Question

My task is to devise a regular expression that will recognize the indefinite article in English – the word “a” or “an” i.e. to write a regular expression to identify the word a or the word an. I must test the expression by writing a test driver which reads a file containing approximately ten lines of text. Your program should count the occurrences of the words “a” and “an”. I shall not match the characters a and an in words such as than.

This is my code so far:

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");
      }
   }
}

What do I have to use to find those words within my text?

Answer 1

Here’s the regex that will match “a” or “an”:

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

Let’s break that regex down:

• \b means word boundary (a single back slash is written as "\\" in java)
• a is simply a literal "a"
• n? means zero or one literal "n"