How can i write the regex "All characters are the same"?

Question

I would like it to match:

aaaaaa
bb
c

but not:

aaabaaa
cd

...

Answer 1

Assuming the regex engine supports back-references,

^(.)\1*$

---

In Java it would be

theString.matches("(.)\\1*")

Answer 2

Using back references:

(.)(\1)*

Read: match any character followed by that same character 0 or more times.

Depending on the regexp engine and your needs, you might want to anchor the regex to only match the whole string, not substrings.