How can we add an underscore before each capital letter inside a java String?

Using the StringBuffer class

To add underscore before each capital letter in a String using StringBuffer −

• Create an empty StringBuffer object.

• The isUpperCase() method of Character class accepts a character and verifies whether it is in upper case, if so, this method returns true. Using this method, verify each character in the String.

• In case of upper case letter append underscore before it, using the append() method.

Example

public class Adding_BeforeCapital {
   public static void main(String args[]) {
      String str = "HelloHowAreYouWelcome";
      StringBuffer sb = new StringBuffer();
      for (int i = 0; i < str.length(); i++) {
         if(Character.isUpperCase(str.charAt(i))) {
            sb.append("_");
            sb.append(str.charAt(i));
         } else {
            sb.append(str.charAt(i));
         }
      }
      String result = sb.toString();
      System.out.println(result);
   }
}

Output

_Hello_How_Are_You_Welcome

Using regular expression

The replaceAll() method of the String class accepts two strings representing a regular expression and a replacement String and replaces the matched values with given String.

Example

public class Adding_BeforeCapital {
   public static void main(String args[]) {
      String str = "HelloHowAreYouWelcome";
      String result = str.replaceAll("()([A-Z])", "$1_$2");
      System.out.println(result);
   }
}

Output

_Hello_How_Are_You_Welcome