java - 如何识别不定冠词 "a"或 "an"？

Question

我的任务是设计一个正则表达式来识别英语中的不定冠词——单词“a”或“an”，即编写一个正则表达式来识别单词 a 或单词 an。我必须通过编写一个测试驱动程序来测试表达式，该驱动程序读取一个包含大约十行文本的文件。您的程序应该计算单词“a”和“an”的出现次数。 I 将不匹配字符 a 和 an 在诸如 th 之类的单词中 .
到目前为止，这是我的代码:

import java.io.IOException;
import java.util.Arrays;
import java.util.regex.Pattern;
import java.util.regex.Matcher;

public class RegexeFindText {
   public static void main(String[] args) throws IOException {

      // Input for matching the regexe pattern
       String file_name = "Testing.txt";
   
           ReadFile file = new ReadFile(file_name);
           String[] aryLines = file.OpenFile();  
           String asString = Arrays.toString(aryLines);
       
            // Regexe to be matched
               String regexe = ""; //<<--this is where the problem lies
       
           int i;
           for ( i=0; i < aryLines.length; i++ ) {
           System.out.println( aryLines[ i ] ) ;
           }
   

      // Step 1: Allocate a Pattern object to compile a regexe
      Pattern pattern = Pattern.compile(regexe);
      //Pattern pattern = Pattern.compile(regexe, Pattern.CASE_INSENSITIVE);  
      // case-        insensitive matching

      // Step 2: Allocate a Matcher object from the compiled regexe pattern,
      //         and provide the input to the Matcher
      Matcher matcher = pattern.matcher(asString);

      // Step 3: Perform the matching and process the matching result

      // Use method find()
      while (matcher.find()) {     // find the next match
         System.out.println("find() found the pattern \"" + matcher.group()
               + "\" starting at index " + matcher.start()
               + " and ending at index " + matcher.end());
      }

      // Use method matches()
      if (matcher.matches()) {
         System.out.println("matches() found the pattern \"" + matcher.group()
               + "\" starting at index " + matcher.start()
               + " and ending at index " + matcher.end());
      } else {
         System.out.println("matches() found nothing");
      }

      // Use method lookingAt()
      if (matcher.lookingAt()) {
         System.out.println("lookingAt() found the pattern \"" + matcher.group()
               + "\" starting at index " + matcher.start()
               + " and ending at index " + matcher.end());
      } else {
         System.out.println("lookingAt() found nothing");
      }
   }
}

我必须用什么来在我的文本中找到这些词？

Answer 1

这是将匹配“a”或“an”的正则表达式:

String regex = "\\ban?\\b";

让我们分解这个正则表达式:

\b表示词边界(单反斜杠在java中写为"\\")

a只是一个文字 "a"

n?表示零或一个文字 "n"