Java 中的 Regex 边界匹配器
原文:https://www . geesforgeks . org/regex-border-matchers-in-Java/
先决条件-Java 中的正则表达式
边界匹配可以帮助我们找到字符串匹配发生的位置。通过用边界匹配器指定这样的信息，可以使模式匹配更加精确。例如，也许你对找到一个特定的单词感兴趣，但前提是它出现在一行的开头或结尾。或者你可能想知道匹配是发生在单词边界上，还是在上一次匹配结束时。
边界匹配者列表
- ^–放在要匹配的单词之前
- $–放在要匹配的单词末尾
- \ b–检查模式是在单词边界开始还是结束
- \ B–匹配非单词边界上的表达式
- \ A–输入的开始
- \ G–要求只在前一场比赛结束时才进行比赛
- \ Z–输入的结束，但对于最终的结束符，如果有的话
- \z — 输入结束
情况 1:将单词与^和$ 匹配
- ^–匹配一行的开头
-
$–匹配结束。
-
java Input : txt = "geeksforgeeks", regex = "^geeks" Output : Found from index 0 to 3 Explanation : Note that the result doesn't include "geeks" after "for" as we have used ^ in regex. -
java Input : txt = "geeksforgeeks", regex = "geeks{content}quot; Output : Found from index 8 to 13. Explanation : Note that the result doesn't include "geeks" before "for" as we have used $ in regex. -
java Input : txt = "geeksforgeeks", regex = "^geeks{content}quot; Output : No match found Explanation : The given regex would only matches with "geeks". -
java Input : txt = " geeksforgeeks", regex = "^geeks" Output: No match found. Explanation : The input string contains extra whitespace at the beginning. -
java // Extra \ is used to escape one \ Input : txt = " geeksforgeeks", regex : "^\\s+geeks" Output: Found from index 0 to 6. Explanation : The pattern specifies geeks after one or more spaces.
// Java program to demonstrate that ^ matches the beginning of
// a line, and $ matches the end.
import java.util.regex.Matcher;
import java.util.regex.Pattern;
class Reg
{
public static void main(String[] args)
{
String txt = "geeksforgeeks";
// Demonstrating ^
String regex1 = "^geeks";
Pattern pattern1 = Pattern.compile(regex1, Pattern.CASE_INSENSITIVE);
Matcher matcher1 = pattern1.matcher(txt);
while (matcher1.find())
{
System.out.println("Start index: " + matcher1.start());
System.out.println("End index: " + matcher1.end());
}
// Demonstrating $
String regex2 = "geeks{content}quot;;
Pattern pattern2 = Pattern.compile(regex2, Pattern.CASE_INSENSITIVE);
Matcher matcher2 = pattern2.matcher(txt);
while (matcher2.find())
{
System.out.println("\nStart index: " + matcher2.start());
System.out.println("End index: " + matcher2.end());
}
}
}
输出:
Start index: 0
End index: 5
Start index: 8
End index: 13
*情况 2:使用\b* 检查模式是在单词边界开始还是结束
-
```java Input: txt = "geeksforgeeks geekspractice", pat = "\bgeeks" Output: Found from index 0 to 5 and from index 14 to 19 Explanation : The pattern "geeks" is present at the beginning of two words "geeksforgeeks" and "geekspractice"
```
-
```java Input: txt = "geeksforgeeks geekspractice", pat = "geeks\b" Output: Found from index 8 to 13 Explanation : The pattern "geeks" is present at the end of one word "geeksforgeeks"
```
// Java program to demonstrate use of \b to match
// regex at beginning and end of word boundary
import java.util.regex.Matcher;
import java.util.regex.Pattern;
class Reg
{
public static void main(String[] args)
{
String txt = "geeksforgeeks geekspractice";
// Demonstrating beginning of word boundary
String regex1 = "\\bgeeks"; // Matched at two places
Pattern pattern1 = Pattern.compile(regex1, Pattern.CASE_INSENSITIVE);
Matcher matcher1 = pattern1.matcher(txt);
while (matcher1.find())
{
System.out.println("Start index: " + matcher1.start());
System.out.println("End index: " + matcher1.end());
}
// Demonstrating end of word boundary
String regex2 = "geeks\\b"; // Matched at one place
Pattern pattern2 = Pattern.compile(regex2, Pattern.CASE_INSENSITIVE);
Matcher matcher2 = pattern2.matcher(txt);
while (matcher2.find())
{
System.out.println("\nStart index: " + matcher2.start());
System.out.println("End index: " + matcher2.end());
}
}
}
输出:
Start index: 0
End index: 5
Start index: 14
End index: 19
Start index: 8
End index: 13
*例 3:在非单词边界上匹配表达式，用\B 代替*
-
```java Input: txt = "geeksforgeeks geekspractice", pat = "\Bgeeks" Output: Found from index 8 to 13 Explanation : One occurrence of pattern "geeks" is not present at the beginning of word which is end of "geeksforgeeks"
```
-
```java Input: txt = "geeksforgeeks geekspractice", pat = "geeks\B" Output: Found from index 0 to 5 and from index 14 to 19 Explanation : Two occurrences of "geeks" are not present at the end of word.
```
// Java program to demonstrate use of \B to match
// regex at beginning and end of non word boundary
import java.util.regex.Matcher;
import java.util.regex.Pattern;
class Reg
{
public static void main(String[] args)
{
String txt = "geeksforgeeks geekspractice";
// Demonstrating Not beginning of word
String regex1 = "\\Bgeeks"; // Matches with two
Pattern pattern1 = Pattern.compile(regex1, Pattern.CASE_INSENSITIVE);
Matcher matcher1 = pattern1.matcher(txt);
while (matcher1.find())
{
System.out.println("Start index: " + matcher1.start());
System.out.println("End index: " + matcher1.end() + "\n");
}
// Demonstrating Not end of word
String regex2 = "geeks\\B"; // Matches with one
Pattern pattern2 = Pattern.compile(regex2, Pattern.CASE_INSENSITIVE);
Matcher matcher2 = pattern2.matcher(txt);
while (matcher2.find())
{
System.out.println("Start index: " + matcher2.start());
System.out.println("End index: " + matcher2.end());
}
}
}
输出:
Start index: 8
End index: 13
Start index: 0
End index: 5
Start index: 14
End index: 19
情况 4:匹配只在前一场比赛结束时发生，使用\G:
-
```java Input: txt = "geeksgeeks geeks", pat = "\Ggeeks" Output: Found from index 0 to 5 and from 5 to 10 Explanation : Only first two occurrences of "geeks" in text match. the occurrence after space doesn't match as it is not just after previous match.
```
// Java program to demonstrate use of \G to match
// to occur only at the end of the previous match
import java.util.regex.Matcher;
import java.util.regex.Pattern;
class Reg
{
public static void main(String[] args)
{
String txt = "geeksgeeks geeks";
// Demonstrating \G
String regex1 = "\\Ggeeks"; // Matches with first two geeks
Pattern pattern1 = Pattern.compile(regex1, Pattern.CASE_INSENSITIVE);
Matcher matcher1 = pattern1.matcher(txt);
while (matcher1.find())
{
System.out.println("Start index: " + matcher1.start());
System.out.println("End index: " + matcher1.end());
}
}
}
输出:
Start index: 0
End index: 5
Start index: 5
End index: 10
参考文献:https://docs . Oracle . com/javase/tutorial/essential/regex/bounds . html
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