Is there any way to convert first word in the string to camelcase format?

Here is a solution:

public static void main(String[] args) {
    try {
        System.out.println(convertString("1234_hello"));
        System.out.println(convertString("hello Data"));
        System.out.println(convertString("hELLO data"));
        System.out.println(convertString("1234hEllo"));
        System.out.println(convertString("heLLO1234hEllo"));
        System.out.println(convertString("$hello"));
        System.out.println(convertString("$1234hEllo_TTHjjZ"));
    }
    catch (Exception e) {
        e.printStackTrace();
    }
}

private static String convertString(String string) {

    String result = string;

    final String regex1 = "^([^a-zA-Z]+)([a-zA-Z])([a-zA-Z]*)([^a-zA-Z].*)$";
    final String regex2 = "^([a-zA-Z])([a-zA-Z]*)([^a-zA-Z].*)$";
    final String regex3 = "^([^a-zA-Z]+)([a-zA-Z])([a-zA-Z]*)$";

    final Pattern pattern1 = Pattern.compile(regex1, Pattern.MULTILINE);
    final Pattern pattern2 = Pattern.compile(regex2, Pattern.MULTILINE);
    final Pattern pattern3 = Pattern.compile(regex3, Pattern.MULTILINE);

    Matcher matcher1 = pattern1.matcher(string);
    Matcher matcher2 = pattern2.matcher(string);
    Matcher matcher3 = pattern3.matcher(string);

    if (matcher1.find()) {
        result = matcher1.group(1) + matcher1.group(2).toUpperCase() + matcher1.group(3).toLowerCase() + matcher1.group(4);
    }
    else if (matcher2.find()) {
        result = matcher2.group(1).toUpperCase() + matcher2.group(2).toLowerCase() + matcher2.group(3);
    }
    else if (matcher3.find()) {
        result = matcher3.group(1) + matcher3.group(2).toUpperCase() + matcher3.group(3).toLowerCase();
    }

    return result;
}

The result is as expected:

1234_Hello
Hello Data
Hello data
1234Hello
Hello1234hEllo
$Hello
$1234Hello_TTHjjZ

I have a solution for you but it is not efficient:

public static String toCamelCase(String input) {
    StringBuilder output = new StringBuilder();
    for(int i = 0; i < input.length(); i++) {
        if(i == 0) {
            output.append(Character.toUpperCase(input.charAt(i)));
            continue;
        }
        if(Character.isLetter(input.charAt(i))) {
            if(Character.isLetter(input.charAt(i-1))) {
                output.append(Character.toLowerCase(input.charAt(i)));
            } else {
                output.append(Character.toUpperCase(input.charAt(i)));
            }
        } else {
            output.append(input.charAt(i));
        }
    }
    return output.toString();
}