referring to the matched string in pattern replacement

[Peng Yu]

I want to replace a character matching a character class regex by a
pair of brackets enclosing the matched strings.

For example, if the regex is [A-Z], and the string is aXz, the output is a(X)z.

I don't see this syntax is capable of doing this. I could spell out
the characters in the character class, but that is tedious.

${parameter/pattern/string}

Is there any other smart way to achieve this in bash efficiently
without calling external programs?

-- 
Regards,
Peng