Add delimiter if String contains a character?

I'm trying to figure out how to use a loop to append the delimiter ":" before the String s is found to contain a letter A-F.

If I have s = "10584f" then I would want the output to be "10584:f"

How should I go about doing this?

This is a simple solution that checks if it contains letters between a to f and put `:' accordingly.

public static void main(String[] args) {
    		String s = "10584f";
    		String newString = "";
    		for(int i=0;i<s.length();i++) {
    			if((int)s.charAt(i)>= (int)'a' &&  (int)s.charAt(i)<= (int)'f') {
    				newString = newString + ":" + s.charAt(i);
    			}else {
    				newString = newString  + s.charAt(i);
    			}
    		}
    		System.out.println(newString);
    	}

You need to check for the ASCII code of each character and append the delimiter before it on this condition:

public class Main {
	public static void main(String[] args) {
		String s = "10584f";
		System.out.println(appendDelimiter(s));
	}

	private static String appendDelimiter(String s) {
		StringBuilder sb = new StringBuilder();
		for(char c : s.toCharArray()) {
			int charCode = (int) c;
			if((charCode >= 65 && charCode <= 70) || (charCode >= 97 && charCode <= 102))
				sb.append(":");
			sb.append(c);
		}
		return sb.toString();
	}
}

Output:

10584:f

If your target string consists of digits followed by a single letter from A-F or a-f, a simple solution can be: replacement of the regex-match, (\\d+)([A-Fa-f]). This regex means group(1) consisting of digits and group(2) consisting of a letter from A-F or a-f. The regex can be replaced with $1:$2 where $1 and $2 specify group(1) and group(2) respectively.

import java.util.Arrays;

public class Main {
	public static void main(String[] args) {
		// Test strings
		String[] arr = { "10584f", "12345A", "13456b", "23456F" };
		for (int i = 0; i < arr.length; i++) {
			arr[i] = arr[i].replaceAll("(\\d+)([A-Fa-f])", "$1:$2");
		}

		// After replacement
		System.out.println(Arrays.toString(arr));
	}
}

Output:

[10584:f, 12345:A, 13456:b, 23456:F]

However, if the string can have multiple letters followed by digits and you want each letter to be prepended with a :, you can iterate the string starting from the index, 1 until the last character of the string and if you come across a letter, prepend it with a : as done in the function, withColonBeforeLetters given below:

Demo:

import java.util.Arrays;

public class Main {
	public static void main(String[] args) {
		// Test strings
		String[] arr = { "10584f", "12345A", "13456b", "23456F", "1abc", "123aBc" };
		for (int i = 0; i < arr.length; i++) {
			arr[i] = withColonBeforeLetters(arr[i]);
		}

		// After replacement
		System.out.println(Arrays.toString(arr));
	}

	static String withColonBeforeLetters(String s) {
		StringBuilder sb = new StringBuilder();

		// Put the first letter into sb
		sb.append(s.charAt(0));

		for (int i = 1; i < s.length(); i++) {
			char ch = s.charAt(i);
			if ((ch >= 'A' && ch <= 'F') || (ch >= 'a' && ch <= 'f')) {
				sb.append(':').append(ch);
			} else {
				sb.append(ch);
			}
		}
		return sb.toString();
	}
}

Output:

[10584:f, 12345:A, 13456:b, 23456:F, 1:a:b:c, 123:a:B:c]