Find/replace number with commas in Java

Question

I have a requirement to put a comma between each digit of a number in a Java string.

Example input:

the number is 12345 and the number is 45678

The output should be:

the number is 1,2,3,4,5 and the number is 4,5,6,7,8

I wrote the following and I am not sure if it is optimized. If someone reviews it and help me fine tune it, it will be helpful.

public static void main(String args[]) {
 String str = "the number is 12345 and the number is 45678";
 System.out.println("Original Text:\n" + str);
 System.out.println("Updated Text:\n" + formatNumber(str, ','));
 }
 public static String formatNumber(String text, char separator) {
 Pattern p = Pattern.compile("\\d+");
 Matcher m = p.matcher(text);
 StringBuffer sb = new StringBuffer();
 while(m.find()) {
 String num = m.group();
 num = num.replaceAll(".(?!$)", "0ドル"+separator);
 m.appendReplacement(sb,num);
 }
 return m.appendTail(sb).toString();
 }

Output:

Original Text:
 the number is 12345 and the number is 45678
 Updated Text:
 the number is 1,2,3,4,5 and the number is 4,5,6,7,8
 Process finished with exit code 0

Though it gave me the output I wanted, I have a feeling that it is not highly optimized when the string size grows larger and larger.

Answer 1

Two of the really nice things about regular expressions are:

1. they can be compiled, which makes them fast, and reusable
2. they can replace what's between characters, not just the characters themselves.

In your case, you can reduce it to a "simple" lookaround regular expression. What you are looking for, is the gap between two digits...

in a regular expression, the gap can have no width. So, look for a place that follows a digit, and is also followed by a digit. Then, replace that place with a separator.

Then, compile and save that expression away, and reuse it:

private static final Pattern BETWEEN_DIGITS = Pattern.compile("(?<=\\d)(?=\\d)");
public static String formatNumber(String text, char separator) {
 return BETWEEN_DIGITS.matcher(text).replaceAll("" + separator);
}

That pattern is complicated.... but read up on look-behinds, and look-aheads: http://www.regular-expressions.info/lookaround.html

• Simon Forsberg: "" + separator, seriously? I used to do those ugly-tricks myself once. These days I always do String.valueOf or Character.toString. I don't think I need to tell you what goes on behind the scenes of "" + x (StringBuilder bla bla bla)