How can I find repeated characters with a regex in Java?

Question

Can anyone give me a Java regex to identify repeated characters in a string? I am only looking for characters that are repeated immediately and they can be letters or digits.

Example:

abccde <- looking for this (immediately repeating c's)

abcdce <- not this (c's seperated by another character)

Answer 1

Try "(\\w)\\1+"

The \\w matches any word character (letter, digit, or underscore) and the \\1+ matches whatever was in the first set of parentheses, one or more times. So you wind up matching any occurrence of a word character, followed immediately by one or more of the same word character again.

(Note that I gave the regex as a Java string, i.e. with the backslashes already doubled for you)

Answer 2

Regular Expressions are expensive. You would probably be better off just storing the last character and checking to see if the next one is the same. Something along the lines of:

String s;
char c1, c2;
c1 = s.charAt(0);
for(int i=1;i<s.length(); i++){
    char c2 = s.charAt(i);

    // Check if they are equal here

    c1=c2;
}

Answer 3

String stringToMatch = "abccdef";
Pattern p = Pattern.compile("(\\w)\\1+");
Matcher m = p.matcher(stringToMatch);
if (m.find())
{
    System.out.println("Duplicate character " + m.group(1));
}