Regex to determine if string is a single repeating character [duplicate]

Question

What is the regex pattern to determine if a string solely consists of a single repeating character?

e.g.

"aaaaaaa" = true
"aaabbbb" = false
"$$$$$$$" = true

This question checks if a string only contains repeating characters (e.g. "aabb") however I need to determine if it is a single repeating character.

Answer 1

You can try with back reference

^(.)\1{1,}$

DEMO

Pattern Explanation:

  ^                        the beginning of the string
  (                        group and capture to \1:
    .                        any character except \n
  )                        end of \1
  \1{1,}                   what was matched by capture \1 (at least 1 times)
  $                        the end of the string

---

Backreferences match the same text as previously matched by a capturing group. The backreference \1 (backslash one) references the first capturing group. \1 matches the exact same text that was matched by the first capturing group.

---

In Java you can try

"aaaaaaaa".matches("(.)\\1+") // true

There is no need for ^ and $ because String.matches() looks for whole string match.