What is a regular expression to find if string contains only repeating characters?

Question

I've already gone through: Regex to match four repeated letters in a string using a Java pattern and Regular expression to match any character being repeated more than 10 times

But they aren't useful in my case. They are fine if I just want to check if a string is containing repeated characters (like 1111, abccccd, 12aaaa3b, etc.). What I want is to check if string is comprising entirely of repeated characters only i.e. aabb111, 1111222, 11222aaa, etc.

Can anyone help me out with this?

Answer 1

Use ((.)\2+)+ as pattern:

String pattern = "((.)\\2+)+";
System.out.println("a".matches(pattern));        // false
System.out.println("1aaa".matches(pattern));     // false
System.out.println("aa".matches(pattern));       // true
System.out.println("aabb111".matches(pattern));  // true
System.out.println("1111222".matches(pattern));  // true
System.out.println("11222aaa".matches(pattern)); // true
System.out.println("etc.".matches(pattern));     // false

About the pattern:

• (...): capture matched part as group. (starting from 1)

  ((.)\2+)+
  ^^ 
  |+----- group 2
  +----- group 1
  

• (.): match any character (except newline) and capture it as group 2 (because it come after enclosing parenthesis).
• \2: backreference to the matched group. If (.) matched a character x, \2 matches another x (not any character, but only x).
• PATTERN+: matches one or more matches of PATTERN.
• (.)\2+: match repeating characters greedy.