Expand character ranges like 0-9, a-z, and A-Z

CharacterRange.java

package your.package;

public class CharacterRange {

	/**
	 * Expands character ranges similar to 0-9, a-z and A-Z.
	 * 
	 * @param string a string to be expanded
	 * @return a string
	 */
	public static String expand(String string) {

		StringBuilder buffer = new StringBuilder();

		int i = 1;
		while (i <= string.length()) {
			final char a = string.charAt(i - 1); // previous char
			if ((i < string.length() - 1) && (string.charAt(i) == '-')) {
				final char b = string.charAt(i + 1); // next char
				char[] expanded = expand(a, b);
				if (expanded.length != 0) {
					i += 2; // skip
					buffer.append(expanded);
				} else {
					buffer.append(a);
				}
			} else {
				buffer.append(a);
			}
			i++;
		}

		return buffer.toString();
	}

	private static char[] expand(char a, char b) {
		char[] expanded = expand(a, b, '0', '9'); // digits (0-9)
		if (expanded.length == 0) {
			expanded = expand(a, b, 'a', 'z'); // lower case letters (a-z)
		}
		if (expanded.length == 0) {
			expanded = expand(a, b, 'A', 'Z'); // upper case letters (A-Z)
		}
		return expanded;
	}

	private static char[] expand(char a, char b, char min, char max) {

		if ((a > b) || !(a >= min && a <= max && b >= min && b <= max)) {
			return new char[0];
		}

		char[] buffer = new char[(b - a) + 1];
		for (int i = 0; i < buffer.length; i++) {
			buffer[i] = (char) (a + i);
		}
		return buffer;
	}

	public static void main(String[] args) {

		String[] ranges = { //
				"0-9", "a-z", "A-Z", "0-9a-f", "a-z2-7", "0-9a-v", //
				"0-9a-hj-kmnp-tv-z", "0-9a-z", "1-9A-HJ-NP-Za-km-z", //
				"A-Za-z0-9", "A-Za-z0-9+/", "A-Za-z0-9-_" };

		for (int i = 0; i < ranges.length; i++) {

			String input = ranges[i];
			String output = CharacterRange.expand(ranges[i]);

			System.out.println("input:  " + input);
			System.out.println("output: " + output);
			System.out.println();
		}
	}
}

Output:

input:  0-9
output: 0123456789

input:  a-z
output: abcdefghijklmnopqrstuvwxyz

input:  A-Z
output: ABCDEFGHIJKLMNOPQRSTUVWXYZ

input:  0-9a-f
output: 0123456789abcdef

input:  a-z2-7
output: abcdefghijklmnopqrstuvwxyz234567

input:  0-9a-v
output: 0123456789abcdefghijklmnopqrstuv

input:  0-9a-hj-kmnp-tv-z
output: 0123456789abcdefghjkmnpqrstvwxyz

input:  0-9a-z
output: 0123456789abcdefghijklmnopqrstuvwxyz

input:  1-9A-HJ-NP-Za-km-z
output: 123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz

input:  A-Za-z0-9
output: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789

input:  A-Za-z0-9+/
output: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/

input:  A-Za-z0-9-_
output: ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_