb)

Implements a terminal append-and-replace step.

This method reads characters from the input sequence, starting at the append position, and appends them to the given string buffer. It is intended to be invoked after one or more invocations of the appendReplacement method in order to copy the remainder of the input sequence.

Parameters:

sb - The target string buffer

Returns:

The target string buffer

replaceAll

public String replaceAll(String replacement)

Replaces every subsequence of the input sequence that matches the pattern with the given replacement string.

This method first resets this matcher. It then scans the input sequence looking for matches of the pattern. Characters that are not part of any match are appended directly to the result string; each match is replaced in the result by the replacement string. The replacement string may contain references to captured subsequences as in the appendReplacement method.

Given the regular expression a*b, the input "aabfooaabfooabfoob", and the replacement string "-", an invocation of this method on a matcher for that expression would yield the string "-foo-foo-foo-".

Invoking this method changes this matcher's state. If the matcher is to be used in further matching operations then it should first be reset.

Parameters:

replacement - The replacement string

Returns:

The string constructed by replacing each matching subsequence by the replacement string, substituting captured subsequences as needed

replaceFirst

public String replaceFirst(String replacement)

Replaces the first subsequence of the input sequence that matches the pattern with the given replacement string.

This method first resets this matcher. It then scans the input sequence looking for a match of the pattern. Characters that are not part of the match are appended directly to the result string; the match is replaced in the result by the replacement string. The replacement string may contain references to captured subsequences as in the appendReplacement method.

Given the regular expression dog, the input "zzzdogzzzdogzzz", and the replacement string "cat", an invocation of this method on a matcher for that expression would yield the string "zzzcatzzzdogzzz".

Invoking this method changes this matcher's state. If the matcher is to be used in further matching operations then it should first be reset.

Parameters:

replacement - The replacement string

Returns:

The string constructed by replacing the first matching subsequence by the replacement string, substituting captured subsequences as needed