Get the Integer from the end of a string (variable length)

Question

I have a string of a variable length and at the end of the string are some digits. What would be the best / efficient way, to parse the string and get the number from the end as an Integer?

The String and the digits at the end can can be of any length. For example:

abcd123 --> 123
abc12345 --> 12345
ab4cd1 --> 1

Answer 1

Something along the line of:

final static Pattern lastIntPattern = Pattern.compile("[^0-9]+([0-9]+)$");
String input = "...";
Matcher matcher = lastIntPattern.matcher(input);
if (matcher.find()) {
    String someNumberStr = matcher.group(1);
    int lastNumberInt = Integer.parseInt(someNumberStr);
}

could do it.

This isn't necessary the "most efficient" way, but unless you have a critical bottleneck around this code (as: extract int from millions of String), this should be enough.

Answer 2

Other solutions provided here are fine, so I'll provide this one just to be a bit different:

public static BigInteger lastBigInteger(String s) {
    int i = s.length();
    while (i > 0 && Character.isDigit(s.charAt(i - 1))) {
        i--;
    }
    return new BigInteger(s.substring(i));
}

• It manually looks for the position of the last non-Character.isDigit(char)
  • It still works if the input is all digit
• It uses BigInteger, so it can handle really large numbers at the end of really long strings.
  • Use Integer.parseInt or Long.parseLong if either sufffice