JS regular expression for repeated unique character

Question

I am trying to determine whether a string contains exactly one given distinct character, repeated at least once. For example "bbbbb" should match but "ccMMcc" should not as it contains two different letters, c and m respectively. Presuming regular expressions would be the simplest way to do so, shame I suck at them, what will I need to match my string against?

Answer 1

The regex is:

\b(\w)\1*\b

that is:

• \b: word boundary
• (\w): the first char, group
• \1*: any number of repetition of the first char

Answer 2

"tttt".match(/^(.)\1*$/) returns ["tttt","t"] but "test".match(/^(.)\1*$/) returns null

Answer 3

You can use a backreference:

^(.)\1+$