strings - final class Matcher's Groups

骆嘉石

2023-12-01

Groups are regular expressions set off by parentheses that can be called up later with their group number. Group 0 indicates the whole expression match, group 1 is the first parenthesized group, etc. Thus, in A ( B (C))D there are three groups: Group 0 is ABCD , group 1 is BC , and group 2 is C .

// strings/Groups.java
// (c)2017 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.

import java.util.regex.*;

public class Groups {
  public static final String POEM =
      "Twas brillig, and the slithy toves\n"
          + "Did gyre and gimble in the wabe.\n"
          + "All mimsy were the borogoves,\n"
          + "And the mome raths outgrabe.\n\n"
          + "Beware the Jabberwock, my son,\n"
          + "The jaws that bite, the claws that catch.\n"
          + "Beware the Jubjub bird, and shun\n"
          + "The frumious Bandersnatch.";

  public static void main(String[] args) {
    Matcher m = Pattern.compile("(?m)(\\S+)\\s+((\\S+)\\s+(\\S+))$").matcher(POEM);
    // match last three words of each line
    while (m.find()) {
      for (int j = 0; j <= m.groupCount(); j++) {
        System.out.print("[" + m.group(j) + "]");
      }
      System.out.println();
    }
  }
}
/* Output:
[the slithy toves][the][slithy toves][slithy][toves]
[in the wabe.][in][the wabe.][the][wabe.]
[were the borogoves,][were][the
borogoves,][the][borogoves,]
[mome raths outgrabe.][mome][raths
outgrabe.][raths][outgrabe.]
[Jabberwock, my son,][Jabberwock,][my son,][my][son,]
[claws that catch.][claws][that catch.][that][catch.]
[bird, and shun][bird,][and shun][and][shun]
[The frumious Bandersnatch.][The][frumious
Bandersnatch.][frumious][Bandersnatch.]
*/

\S [^ \t\r\n\v\f] Non-whitespace characters

\p{Space} or \s [ \t\r\n\v\f] Whitespace characters

Multiline mode enabled via the embedded flag expression (?m)

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 this matcher's region, 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

groupCount

public int groupCount()

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

Group zero denotes the entire pattern by convention. It is not included in this count.

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

Specified by:

groupCount in interface MatchResult

Returns:

The number of capturing groups in this matcher's pattern

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 empty string in the input.

Specified by:

group in interface MatchResult

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

references:

1. On Java 8 - Bruce Eckel

2. https://github.com/wangbingfeng/OnJava8-Examples/blob/master/strings/Groups.java